 Section 1 of Grey's Anatomy Part 3. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recorded by Laurie Ann Walden. Anatomy of the Human Body Part 3 by Henry Gray. Section 1. Angiology Introduction. The vascular system is divided for descriptive purposes into A, the blood vascular system, which comprises the heart and blood vessels for the circulation of the blood, and B, the lymph vascular system, consisting of lymph glands and lymphatic vessels through which a colorless fluid, the lymph, circulates. It must be noted, however, that the two systems communicate with each other and are intimately associated developmentally. The heart is the central organ of the blood vascular system and consists of a hollow muscle. By its contraction, the blood is pumped to all parts of the body through a complicated series of tubes termed arteries. The arteries undergo enormous ramification in their course throughout the body and end in minute vessels called arterioles, which in their turn open into a close meshed network of microscopic vessels termed capillaries. After the blood has passed through the capillaries, it is collected into a series of larger vessels called veins, by which it is returned to the heart. The passage of the blood through the heart and blood vessels constitutes what is termed the circulation of the blood, of which the following is an outline. The human heart is divided by septa into right and left halves, and each half is further divided into two cavities. An upper termed the atrium and a lower, the ventricle. The heart therefore consists of four chambers, two, the right atrium and right ventricle, forming the right half, and two, the left atrium and left ventricle, the left half. The right half of the heart contains venous or impure blood, the left arterial or pure blood. The atria are receiving chambers and the ventricles distributing ones. From the cavity of the left ventricle, the pure blood is carried into a large artery, the aorta, through the numerous branches of which it is distributed to all parts of the body, with the exception of the lungs. In its passage through the capillaries of the body, the blood gives up to the tissues, the materials necessary for their growth and nourishment, and at the same time receives from the tissues the waste products resulting from their metabolism. In so doing it is changed from arterial into venous blood, which is collected by the veins, and through them returned to the right atrium of the heart. From this cavity the impure blood passes into the right ventricle and is then conveyed through the pulmonary arteries to the lungs. Through the capillaries of the lungs it again becomes arterialized and is then carried to the left atrium by the pulmonary veins. From the left atrium it passes into the left ventricle from which the cycle once more begins. The course of the blood from the left ventricle through the body, generally to the right side of the heart, constitutes the greater or systemic circulation. While its passage from the right ventricle through the lungs to the left side of the heart is termed the lesser or pulmonary circulation. It is necessary however to state that the blood which circulates through the spleen, pancreas, stomach, small intestine, and the greater part of the large intestine is not returned directly from these organs to the heart but is conveyed by the portal vein to the liver. In the liver this vein divides like an artery and ultimately ends in capillary-like vessels or sinusoids from which the rootlets of a series of veins called the hepatic veins arise. These carry the blood into the inferior vena cava once it is conveyed to the right atrium. From this it will be seen that the blood contained in the portal vein passes through two sets of vessels. One, the capillaries in the spleen, pancreas, stomach, etc. and two, the sinusoids in the liver. The blood in the portal vein carries certain of the products of digestion the carbohydrates which are mostly taken up by the liver cells and stored as glycogen and the protein products which remain in solution and are carried into the general circulation to the various tissues and organs of the body. Speaking generally the arteries may be said to contain pure and the veins impure blood. This is true of the systemic but not of the pulmonary vessels since it has been seen that the impure blood is conveyed from the heart to the lungs by the pulmonary arteries and the pure blood return from the lungs to the heart by the pulmonary veins. Arteries therefore must be defined as vessels which convey blood from the heart and veins as vessels which return blood to the heart. The structure of arteries. The arteries are composed of three coats, an internal or endothelial coat, tunica intima of coulaker, a middle or muscular coat, tunica media, and an external or connective tissue coat, tunica adventitia. The two inner coats together are very easily separated from the external as by the ordinary operation of tying a ligature around an artery. A fine string be tied forcibly upon an artery and then taken off. The external coat will be found undivided but the two inner coats are divided in the track of the ligature and can easily be further dissected from the outer coat. The inner coat, tunica intima, can be separated from the middle by a little maceration or it may be stripped off in small pieces but on account of its friability it cannot be separated as a complete membrane. It is a fine, transparent, colorless structure which is highly elastic and after death is commonly corrugated into longitudinal wrinkles. The inner coat consists of, one, a layer of pavement endothelium, the cells of which are polygonal, oval or fusiform and have very distinct round or oval nuclei. This endothelium is brought into view most distinctly by staining with nitrate of silver. Two, a sub-endothelial layer consisting of delicate connective tissue with branched cells lying in the interspaces of the tissue. In arteries of less than two millimeters in diameter the sub-endothelial layer consists of a single stratum of stellate cells and the connective tissue is only largely developed in vessels of a considerable size. Three, an elastic or fenestrated layer which consists of a membrane containing a network of elastic fibers having principally a longitudinal direction and in which under the microscope small elongated apertures or perforations may be seen giving it a fenestrated appearance. It was therefore called by Henley the fenestrated membrane. This membrane forms the chief thickness of the inner coat and can be separated into several layers some of which present the appearance of a network of longitudinal elastic fibers and others a more membranous character marked by pale lines having a longitudinal direction. In minute arteries the fenestrated membrane is a very thin layer but in the larger arteries and especially the aorta it has a very considerable thickness. The middle coat tunica media is distinguished from the inner by its color and by the transverse arrangement of its fibers. In the smaller arteries it consists principally of plain muscle fibers in fine bundles arranged in lamellae and disposed circularly around the vessel. These lamellae vary in number according to the size of the vessel the smallest arteries having only a single layer and those slightly larger three or four layers. It is to this coat that the thickness of the wall of the artery is mainly due. In the larger arteries as the iliac, femoral and carotid elastic fibers unite to form lamellae which alternate with the layers of muscular fibers. These lamellae are united to one another by elastic fibers which pass between the muscular bundles and are connected with the fenestrated membrane of the inner coat. In the largest arteries as the aorta and inominate the amount of elastic tissue is very considerable. In these vessels a few bundles of white connective tissue have also been found in the middle coat. The muscle fiber cells are about 50 microns in length and contain well-marked rod-shaped nuclei which are often slightly curved. The external coat tunica adventitia consists mainly of fine and closely felted bundles of white connective tissue but also contains elastic fibers in all but the smallest arteries. The elastic tissue is much more abundant next the tunica media and it is sometimes described as forming here between the adventitia and media a special layer the tunica elastica externa of Henley. This layer is most marked in arteries of medium size. In the largest vessels the external coat is relatively thin but in small arteries it is of greater proportion at thickness. In the smaller arteries it consists of a single layer of white connective tissue and elastic fibers. While in the smallest arteries just above the capillaries the elastic fibers are wanting and the connective tissue of which the coat is composed becomes more nearly homogenous the nearer it approaches the capillaries and is gradually reduced to a thin membranous envelope which finally disappears. Some arteries have extremely thin walls in proportion to their size. This is especially the case in those situated in the cavity of the cranium and vertebral canal the difference depending on the thinness of the external and middle coats. The arteries in their distribution throughout the body are included in thin fibro areolar investments which form their sheaths. The vessel is loosely connected with its sheath by delicate areolar tissue and the sheath usually encloses the accompanying veins and sometimes a nerve. Some arteries, as those in the cranium, are not included in sheaths. All the larger arteries, like the other organs of the body are supplied with blood vessels. These nutrient vessels called the vase of vasorum arise from a branch of the artery or from a neighboring vessel at some considerable distance from the point at which they are distributed. They ramify in the loose areolar tissue connecting the artery with its sheath and are distributed to the external coat but do not, in man, penetrate the other coats. In some of the larger mammals a few vessels have been traced into the middle coat. Minute veins return the blood from these vessels. They empty themselves into the vein or veins accompanying the artery. Lymphatic vessels are also present in the outer coat. Arteries are also supplied with nerves which are derived from the sympathetic but may pass through the cerebrospinal nerves. They form intricate plexuses upon the surfaces of the larger trunks and run along the smaller arteries as single filaments or bundles of filaments which twist around the vessel and unite with each other in a plexiform manner. The branches derived from these plexuses penetrate the external coat and are distributed principally to the muscular tissue of the middle coat and thus regulate by causing the contraction and relaxation of this tissue the amount of blood sent to any part. The capillaries the smaller arterial branches accepting those of the cavernous structure of the sexual organs of the splenic pulp and of the placenta terminate in networks of vessels which pervade nearly every tissue of the body. These vessels from their minute size are termed