 Section 0 of Gray's Anatomy Part 1. 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 1 by Henry Gray. Preface to the 20th edition. Since the publication of the first English edition of this work in 1858 and the first American edition in 1859, great advances in the subject of anatomy have been made, especially in microscopic anatomy and the anatomy of the embryo. This knowledge was embodied from time to time in the successive editions until finally considerable portions of the text, sometimes sections, were devoted to these subjects. However, the main text has always remained primarily a descriptive anatomy of the human body. In the present edition, the special sections on embryology and histology have been distributed among the subjects under which they naturally belong. New matter on physiological anatomy, laws of bone architecture, the mechanics and variations of muscles have been added, occupying much of the space formerly devoted to the sections on applied anatomy. The sections on the ductalus glands in the nervous system have been largely rewritten. In the latter, a more rational presentation of the sympathetic nervous system has been achieved through the use of diagrams and descriptions based on physiological and pharmacological work. The central connections of the spinal and cranial nerves are also emphasized. Illustrations have been added wherever important points could be made more clear and throughout the work, colored pictures have been even more extensively used than here to fore. In this respect, special mention might be made of the central nervous system and the section on the muscles. In the section on syndesmology, six illustrations are used from Quain's Anatomy through the courtesy of the publishers Messer's Longmans Greening Company of London. The use of the BNA nomenclature in English has been retained practically unchanged in this edition and important references to the literature have been added at the end of each section. As a practical work on the subject for the student, Gray's Anatomy has always been recognized and appreciated. The plan originally formulated, which has proved so successful, has been adhered to as much as possible. It is interesting to note that although Henry Gray saw only the first edition, much of the original text persists and many of his illustrations are still in use. Bearing this in mind, it has been the endeavor of the editor to supply only such changes as advances in the science made necessary in order that this work may reflect the latest accessions to anatomical knowledge. W.H.L. Baltimore, 1918 Introduction The term human anatomy comprises a consideration of the various structures which make up the human organism. In a restricted sense, it deals merely with the parts which form the fully developed individual and which can be rendered evident to the naked eye by various methods of dissection. Regarded from such a standpoint, it may be studied by two methods. One, the various structures may be separately considered, systematic anatomy. Or, two, the organs and tissues may be studied in relation to one another, topographical or regional anatomy. It is, however, of much advantage to add to the facts ascertained by naked eye dissection, those obtained by the use of the microscope. This introduces two fields of investigation. That is, the study of the minute structure of the various component parts of the body, histology, and the study of the human organism in its immature condition, that is, the various stages of its intrauterine development from the fertilized ovum up to the period when it assumes an independent existence. Embryology Owing to the difficulty of obtaining material illustrating all the stages of this early development, gaps must be filled up by observations on the development of lower forms, comparative embryology, or by consideration of adult forms in the line of human ancestry, comparative anatomy. The direct application of the facts of human anatomy to the various pathological conditions which may occur constitutes the subject of applied anatomy. Finally, the appreciation of structures, own or immediately underlying the surface of the body, is frequently made the subject of special study, surface anatomy. Systematic anatomy The various systems of which the human body is composed are grouped under the following headings. Osteology The bony system or skeleton. Syndesmology The articulations or joints. Myology The muscles. With the description of the muscles it is convenient to include that of the fascia which are so intimately connected with them. Angiology The vascular system comprising the heart, blood vessels, lymphatic vessels, and lymph glands. Neurology The nervous system. The organs of sense may be included in this system. Splanknology The visceral system. Topographically the viscera form two groups, that is the thoracic viscera and the abdominal pelvic viscera. The heart, a thoracic viscous, is best considered with the vascular system. The rest of the viscera may be grouped according to their functions. A. The respiratory apparatus. B. The digestive apparatus. And C. The urogenital apparatus. Strictly speaking the third subgroup should include only such components of the urogenital apparatus as are included within the abdominal pelvic cavity. But it is convenient to study under this heading certain parts which lie in relation to the surface of the body. That is the testes and the external organs of generation. For descriptive purposes the body is supposed to be in the erect posture with the arms hanging by the sides and the palms of the hands directed forward. The median plane is a vertical antero-posterior plane passing through the center of the trunk. This plane will pass approximately through the sagittal suture of the skull and hence any plane parallel to it is termed a sagittal plane. A vertical plane at right angles to the median plane passes roughly speaking through the central part of the coronal suture or through a line parallel to it. Such a plane is known as a frontal plane or sometimes as a coronal plane. A plane at right angles to both the median and frontal planes is termed a transverse plane. The terms anterior or ventral and posterior or dorsal are employed to indicate the relation of parts to the front or back of the body or limbs and the terms superior or cephalic and inferior or caudal to indicate the relative levels of different structures. Structures nearer to or farther from the median plane are referred to as medial or lateral respectively. The terms superficial and deep are strictly confined to descriptions of the relative depth from the surface of the various structures. External and internal are reserved almost entirely for describing the walls of cavities or of hollow viscera. In the case of the limbs the words proximal and distal refer to the relative distance from the attached end of the limb. End of Section 0 Section 1 of Grey's Anatomy, Part 1 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 Ruth Golding Anatomy of the Human Body, Part 1 by Henry Gray Section 1 Embryology The term embryology in its widest sense is applied to the various changes which take place during the growth of an animal from the egg to the adult condition. It is, however, usually restricted to the phenomena which occur before birth. Embryology may be studied from two aspects. One, that of ontogeny which deals only with the development of the individual and two, that of phylogeny which concerns itself with the evolutionary history of the animal kingdom. In vertebrate animals the development of a new being can only take place when a female germ cell or ovum has been fertilized by a male germ cell or spermatozoan. The ovum is a nucleated cell and all the complicated changes by which the various tissues and organs of the body are formed from it after it has been fertilized are the result of two general processes, vis segmentation and differentiation of cells. Thus the fertilized ovum undergoes repeated segmentation into a number of cells which at first closely resemble one another but are sooner or later differentiated into two groups. One, somatic cells, the function of which is to build up the various tissues of the body and two, germinal cells which become embedded in the sexual glands, the ovaries in the female and the testes in the male and are destined for the perpetuation of the species. Having regard to the main purpose of this work it is impossible in the space available in this section to describe fully or illustrate adequately all the phenomena which occur in the different stages of the development of the human body. Only the principal facts are given and the student is referred for further details to one or other of the textbooks on human embryology. One, the animal cell. All the tissues and organs of the body originate from a microscopic structure, brackets, the fertilized ovum, closed brackets, which consists of a soft jelly-like material enclosed in a membrane and containing a vesicle or small spherical body inside which are one or more denser spots. This may be regarded as a complete cell. All the solid tissues consist largely of cells essentially similar to it in nature but differing in external form. In the higher organisms a cell may be defined as a nucleated mass of protoplasm of microscopic size. Its two essentials therefore are a soft jelly-like material similar to that found in the ovum and usually styled cytoplasm and a small spherical body embedded in it