capillaries. They are interposed between the smallest branches of the arteries and the commencing veins constituting a network the branches of which maintain the same diameter throughout. The meshes of the network are more uniform in shape and size than those formed by the anastomoses of the small arteries and veins. The diameters of the capillaries vary in the different tissues of the body the usual size being about 8 microns. The smallest are those of the brain and the mucous membrane of the intestines and the largest, those of the skin and the marrow of bone where they are stated to be as large as 20 microns in diameter. The form of the capillary net varies in the different tissues the meshes being generally rounded or elongated. The rounded form of mesh is most common and prevails in this network as in the lungs in most glands in mucous membranes and in the cutis. The meshes are not of an absolutely circular outline but more or less angular sometimes nearly quadrangular or polygonal or more often irregular. Elongated meshes are observed in the muscles and nerves the meshes resembling parallelograms in form the long axis of the mesh running parallel with the long axis of the muscle. Sometimes the capillaries have a looped arrangement a single vessel projecting from the common network and returning after forming one or more loops as in the papillae of the tongue and skin. The number of the capillaries in the size of the meshes determine the degree of vascularity of a part. The closest network in the smallest interspaces are found in the lungs and in the coroid coat of the eye. In the interspaces are smaller than the capillary vessels themselves. In the intertubular plexus of the kidney in the conjunctiva and in the cutis the interspaces are from 3 to 4 times as large as the capillaries which form them and in the brain from 8 to 10 times as large as the capillaries in their long diameters and from 4 to 6 times as large in their transverse diameters. In the adventitia of arteries the width of the meshes is 10 times that of the capillary vessels. As a general rule the more active the function of the organ the closer is its capillary net and the larger its supply of blood. The meshes of the network are very narrow in all growing parts in the glands and in the mucous membranes wider in bones and ligaments which are comparatively inactive. Blood vessels are nearly altogether absent in tendons in which very little organic change occurs after their formation. In the liver the capillaries take a more or less radial course toward the intralobular vein and their walls are incomplete so that the blood comes into direct contact with the liver cells. These vessels in the liver are not true capillaries but sinusoids. They are developed by the growth of columns of liver cells into the blood spaces of the embryonic organ. Structure. The wall of a capillary consists of a fine transparent endothelial layer composed of cells joined edge to edge by an interstitial cement substance and continuous with the endothelial cells which line the arteries and veins. When stained with nitrate of silver the edges which bound the epithelial cells are brought into view. These cells are of large size and of an irregular polygonal or lancelate shape each containing an oval nucleus which may be displayed by carmine or hematoxilin. Between their edges, at various points of their meeting, roundish dark spots are sometimes seen which have been described as stomata though they are closed by intercellular substance. They have been believed to be the situations through which the colorless corpuscles of the blood when migrating from the blood vessels emerge. But this view though probable is not universally accepted. Colossal describes these cells as having a rather more complex structure. He states that each consists of two parts of hyaline ground plates and of a protoplasmic granular part in which is embedded the nucleus on the outside of the ground plates. The hyaline internal coat of the capillaries does not form a complete membrane but consists of plates which are inelastic and though in contact with each other are not continuous. When therefore the capillaries are subjected to intravascular pressure the plates become separated from each other. The protoplasmic portions of the cells on the other hand are united together. In some organs, for example the glomeruli of the kidneys, intercellular cement cannot be demonstrated in the capillary wall and the cells are believed to form a sensation. In many situations a delicate sheath or envelope of branched nucleated connective tissue is found around the simple capillary tube particularly in the larger ones and in other places especially in the glands the capillaries are invested with retiform connective tissue. Sinusoids In certain organs for example the heart, the liver the suprarenal and parathyroid glands the glomus caroticum and glomus coxidium the smallest blood vessels present various differences from true capillaries. They are wider with an irregular lumen and have no connective tissue covering their endothelial cells being in direct contact with the cells of the organ. Moreover they are either arterial or venous and not intermediate as are the true capillaries. These vessels have been called sinusoids by Minot. They are formed by columns of cells or trabeculae pushing their way into a large blood vessel and carrying its endothelium before them. At the same time the wall of the vessel or space grows out between the cell columns. Structure of veins The veins like the arteries are composed of three coats internal, middle and external. And these coats are with the necessary modifications analogous to the coats of the arteries the internal being the endothelial the middle the muscular and the external connective tissue or areolar. The main difference between the veins and the arteries is in the comparative weakness of the middle coat in the former. In the smallest veins the three coats are hardly to be distinguished. The endothelium is supported on a membrane separable into two layers the outer of which is the thicker and consists of a delicate nucleated membrane, adventitia. While the inner is composed of a network of longitudinal elastic fibers, media. In the veins next above these in size 0.4 mm in diameter according to Kulaker a connective tissue layer containing numerous muscle fibers circularly disposed can be traced forming the middle coat while the elastic and connective tissue elements of the outer coat become more distinctly perceptible. In the middle sized veins the typical structure of these vessels becomes clear. The endothelium is of the same character as in the arteries but its cells are more oval and less fusiform. It is supported by a connective tissue layer consisting of a delicate network of branched cells and external to this is a layer of elastic fibers disposed in the form of a network in place of the definite fenestrated membrane seen in the arteries. This constitutes the internal coat. The middle coat is composed of a thick layer of connective tissue with elastic fibers intermixed in some veins with a transverse layer of muscular tissue. The white fibers element is in considerable excess and the elastic fibers are in much smaller proportion in the veins than in the arteries. The outer coat consists as in the arteries of areolar tissue with longitudinal elastic fibers. In the largest veins the outer coat is from two to five times thicker than the middle coat and contains a large number of longitudinal muscle fibers. These are most distinct in the inferior vena cava especially at the termination of this vein in the heart, in the trunks of the hepatic veins and all the large trunks of the portal vein and in the external iliac, renal and azygos veins. In the renal and portal veins they extend through the whole thickness of the outer coat but in the other veins mentioned a layer of connective and elastic tissue is found external to the muscular fibers. All the large veins which open into the heart are covered for a short distance with a layer of striped muscular tissue continued onto them from the heart. Muscular tissue is wanting 1. In the veins of the maternal part of the placenta 2. In the venous sinuses of the dura mater and the veins of the pia mater of the brain and medulla 3. In the veins of the retina 4. In the veins of the cancellus tissue of bones 5. In the venous spaces of the corpora cavernosa The veins of the above mentioned parts consist of an internal endothelial lining supported on one or more layers of areolar tissue. Most veins are provided with valves which serve to prevent the reflux of the blood. Each valve is formed by a reduplication of the inner coat strengthened by connective tissue and elastic fibers and is covered on both surfaces with endothelium the arrangement of which differs on the two surfaces. On the surface of the valve next to the wall of the vein the cells are arranged transversely while on the other surface over which the current of blood flows the cells are arranged longitudinally in the direction of the current. Most commonly two such valves are found placed opposite one another more especially in the smaller veins or in the larger trunks at the point where they are joined by smaller branches. Occasionally there are three and sometimes only one. The valves are semi-lunar they are attached by their convex edges to the wall of the vein the concave margins are free directed in the course of the venous current and lie in close opposition with the wall of the vein as long as the current of blood takes its natural course. If however any regurgitation takes place the valves become distended their opposed edges are brought into contact and the current is interrupted. The wall of the vein on the cardiac side of the point of attachment of each valve is expanded into a pouch or sinus which gives to the vessel when injected or distended with blood a knotted appearance. The valves are very numerous in the veins of the extremities especially of the lower extremities these vessels having to conduct the blood against the force of gravity. They are absent in the very small veins that is those less than two millimeters in diameter also in the vena cavae hepatic, renal, uterine and ovarian veins. A few valves are found in each spermatic vein and one also at its point of junction with the renal vein or inferior vena cavae respectively. The cerebral and spinal veins the veins of the cancelated tissue of bone, the pulmonary veins and the umbilical vein and its branches are also destitute of valves. A few valves are occasionally found in the asygus and intercostal veins. Rudimentary valves are found in the tributaries of the portal vena system. The veins like the arteries are supplied with nutrient vessels vasophasorum. Nerves also are distributed to them in the same manner as to the arteries but in much less abundance. End of section one. The blood. The blood is an opaque, rather viscous fluid of a bright red or scarlet color when it flows from the arteries of a dark red or purple color when it flows from the veins. It is salt to the taste and has a peculiar faint odor and an alkaline reaction. Its specific gravity is about 1.06 and its temperature is generally about 37 degrees though varying slightly in different parts of the body. General composition of the blood Blood consists of a fairly yellow fluid the plasma or liquor sanguinous in which are suspended numerous minute particles, the blood corpuscles. The majority of which are colored and give the blood its red tint. If a drop of blood be placed on a thin layer on a glass slide and examined under the microscope a number of these corpuscles will be seen floating in the plasma. The blood corpuscles are of three kinds. One, colored corpuscles or erythrocytes. Two, colorless corpuscles or leukocytes and three, blood platelets. One, colored or red corpuscles erythrocytes. When examined under the microscope are seen to be circular discs biconcave in profile. The disc has no nucleus but in consequence of its biconcave shape presents according to the alterations of focus under an ordinary high power a central part sometimes bright sometimes dark which has the appearance of a nucleus. It is to the aggregation of the red corpuscles that the blood owes its red hue. Although when examined by transmitted light their color appears to be only a faintly reddish yellow. The corpuscles vary slightly in size even in the same drop of blood but the average diameter is about 7.5 microns and the thickness about 2 microns. Footnote a micromilometer or micron is one one thousandth of a millimeter or one twenty five thousand of an inch. Endnote. Besides these there are found certain smaller corpuscles of about one half of the size just indicated. These are termed microcytes and are very scarce in normal blood. In disease conditions, for example anemia however, they are more numerous. The number of red corpuscles in the blood is enormous between four million and five million are contained in a cubic millimeter. Power states that the red corpuscles of an adult would present an aggregate surface of about three thousand square yards. If the web of a living frog's foot be spread out and examined under the microscope the blood is seen to flow in a continuous stream through the vessels and the corpuscles show no tendency to adhere to each other or to the walls of the vessel. Doubtless the same is the case in the human body but when human blood is drawn and examined on a slide without reagents the corpuscles tend to collect into heaps like rouleaux of coins. It has been suggested that this phenomenon may be explained by alteration and surface tension. During life the red corpuscles may be seen to change their shape under pressure so as to adapt themselves to some extent to the size of the vessel. They are however highly elastic and speedily recover their shape when the pressure is removed. They are readily influenced by the medium in which they are placed. In water they swell up lose their shape and become globular and osmosis. Subsequently the hemoglobin is dissolved out and the envelope can barely be distinguished as a faint circular outline. Solutions of salt or sugar denser than the plasma give them a stellate or creanated appearance exosmosis. But the usual shape may be restored by diluting the solution to the same tenacity as the plasma. The creanated outline may be produced as the first effect of the passage of an electric shock. Subsequently if sufficiently strong shock ruptures the envelope. A solution of salt isotonic with the plasma merely separates the blood corpuscles mechanically without changing their shape. Two views are held with regard to the structure of the erythrocytes. The older view, that of Rowlett, supposes that the corpuscle consists of a sponge work or stroma permeated by a solution of hemoglobin. Schaefer on the other hand believes that the hemoglobin solution is contained within an envelope or membrane and the fact stated above with regard to the osmotic behavior of the erythrocyte support this belief. The envelope consists mainly of lechicin, cholesterol and nucleoprotein. The colotus corpuscles or leukocytes are of various sizes some no larger other smaller than the red corpuscles. In human blood however the majority are rather larger than the red corpuscles and measure about 10 microns in diameter. On the average from 7000 to 12000 leukocytes are found in each cubic millimeter of blood. They consist of minute masses of nucleated protoplasm and exhibit several varieties which are differentiated from each other chiefly by the occurrence or non occurrence of granules in their protoplasm and by the staining reactions of these granules when present. The most numerous, 60% and important are irregular in shape possessed of the power of amoeboid movement and are characterized by nuclei which often consist of two or three parts, multi-partite connected together by fine threads of chromatin. The protoplasm is clear and contains a number of very fine granules which stain with acid dyes such as eosin or with neutral dyes and are therefore called oxafil or neutrophil. These cells are termed the polymorphonuclear leukocytes. Two, a second variety comprises from 1 to 4% of the leukocytes. They are larger than the previous kind and are made up of polygranular protoplasm, the granules being highly refractile and grouped around single nuclei of horseshoe shape. The granules stain deeply with eosin and the cells are therefore often termed eosinophil corpuscles. Three, the third variety is called the hyaline cell or macrosite. This is usually about the same size as the eosinophil cell and when at rest is spherical in shape and contains a single round or oval nucleus. The protoplasm is free from granules but it is not quite transparent having the appearance of ground glass. Four, the fourth kind of colorless corpuscle is designated the lymphocyte because it is identical with the cell derived from the lymph glands or other lymphoid tissue. It is the smallest of the leukocytes and consists chiefly of a spheroidal nucleus with a very little surrounding protoplasm of a homogeneous nature. It is regarded as the immature form of the hyaline cell. The third and fourth varieties together constitute from 20 to 30% of the colorless corpuscles but of these two varieties the lymphocytes are by far the more numerous. Lucocytes having in their protoplasm granules which stain with basic dyes basophil have been described as occurring in human blood but they are rarely found except in disease. The colorless corpuscles are very various in shape in living blood because many of them have the power of constantly changing their form by protruding finger-shaped or filamentous processes of their substance by which they move and take up granules from the surrounding medium. In locomotion the corpuscle pushes out a process of its substance a pseudopodium as it is called and then shifts the rest of the body into it. In the same way when any granule or particle comes in its way the corpuscle wraps a pseudopodium around it and then withdraws the pseudopodium with the contained particle into its own substance. By means of these amoeboid properties the cells have the power of wandering or emigrating from the blood vessels by penetrating their walls and thus finding their way into the extravascular spaces. A chemical investigation of the protoplasm of the lucocytes shows the presence of nucleoprotein and of globulin. The occurrence of small amounts of fat, lechithin and glycogen may also be demonstrated. The blood platelets are discoyed or irregularly shaped colorless refractile bodies much smaller than the red corpuscles. Each contains a central chromatin mass resembling a nucleus. Blood platelets possess the power of amoeboid movement. When blood is shed they rapidly disintegrate and form granular masses setting free prothrombin and the substance called by howl thromboplastin. It is doubtful whether they exist normally in circulating blood. End of section 2 Section 3 of Grey's Anatomy Part 3 This is a LibriVox recording All LibriVox recordings are in the public domain. For more information or to volunteer please visit LibriVox.org Recording by Shaleefa Malikim Anatomy of the Human Body Part 3 by Henry Gray Development of the Vascular System Part 1 Blood vessels first make their appearance in several scattered vascular areas which are developed simultaneously between the enteroderm and the mesoderm of the yolk sac. It ests outside the body of the embryo. Here a new type of cell the angioblast or vasoformative cell is differentiated from the mesoderm. These cells as they divide form small dense syncytial masses which is soon joined with similar masses by mesofine processes to form plexuses. These plexuses increase both by division and growth of its cells and by the addition of new angioblasts which differentiate from the mesoderm. Within the solid plexuses and also within the isolated masses of angioblasts vacuoles appear through liquefaction of the center part of the syncytium into plasma. The lumen of the blood vessels thus formed is probably intracellular. The flattened cells at the periphery form the anothelium. The nucleated red blood corpuscles develop either from small masses of the original angioblast left attached to the inner wall of the lumen or directly from the flat anothelial cells. In either case the syncytial mass thus formed projects from 2 the wall of the vessel. Such a mass is known as a blood island and hemoglobin gradually accumulates within it. Later the cells on the surface round up giving the mass a mulberry-like appearance. Then the red blood cells break loose and are carried away in the plasma. Such free blood cells continue to divide. The term blood island was originally used for the syncytial masses of angioblasts found in the area vascular, but it is probably best to limit the term to the masses within the lumen from which the red blood cells arise as Sabin has done. Blood islands have been seen in the area vascular in the onphalosomanteric fiend and arteries and in the dorsal aorta. The differentiation of angioblasts from the mesoderm occurs not only in the area vascular, but within the embryo and probably most of the larger blood vessels are developed in situ in this manner. This process of the differentiation of angioblasts from the mesoderm probably ceases in different regions of the embryo at different periods and after its cessation new vessels are formed by rounds from vessel already laid down in the form of capillary plexuses. The first rudiment of the heart appears as a pair of tubular vessels which are developed in this planchnopla of the pericordial area. These are named the primidif aortae and a direct continuity as soon established between them and the vessels of the orc sac. Each receives anteriorly a vein, the vitalin vein, from the orc sac and is prolonged backward on the lateral aspect of the notochord under the name of the dorsal aortae. The dorsal aortae give branches to the orc sac and are continued backward through the body-stalk as the umbilical arteries to the vile of the corian. Aeternid describes the circulation in an embryo which he estimates to be about 13 days old. The rudiment of the heart is situated immediately below the foregut and consists of a short stem. It gives off two vessels, the primidif aortae which run backward one on either side of the notochord and then pass into the body-stalk along which they are carried to the corian. From the corianic vile the blood is returned by a pair of umbilical veins which unite in the body-stalk to form a single vessel and subsequently encircle the mouth of the orc sac and open into the heart. At the junction of the orc sac and body-stalk each vein is joined by a branch from the vascular plexus of the orc sac. From his observations it seems that in the human embryo the corianic circulation is established before that on the orc sac. By the forward growth and flexure of the head the pericordial area the dimensions of the primidif aortae are folded backward on the ventral aspect of the foregurt and the original relation of the somatoplae and splenchnoplae layers of the pericordial area is reversed. Each primidif aorta now consists of a ventral and a dorsal part connected anteriorly by an arch. These three parts are named respectively the anterior ventral aorta and the first cephalic arch. The vitaline veins which enter the embryo through the anterior ball of the umbilical orifice are now continued with the posterior ends of the anterior ventral aorta. With the formation of the tailfold the posterior parts of the primitive aortae are carried forward in a ventral direction to form the posterior ventral aortae and primary caudal arches. In the pericardial region the two primitive aortae grow together and fuse to form a single tubular heart the posterior end of which receives the two vitaline veins while from its anterior end the two anterior ventral aortae emerge. The first cephalic arches pass through the mandibular arches and behind them five additional pairs subsequently develop so that all together six pairs of aortic arches are formed. The fifth arches are very transitory vessels connecting the ventral aortae with the dorsal ends of the sixth arches. By the rhythmical contraction of the tubular heart the blood is forced through the aortae and blood vessels of the vascular area from which it is returned to the heart by the vitaline veins. This constitutes the vitaline circulation and by means of it nutriment is absorbed from the orc vitalis. The vitaline veins at first open separately into the posterior end of the tubular heart but after a time their terminal portions fuse to form a single vessel. The vitaline veins ultimately drain the blood from the digestive tube and are modified to form the portal vein. This is caused by the growth of the liver which interrupts their direct continuity with the heart and the blood returned by them circulate through the liver before reaching the heart. With the atrophy of the orc sac the vitaline circulation diminishes and ultimately seizes while an increasing amount of blood is carried through the umbilical arteries to the villi of the corian. Subsequently, as a non-placental corionic villi atrophy their vessels disappear and then the umbilical arteries convey the whole of their content to the placenta whence it is returned to the heart by the umbilical veins. In this manner the placental circulation is established and by means of it nutritive materials are absorbed from and waste products given up to the maternal blood. The umbilical veins, like the vitaline undergo interruption in the developing liver and the blood returned by them passes through this organ before reaching the heart. Ultimately the ride umbilical vein shrivels up and disappears. During the occurrence of these changes great alterations take place in primitive heart and blood vessels. Further development of the heart between the endothelian lining and the outer wall of the heart there exists for a time an intricate, trabicular network of masodermal tissue from which at a later stage the muscule papillaris cordae tanninii and trabecule are developed. The simple tubular heart already described is elongated and bends on itself so as to form an S shaped loop the interior part bending to the right and the posterior part to the left. The intermediate portion arches transversely from left to right and then turns sharply forward into the interior part of the loop. Slight contractions make their appearance in the tube and divide it from behind forward into five parts. These 1. The sinus finosis 2. The primitive atrium 3. The primitive ventricle 4. The bulbous cordus and 5. The trancous arteriosis The constriction between the atrium and ventricle constitutes the arterial canal and indicates the side of the future atrioventricular valves. The sinus finosis is at first situated in the septum transversum a layer of mesoderm in which the liver and the central tendon of the diaphragm are developed behind the primitive atrium and is formed by the union of the vital and veins. The veins or ducts of cavier from the body of the embryo and the umbilical veins from the placenta subsequently open into it. The sinus is at first transversely and opens by median aperture into the primitive atrium Soon, however, it assumes an oblique position and becomes crescentic in form. Its right half or horn increases more rapidly than the left and the opening into the atrium now communicates with the right portion of the atrial cavity. The right horn and transverse portion of the sinus ultimately become incorporated with and form a part of the adult right atrium. The sign of union between it and the auricular being indicated in the interior of the atrium by a vertical crest the crystal terminalis of his. The left horn, which ultimately receives only the left duct of cavier, persists as the coronary signers. The vital and umbilical veins are soon replaced by a single vessel, the aviric vena cava and the three veins of the vena cava and right and left cavierian ducts open into the dorsal aspect of the atrium by a common slit-like aperture. The upper part of this aperture represents the opening of the permanent superior vena cava the lower, that of the inferior vena cava and the intermediate part the orophys of the coronary signers. The slit-like aperture lies oblique and is guarded by two halves the right and left venous valves above the opening these unite with each other and are continuous with the fold named the septum spherium below the opening they fuse to form a triangular thickening the spina vestibuli the right venous valve is retained a small septum the sinus septum grows from the posterior wall of the sinus venosis diffuse with the valve and divide it into two parts an upper, the valve of the inferior vena cava and a lower, the valve of the coronary sinus the extreme upper portion of the right venous valve, together with the septum spherium form the crista terminalis already mentioned the upper and middle thirds of the left venous valve disappear the lower third is continued into the spina vestibuli fuses with the septum secundum of the atria and takes part in the formation of the limbus fossae of valis the atrial canal is at first a short straight tube connecting the atrial with the ventricular portion of the heart but its growth is relatively slow and it becomes overleapt by the atria and ventricles so that its position on the surface of the heart is indicated only by an annular constriction its lumen has reduced to transverse slit and two thickenings appear one on its daughter and another on its ventral wall or anicardial cushions as they are termed projecting to the canal and meeting in the middle line unite to form the septum intermedium which divides the canal into two channels the future right and left atrial ventricular orifices the primitive atrium grows rapidly and partially encircles the bulbous cordus the groove against which the bulbous cordus lies is the first indication of a division into right and left atria the cavity of the primitive atria becomes subdivided into right and left chambers by septum the septum primum which grows downward and choose a cavity for a time the atria they communicate with each other by an opening the ostium primum of born below the free margin of the septum this opening is closed by the union of the septum primum with the septum intermedium and the communication between the atrias re-established through an opening which is developed in the upper part of the septum primum this opening is known as a foramen ovale ostium secundum of born and persists until birth a second septum the septum secundum semiluna in shape grows downward from the upper wall of the atrium immediately to the right of the primary septum and foramen ovale shortly after birth it fuses with the primary septum and by this means the foramen ovale is closed but sometimes the fusion is incomplete and the upper part of the foramen remains patent the limbus fossae of valis denotes the free margin of the septum secundum issuing from each lung is a pair of pulmonary veins each pair unites to form a single vessel and these in turn join in a common trunk which opens into the left atrium subsequently the common trunk and the two vessels forming it expand to the right of the atrium or greater part of the atrium the expansion reaching as far as the openings of the four vessels so that in the adult all four veins open separately into his left atrium the primitive ventricle becomes divided by septum the septum inferior or ventricular septum which grows upward from the lower part of the ventricle its position being indicated so its dorsal part increases more rapidly than its ventral portion and fuses with the dorsal part of the septum intermedium for a time an interventricular foramen exists above its ventral portion but this foramen is ultimately closed by the fusion of the altic septum with the ventricular septum when the heart assumes its S shaped form the bulbous cord is two and in front of the primitive ventricle the adjacent walls of the bulbous cord is in ventricle approximate, fuse and finally disappear and the bulbous cord is now communicated freely with the right ventricle while the junction of the bulbous with the trunk as arteriosus is brought directly to ventral two and applied to the atrial canal by the upgrowth of the ventricular septum this cord is is in great measure separated from the left ventricle but remains an integral part of the right ventricle of which it forms the infantibulum the trunk is arteriosus and bulbous cord is are divided by the aortic septum this makes its appearance in three portions one two distal rich like thickenings project into the lumen of the tube these increase in size and ultimately meet and fuse to form a septum which takes a spiral course towards the proximal end of the trunk as arteriosus it divides the distal part of the trunk into two vessels the aorta and pulmonary artery which lies side by side above but near the heart the pulmonary artery is in front of the aorta two four endocardial cushions appear in the proximal part of the trunk as arteriosus in the region of the future semiluna valves the manner in which these are related to the aortic septum is described below three two endocardial thickenings and tyra and posterior develop in the bulbous cord is and unite to form a short septum this joins above with the aortic septum and below with the ventricular septum the septum grows down into the ventricle as an oblique partition which ultimately blends with the ventricular septum in such a way as to bring the bulbous cord into a communication with the pulmonary artery and through the letter with the sixth pair of aortic arches while the left ventricle is brought into continuity with the aorta which communicates with the remaining aortic arches end of section three section four of Grey's Anatomy part three this is LibriVox recording or LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by Shulif Amalakim Anatomy of the human body part three by Henry Grey development of the vascular system part two the valves of the heart the atrioventricular valves are developed in relation to the atrial canal by the upward expansion of the basis of the ventricles the canal becomes invaginated into the ventricular cavities the invaginated margin forms the rudiment of the lateral cusps of the atrioventricular valves the medial or septal cusps of the valves are developed as downward prolongations of the septum intermedium the aortic and pulmonary semi-luna valves are formed from four endocardial thickenings in anterior and posterior and two lateral which appear at the proximal end of the truncus arteriosis as the aortic septum grows downward it divides each of the lateral thickenings into two thus giving rise to six thickenings the rudiment of the semi-luna valves three at the aortic and three at pulmonary orifice further development of the arteries recent observations show that practically none of the main vessels of the adult arise as such in the embryo in the site of each vessel a capillary network forms and by the enlargement of definite paths in this the larger arteries and veins are developed the branches of the main arteries are not always simple modifications of the vessels of the capillary network but may arise as new outgrows from the enlarged stem it has been seen that each primitive aorta consists of a ventral and dorsal part which are continuous through the first aortic arch the dorsal aortae at first run backward separately on eyes the side of the notochord but about a third week they fuse from about the level of the fourth thoracic to that of the fourth lumbar segment to form a single trunk the descending aorta the first aortic arches run through the mandibular arches and behind them five additional pairs are developed within the visceral arches so that in all six pairs of aortic arches are formed the first and second arches pass between the ventral and dorsal aortae while the others arise at first by common trunk from the trunk as arduoses but end separately in the dorsal aortae as unlike elongates the ventral aortae are drawn out and the third and fourth arches arise directly from these vessels in fishes these arches persists and give off branches to the gills in which the blood is oxygenated in mammals some of them remain as permanent structures while others disappear or become obliterated the interior ventral aortae these persists on both sides the right forms the enominate artery the right common and external carotid arteries the left gives rise to the short portion of the aortic arch which reaches from the origin of the enominate artery to that of the left common carotid artery B. the left common and external carotid arteries the aortic arches the first and second arches disappear early but the dorsal end of the second gives origin to the stapedial artery a vessel which atrophies in man but persists in some mammals it passes through the ring of the staves and divides into supraorbital infraorbital and mandibular branches which follow the three divisions of the trigeminal nerve the infraorbital and mandibular arise from a common stem the terminal part of which anestemosis with the external carotid on the obliteration of the stapedial artery this anestemosis enlarges and forms the internal maxillary artery and the branches of the stapedial artery are now branches of this vessel the common stem of the infraorbital and mandibular branches passes between the two roots of the auricular temporal nerve and becomes the middle meningial artery the original supraorbital branch of the stapedial was represented by the orbital twigs with the middle meningial the third aortic arch constitutes the commencement of the internal carotid artery and is therefore named the carotid arch the fourth right arch forms the right subclavian as far as the origin of its internal mammary branch while the fourth left arch constitutes the arch of the aorta between the origin of the left carotid artery and the termination of the ductus arteriosis the fifth arch disappears on both sides the sixth right arch disappears the sixth left arch gives off the pulmonary arteries and forms the ductus arteriosis this duct remains pervious during the whole of fetal life but is obliterated a few days after birth His showed that in the early embryo the right and left arches each gives a branch to the lungs but that later both pulmonary arteries take origin from the left arch the dorsal aortae persists and forms the continuations of the internal carotid arteries these arteries pass to the brain and each divides into an anterior and a posterior branch the former giving of the uftalmic and the anterior and middle cerebral arteries while the letter turns back and joins the cerebral part of the vertebral artery behind the third arch the right dorsal aorta disappears as far as the point with the two dorsal aortae fuses to form the descending aorta the part of the left dorsal aorta between the third and fourth arches disappears while the remainder persists to form the descending part of the arch of the aorta a constriction the aortic isthmus is sometimes seen in the aorta between the origin of the left subclavian and the attachment of the ductus arteriosis sometimes the right subclavian artery arises from the aortic arch distal to the origin of the left subclavian it passes upward and to the right behind the trachea and esophagus this condition may be explained by the persistence of the right dorsal aorta and the obliteration of the fourth right arch in birds the fourth right arch forms the arch of the aorta in reptiles the fourth arch on both sides persists and gives rise to the double aortic arch in these animals the heart originally lies on the ventral aspect of the pharynx immediately behind the stomaceum with the elongation of the neck and the development of the lungs it recedes within the toriques and as a consequence the interior ventral aorta are drawn out and the original position of the fourth and fifth arches is greatly modified thus on the right side the fourth recedes to the root of the neck while on the left side it is withdrawn within the thorax the recurrent nerves originally pass to the larynx under the sixth pair of arches and are therefore pulled backward with the descent of these structures so that in the adult the left nerve hooks around the ligamentum arteriosum owing to the disappearance of the fifths and sixths right arches the right nerve hooks around that immediately above them it is the commencement of the subclavian artery segmental arteries arise from the primitive dorsal aorta and cause between successive segments the seventh segmental artery is of special interest since it forms the lower end of the vertebral artery and when the fallen bud appears sends a branch to it the subclavian artery from the seventh segmental arteries the entire left subclavian and the greater part of the right subclavian are formed the second pair of segmental arteries accompany the heboclosal nerves to the brain and are named the heboclosal arteries each sends forward a branch which forms the cerebral part of the vertebral artery and anastomosis with the posterior branch of the internal carotid the two vertebrals unite on the ventral surface of the hindbrain to form the basilar artery later the heboclosal artery atrophies and the vertebral is connected with the first segmental artery the cervical part of the vertebral is developed from a longitudinal anastomosis between the first seven segmental arteries so that the seventh of these ultimately becomes the source of the artery as a result of the growth of the upper limb the subclavian artery increases greatly in size and the vertebral then appears to spring from it recent observations show that several segmental arteries contribute branches to the upper limb bud and form in it a free capillary anastomosis of these branches only one vis that derived from the seventh segmental artery to form the subclavian artery the subclavian artery is prolonged into the limb under the names of the auxiliary and racial arteries and these together constitute the arterial stem for the upper arm the direct continuation of this stem in the forearm is a volar interosseous artery a branch which accompanies the median nerve soon increases in size and becomes the main vessel median artery of the forearm while the volar interosseous diminishes later the radial and ulnar arteries are developed as branches of the brachial part of the stem and coincidentally with it enlargement the median artery recedes occasionally it persists as a vessel of some considerable size and then accompanies the median nerve into the palm of the hand the primary arterial stem for the lower limb is formed by the inferior gluteal sciatic artery which accompanies the sciatic nerve along the posterior aspect of the thigh to the back of the knee when it is continued at perineal artery this arrangement exists in reptiles and amphibians the femoral artery arises later as a branch of the common ilioc moving down the front and medial side of the thigh to the bend of the knee joins the inferior gluteal artery the femoral quickly enlarges and coincidentally with this the part of the inferior gluteal immediately above the knee and a ghost atrophy the anterior and posterior tibial arteries are branches of the main arterial stem further development of the veins the formation of the great veins of the embryo may be best considered by dividing them into two groups visceral and parietal the visceral veins the visceral veins are the two vitalin or ompholomus centric veins bringing the blood from the orc sac and the two umbilical veins returning the blood from the placenta these four veins open close together into the sinus venosis the vitalin veins run upward at first in front and subsequently on either side of the intestinal canal they are united on the ventral aspect of the canal and beyond this are connected to one another by two anesthmotic ranges one on the dorsal and the other on the ventral aspect of the duodenal portion of the intestine which is thus encircled by two venous rings into the middle or dorsal or dorsal anesthmosis the superior mesentric vein opens the portions of the veins above the upper ring become interrupted by the developing liver and broken up by it into a plexus of small capillary like vessels termed sinusoids minno the branches conveying the blood to this plexus are named the venae advientes and become the branches of the portal vein the vessels draining the plexus into the sinus venosis are termed the venae revientes and form the future hepatic veins ultimately the left venous reverence no longer communicates directly with the sinus venosis but opens into the right venous reverence the persistent part of the upper venous ring above the opening of the superior mesentric vein forms the trunk of the portal vein the two umbilical veins fuse early to form a single trunk in the body stark but remain separate within the embryo and pass forward to the sinus venosis in the side walls of the body like the vitalin veins their direct connection with the sinus venosis becomes interrupted by the developing liver and thus at this stage the hole of the blood from the orc sec and placenta goes through the substance of the liver before it reaches the heart the right umbilical and right vitalin veins shrivel and disappear the left umbilical on the other hand becomes enlarged and opens into the upper venous ring of the vitalin veins with the atrophy of the orc sec the left vitalin vein also undergoes atrophy and disappears finally a direct branch is established between this ring and the right hepatic vein this branch is named the ductus venosis and enlarging rapidly it forms a wide channel through which most of the blood returned from the placenta is carried direct to the heart without passing through the liver a small proportion of the blood from the placenta is however conveyed from the left umbilical vein through the liver through the left venous and venous the left umbilical vein and the ductus venosis undergo atrophy and obliteration after birth and form respectively the ligamentum terus and ligamentum venosum of the liver the parietal veins the first indication of a parietal system consists in the appearance of two short transverse veins the ducts of cavier are drawn on eyes aside into his sinus venosis each of these ducts receives an ascending and descending vein the ascending veins return the blood from the parietes of the trunk and from the wolfian bodies and are called cardial veins the descending veins return the blood from the head and are called primitive jugular veins the blood from the lower limbs by the right and left iliac and hypogestric veins which in the earlier stages of development open into the corresponding right and left cardial veins later a transverse branch the left common iliac vein is developed between the lower parts of the two cardial veins and through this the blood is carried into the right cardinal vein the portion of the left cardial vein below the left renal vein and disappears up to the point of entrance of the left spermatic vein the portion above the left renal vein persists as a hammy azegars and accessory hammy azegars veins and the lower portion of the highest left intercostal vein the right cardinal vein which now receives the blood from both lower extremities forms a large venous trunk along the posterior abdominal wall up to the level of the renal veins it forms a lower part of the inferior venacava above the level of the renal veins the right cardinal vein persists as a azegars vein and receives the right intercostal veins while the hammy azegars veins are brought into communication with it by the development of transverse branches in front of the vertebral column inferior venacava the development of the inferior venacava the development of the inferior venacava is associated with the formation of two veins the subcardinal veins these lie parallel to and on the ventral aspect of the cardinal veins and originate as longitudinal and estymosing channels which link up the tributaries from the mesentery to the cardinal veins they communicate with the cardinal veins above and below and also by a series of transverse branches the two subcardinals are for a time connected with each other in front of the aorta by cross branches but these disappear and are replaced by a single transverse channel at a level where the renal veins join the cardinals and at the same level a cross communication is established on either side between the cardinal and subcardinal the portion of the ref subcardinal behind this cross communication disappears while that in front it est the pre-renal part forms a connection with the ductus phenosis at a point of opening of the hepatic veins and rapidly enlarging receives the blood from the post-renal part of the right cardinal through the cross communication referred to in this manner a single trunk the inferior venacava is formed and consists of the proximal part of the ductus phenosis the pre-renal part of the right subcardinal vein the post-renal part of the right cardinal vein and the cross branch which joins these two veins the left subcardinal disappears except the part immediately in front of the renal vein which is retained as a left suprare renal vein the spermatic oropharian vein opens into the post-renal to the post-renal of the corresponding cardinal vein this portion of the right cardinal as already explained forms a lower part of the inferior venacava so that a right spermatic opens directly into that vessel the post-renal segment of the left cardinal disappears with the exception of the portion between this spermatic and renal vein which is retained as a terminal part of the left subcardinal vein in consequence of the atrophy of the wolfian bodies the cardinal veins diminish in size the primitive jugular veins on the other hand become enlarged owing to the rapid development of the head and brain they are further augmented by receiving the veins subclavian from the upper extremities and so come to form the chief veins of the cuverian ducts these ducts gradually assume an almost vertical position in consequence of the descent of the heart into the thorax the right and left cuverian ducts are originally of the same diameter and are frequently termed the right and left superior venacava by the development of a transverse branch the left enominated vein between the two primitive jugular veins the blood is carried across from the left to the right primitive jugular the portion of the right primitive jugular vein between the left enominated and the aziga's vein forms the upper part of the superior venacava of the adult the lower part of this vessel below the entrance of the aziga's vein is formed by the right cuverian duct below the origin of the transverse branch the left primitive jugular vein and left cuverian duct atrophy the former constituting the upper part of the highest left intercostal vein while the letter is represented by the ligament of the left venacava vestigial fold of Marshall and the oblique vein of the left adrium oblique vein of Marshall both right and left superior venacava are present in some animals and are occasionally found in the adult human being the oblique vein of the left adrium passes downward across the back of the left adrium to open into the coronary sinus which, as already indicated, represents the persistent left horn of the sinus finosis venus sinuses of the Duran Martyr the primary arrangement for drainage of the capillaries of the head consists of a primary head vein which it starts in the region of the mid-brain and runs cordal ward along the side of the brain tube to terminate at the duct of cuvier the primary head vein drains three plexuses of capillaries the anterior dural plexus the middle dural plexus and the posterior dural plexus the growth of the cartilaginous capsule of the ear and the growth and alteration in form of the brain bring about changes in the primary arrangement owing to the growth of the otto capsule and middle ear the cause of the primary head vein becomes unfavorable and a segment of it becomes obliterated to make the necessary adjustment an anastomosis is established about the otto capsule and the middle plexus drains into the posterior plexus then the anterior plexus fuses with the middle plexus and drains through it the newly established channel dorsal to the otto capsule all that remains of the primary head vein is a cardinal portion or internal jugular and the part in the region of the trigeminal nerve which may be called the cavernous sinus into it drained orbital veins the drainage from the cavernous sinus is now upward through the original trunk of the middle plexus which is now the superior patrosal sinus into the newly established dorsal channel this dorsal channel is a transverse sinus the inferior patrosal sinus appears later from the anterior plexus a sagittal plexus extends forward from which develops the superior sagittal sinus the straight sinus is formed in the ventral part of the sagittal plexus as a hemispheres extend backward these sinuses elongate by incorporating two more cordon loops of the plexus the anterior part of the sinus is completed first the external jugular vein at first drains the region behind the ear posterior auricular and enters the primitive jugular as a lateral tributary a group of veins from the face and lingual region converge to form a common vein the lingua facial which also terminates in the primitive jugular later first communication developed between the external jugular and the lingua facial with the result that a posterior group of facial veins is transferred to the external jugular end of section 4 section 5 of Grey's Anatomy part 3 this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by Leanne Howlett anatomy of the human body part 3 by Henry Gray the thoracic cavity the heart and lungs are situated in the thorax the walls of which afford them protection the heart lies between the two lungs and is enclosed within a fibrous bag in the heartium while each lung is invested by a serous membrane the pleura the skeleton of the thorax and the shape and boundaries of the cavity have already been described the cavity of the thorax the capacity of the cavity of the thorax does not correspond with its apparent size externally because one the space enclosed by the lower ribs is occupied by some of the abdominal viscera two the cavity extends above the anterior parts of the first ribs into the neck the size of the thoracic cavity is constantly varying during life with the movements of the ribs and diaphragm and with the degree of distention of the abdominal viscera from the collapsed state of the lungs as seen when the thorax is opened in the dead body it would appear as if the viscera only partly filled the cavity but during life there is no vacant space that which is seen after death being filled up by the expanded lungs the upper opening of the thorax the parts which pass through the upper opening of the thorax are from before backward in or near the middle line the sternohioidius and sternothiroidius muscles the remains of the thymus the inferior thyroid veins the trachea esophagus, thoracic duct and the longest colli muscles at the sides the inominate artery the left common carotid left subclavian and internal mammary arteries and the costoservical trunks the inominate veins the vagus, cardiac, phrenic and sympathetic nerves the greater parts of the anterior divisions of the first thoracic nerves and the recurrent nerve of the left side the apex of each lung covered by the pleura also projects through this aperture a little above the level of the sternal end of the first rib the lower opening of the thorax the lower opening of the thorax is wider transversely than from before backward it slopes obliquely downward and backward so that the thoracic cavity is much deeper behind than in front the diaphragm closes the opening and forms the floor of the thorax the floor is flatter at the center than at the sides and higher on the right side than on the left in the dead body the right side reaches the level of the upper border of the fifth costal cartilage while the left extends only to the corresponding part of the sixth costal cartilage from the highest point on each side the floor slopes suddenly downward to the costal and vertebral attachments of the diaphragm this slope is more marked behind than in front so that only a narrow space is left between the diaphragm and the posterior wall of the thorax 4a. the pericardium the pericardium is a conical fibrocerous sac in which the heart and the roots of the great vessels are contained it is placed behind the sternum and the cartilages of the third, fourth fifth, sixth and seventh ribs of the left side in the mediastinal cavity in front it is separated from the anterior wall of the thorax in the greater part of its extent by the lungs and pluri but a small area, somewhat variable in size and usually corresponding with the left half of the lower portion of the body of the sternum and the medial ends of the cartilages of the fourth and fifth ribs comes into direct relationship with the chest wall the lower extremity of the thymus in the child is in contact with the front of the upper part of the pericardium behind it rests upon the bronchi the esophagus the descending thoracic aorta and the posterior part of the mediastinal surface of each lung laterally it is covered by the pluri and is in relation with the mediastinal surfaces of the lungs the phrenic nerve with its accompanying vessels descends between the pericardium and plura on either side structure of the pericardium although the pericardium is usually described as a single sac an examination of its structure shows that it consists essentially of two sacs intimately connected with one another but totally different in structure the outer sac known as the fibrous pericardium consists of fibrous tissue the inner sac or serous pericardium is a delicate membrane which lies within the fibrous sac and lines its walls it is composed of a single layer of flattened cells resting on loose connective tissue the heart invaginates the wall of the serous sac from above and behind and practically obliterates its cavity it is a potential one the fibrous pericardium forms a flask shaped bag the neck of which is closed by its fusion with the external coats of the great vessels while its base is attached to the central tendon and to the muscular fibers of the left side of the diaphragm in some of the lower mammals the base is either completely separated from the diaphragm or joined to it by some loose aureolar tissue and man the diaphragmatic attachment consists of loose fibrous tissue which can be readily broken down but over a small area the central tendon of the diaphragm and the pericardium are completely fused above the fibrous pericardium not only blends with the external coats of the great vessels but is continuous with the pre tracheal layer of the deep cervical fascia by means of these upper and lower connections anchored within the thoracic cavity it is also attached to the posterior surface of the sternum by the superior and inferior sternoparicardiac ligaments the upper passing to the manubrium and the lower to the xiphoid process the vessels receiving fibrous prolongations from this membrane are the aorta the superior vena cava the right and left pulmonary arteries and the four pulmonary veins the inferior vena cava enters the pericardium through the central tendon of the diaphragm and receives no covering from the fibrous layer the serous pericardium is as already stated a closed sac which lines the fibrous pericardium and is invaginated by the heart it therefore consists of a visceral and a parietal portion the visceral portion or epicardium covers the heart and the great vessels and from the latter is continuous with the parietal layer which lines the fibrous pericardium the portion which covers the vessels is arranged in the form of two tubes the aorta and pulmonary artery are enclosed in one tube the arterial mesocardium the superior and inferior vena cavae and the four pulmonary veins are enclosed in a second tube the parietal mesocardium the attachment of which to the parietal layer presents the shape of an inverted U the cul-de-sac enclosed between the limbs of the U lies behind the left atrium and is known as the oblique sinus while the passage between the venus and arterial mesocardia that is between the aorta and pulmonary artery in front and the atria behind is termed the transverse sinus the ligament of the left vena cava between the left pulmonary artery and subjacent pulmonary vein is a triangular fold of the serous pericardium it is known as the ligament of the left vena cava the stigial fold of marshal it is formed by the duplicature of the serous layer over the remnant of the lower part of the left superior vena cava duct of couvier which becomes obliterated during fetal life and remains as a fibrous band stretching from the highest left intercostal vein to the left atrium where it is continuous with a small vein the vein of the left atrium oblique vein of marshal which opens into the coronary sinus the arteries of the pericardium are derived from the internal mammary and its musculofrenic branch and from the descending thoracic aorta the nerves of the pericardium all from the vagus and phrenic nerves and the sympathetic trunks End of Section 5 Recording by Leanne Howlett The heart is a hollow muscular organ of a somewhat conical form that lies between the lungs in the middle mediastanum and is enclosed in the pericardium it is placed obliquely in the chest behind the body of the sternum and adjoining parts of the rib cartilages and projects farther into the left than into the right half of the thoracic cavity so that about one third of it is situated on the right and two thirds on the left of the median plane Size The heart in the adult measures about 12 cm in length 8 to 9 cm in breadth at the broadest point and 6 cm in thickness Its weight in the male varies from 280 to 340 g in the female from 230 to 280 g The heart continues to increase in weight and size up to an advanced period of life this increase is more marked in men than in women Component Parts As has already been stated the heart is subdivided by septa into right and left halves and a constriction subdivides each half of the organ into two cavities the upper cavity being called the atrium, the lower, the ventricle The heart therefore consists of four chambers that is, right and left atria and right and left ventricles The division of the heart into four cavities is indicated on its surface by grooves The atria are separated from the ventricles by the coronary sulcus ariculoventricular groove This contains the trunks of the nutrient vessels of the heart and is deficient in front where it is crossed by the root of the pulmonary artery The interatrial groove separating the two atria is scarcely marked on the posterior surface while anteriorly it is hidden by the pulmonary artery The ventricles are separated by two grooves one of which the anterior longitudinal sulcus is situated on the sternocostal surface of the heart close to its left margin The other posterior longitudinal sulcus on the diaphragmatic surface near the right margin These grooves extend from the base of the ventricular portion to a notch, the incisera apisis cordus on the acute margin of the heart just to the right of the apex The base, basis cordus directed upward, backward and to the right is separated from the fifth, sixth, seventh and eighth thoracic vertebrae by the esophagus, aorta and thoracic duct It is formed mainly by the left atrium and to a small extent by the back part of the right atrium Somewhat quadrilateral in form, it is in relation above with the bifurcation of the pulmonary artery located below by the posterior part of the coronary sulcus containing the coronary sinus On the right it is limited by the sulcus terminalis of the right atrium and on the left by the ligament of the left vena cava and the oblique vein of the left atrium The four pulmonary veins two on either side open into the left atrium while the superior vena cava opens into the upper and the anterior vena cava into the lower of the right atrium The apex, apex cordus The apex is directed downward forward and to the left and is overlapped by the left long and pleura It lies behind the fifth left intercostal space, 8 to 9 cm from the mid-sternal line or about 4 cm below and 2 mm to the medial side of the left mammary papilla The sternocostal surface is directed forward, upward and to the left Its lower part is convex formed chiefly by the right ventricle and traversed near its left margin by the anterior longitudinal sulcus Its upper part is separated from the lower by the coronary sulcus and is formed by the atria It presents a deep concavity occupied by the ascending aorta and the pulmonary artery The diaphragmatic surface directed downward and slightly backward is formed by the ventricles and rests upon the central tendon and a small part of the left muscular portion of the diaphragm It is separated from the base by the posterior part of the coronary sulcus and is traversed obliquely by the posterior longitudinal sulcus The right margin of the heart is long and is formed by the right atrium above and the right ventricle below The atrial portion is rounded and almost vertical It is rounded behind the third, fourth, and fifth right costal cartilages about 1.25 cm from the margin of the sternum The ventricular portion, thin and sharp is named to the acute margin It is nearly horizontal and extends from the sternal end of the sixth right costal cartilage to the apex of the heart The left or obtuse margin is shorter, full and rounded It is formed mainly by the left ventricle at an extent above by the left atrium It extends from a point in the second left intercostal space about 2.5 mm from the sternal margin obliquely downward with a convexity to the left to the apex of the heart Right atrium, atrium dextrum, right oracle The right atrium is larger than the left but its walls are somewhat thinner measuring about 2 mm Its cavity is capable of containing C.C.C. It consists of two parts a principal cavity or sinus venerum situated posteriorly and an anterior, smaller portion the auricula sinus venerum sinus venosis The sinus venorum is the large quadrangular cavity placed between the two vena cavae Its walls, which are extremely thin are connected below with the right ventricle with the left atrium but are free in the rest of their extent auricula auricula dextra, right auricular appendix The auricula is a small conical muscular pouch the margins of which present a dentated edge It projects from the upper and front part of the sinus forward and toward the left side overlapping the root of the aorta The separation of the auricula from the sinus venorum is indicated externally by a groove the terminal sulcus which extends from the front of the superior vena cava to the front of the inferior vena cava and represents the line of union of the sinus venosis of the embryo with the primitive atrium On the inner wall of the atrium the separation is marked by a vertical smooth muscular ridge the terminal crest Behind the crest the internal surface of the atrium is smooth the muscular fibers of the wall are raised into parallel ridges resembling the teeth of a comb and hence named the muscule pectinati Its interior presents the following parts for examination openings superior vena cava inferior vena cava coronary sinus foramina venorum minimorum atrioventricular valves valve of the inferior vena cava coronary sinus fossa ovalis limbus fossovalis intravenous tubercle muscule pectinati crista terminalis The superior vena cava returns the blood from the upper half of the body and opens into the upper and back part of the atrium the direction of its orifice being downward and forward Its opening has no valve The inferior vena cava larger than the superior from the lower half of the body and opens into the lowest part of the atrium near the atrial septum Its orifice being directed upward and backward and guarded by a rudimentary valve the valve of the inferior vena cava Eustachian valve The blood entering the atrium through the superior vena cava is directed downward and forward that is toward the atrioventricular orifice While that entering through the inferior vena cava is directed upward and backward toward the atrial septum This is the normal direction of the two currents in fetal life The coronary sinus opens into the atrium between the orifice of the inferior vena cava and the atrioventricular opening It returns blood from the substance of the heart and is protected by a semicircular valve the valve of the coronary sinus valve of thebesius The foramina vinara minamarum foramina thebesii are the orifices of minute veins venicordis minimii which return blood directly from the muscular substance of the heart The atrioventricular opening tricuspid orifice is the large oval aperture of communication between the atrium and the ventricle It will be described with the right ventricle The valve of the inferior vena cava valvula vena cavi inferioris eustaceae Eustacean valve is situated in front of the orifice of the inferior vena cava It is semi-lunar in form It's convex margin being attached to the anterior margin of the orifice It's concave margin which is free ends in two corneua of which the left is continuous with the anterior edge of the limbus fossovalis while the right is lost on the wall of the atrium The valve is formed by a duplicature of the lining membrane of the atrium containing a few muscular fibers In the fetus this valve is of large size and serves to direct the blood from the inferior vena cava through the foramen of valley into the left atrium In the adult it occasionally persists and may assist in preventing the reflux of blood into the inferior vena cava More commonly it is small and may present a crib reform of the orifice appearance Sometimes it is altogether wanting The valve of the coronary sinus valvula sinus coronary eye or thebesi eye thebesian valve is a semicircular fold of the lining membrane of the atrium at the arthus of the coronary sinus It prevents the regurgitation of blood into the sinus during the contraction of the atrium This valve may be double or it may be crib reform The fossa ovalis is an oval depression on the septal wall of the atrium and corresponds to the situation of the foramen of valley in the fetus It is situated at the lower part of the septum above and to the left of the arthus of the inferior vena cava The limbus fossovalis annulus ovalis is the prominent oval margin of the fossa ovalis It is most distinct above and at the sides of the fossa Below it is deficient A small slit-like valvular opening is occasionally found at the upper margin of the fossa leading upward beneath the limbus into the left atrium It is the remains of the fetal aperture between the two atria The intravenous tubercle tuberculum intravenosum tubercle of lower is a small projection on the posterior wall of the atrium above the fossa ovalis It is distinct in the hearts of quadrupeds but in man is scarcely visible It was supposed by lower to direct the blood from the superior vena cava toward the atrioventricular opening Right ventricle ventriculus dexter The right ventricle is triangular in form and extends from the right atrium to near the apex of the heart Its anteros superior surface is rounded and convex and forms the larger part of the frontal surface of the heart Its under surface is flattened rests upon the diaphragm and forms a small part of the diaphragmatic surface of the heart Its posterior wall is formed by the ventricular septum which bulges into the right ventricle so that a transverse section of the cavity presents a semi-lunar outline Its upper and left angle forms a conical pouch the conus arteriosus from which the pulmonary artery a tendinous band which may be named the tendon of the conus arteriosus extends upward from the right atrioventricular fibrous ring and connects the posterior surface of the conus arteriosus to the aorta The wall of the right ventricle is thinner than that of the left the proportion between them being as one to three It is thickest at the base and gradually becomes thinner toward the apex The cavity equals in size that of the left ventricle and is capable of containing about 85 ccs Its interior presents the following parts for examination The right atrioventricular orifice is the large oval aperture of communication between the right atrium and the ventricle Situated at the base of the ventricle it measures about 4 cm in diameter and is surrounded by a fibrous ring covered by the lining membrane of the heart It is considerably larger than the corresponding aperture on the left side being sufficient to admit the ends of four fingers It is guarded by the tricuspid valve The opening of the pulmonary artery is circular in form and situated at the summit of the conus arteriosus close to the ventricular septum It is placed above and to the left of the atrioventricular opening and is guarded by the pulmonary semi-lunar valves The tricuspid valve valvula tricuspid alas consists of three somewhat triangular cusps or segments The largest cusp is interposed between the atrioventricular orifice and the conus arteriosus and is termed the anterior or infundibular cusp A second, the posterior or marginal cusp is in relation to the right margin of the ventricle and a third, the medial or septal cusp to the ventricular septum They are formed by duplicatures of the lining membrane of the heart strengthened by intervening layers of fibrous tissue Their central parts are thick and strong Their marginal portions thin and translucent and get segments sometimes seen Their bases are attached to a fibrous ring surrounding the atrioventricular orifice and are also joined to each other so as to form a continuous annular membrane while their apices project into the ventricular cavity Their atrial surfaces directed toward the blood current from the atrium are smooth Their ventricular surfaces directed toward the wall of the ventricle are rough and irregular with the apices and margins of the cusps give attachment to a number of delicate tendinous cords the cordy tendony The trabeculi carny, columny carny are rounded or irregular muscular columns which project from the hole of the inner surface of the ventricle with the exception of the conus arteriosus They are of three kinds Some are attached along their entire length on one side and merely form prominent ridges Others are fixed at their extremities but free in the middle While a third set, muscule papillaries are continuous by their bases with the wall of the ventricle while their apices give origin to the cordy tendony which pass to be attached to the segments of the tricuspid valve There are two papillary muscles anterior and posterior of these the anterior is the larger and its cordy tendony are connected with the anterior cusps of the valve The posterior papillary muscle sometimes consists of two or three parts Its cordy tendony are connected with the posterior and medial cusps In addition to these, some cordy tendony spring directly from the ventricular septum or from small papillary immanences on it and pass to the anterior and medial cusps A muscular band well marked in sheep and some other animals begins from the base of the anterior papillary muscle to the ventricular septum From its attachments it may assist in preventing over-distancing of the ventricle and so has been named the moderator band The pulmonary semilunar valves are three in number two in front and one behind formed by duplicatures of the lining membrane strengthened by fibrous tissue They are attached by their convex margins to the wall of the artery with its junction with the ventricle their free borders being directed upward into the lumen of the vessel The free and attached margins of each are strengthened by tendinous fibers and the former presents at its middle a thickened nodule corpus oranti From this nodule tendinous fibers radiate through the segment to its attached margin but are absent from two narrow crescentic portions, the lunuli placed one on either side of the nodule greatly adjoining the free margin Between the semilunar valves and the wall of the pulmonary artery are three pouches or sinuses sinuses of valsalva left atrium, atrium synestrum left oracle The left atrium is rather smaller than the right, but its walls are thicker measuring about three millimeters It consists, like the right of two parts, a principal cavity and an auricula The principal cavity is cuboidal in form and concealed in front by the pulmonary artery and aorta. In front and to the right it is separated from the right atrium by the atrial septum Opening into it on either side are the two pulmonary veins auricula auricula sinestra left auricula appendix The auricula is somewhat constricted at its junction with the principal cavity It is longer, narrower and more curved than that of the right side Its margins are more deeply indented It is directed forward and toward the right and overlaps the root of the pulmonary artery The interior of the left atrium presents the following parts for examination openings of the four pulmonary veins left atrioventricular opening musculi pectinati The pulmonary veins four in number open into the upper part of the posterior surface of the left atrium two on either side of its middle line They are not provided with valves The two left veins frequently end by a common opening The left atrioventricular opening is the aperture between the left atrium and ventricle and is rather smaller than the corresponding opening on the right side The musculi pectinati fewer and smaller than in the right auricula are confined to the inner surface of the auricula On the atrial septum may be seen a lunated impression bounded below by a chrysintic ridge the concavity of which is turned upward The depression is just above the fossa ovalis of the right atrium End of section 6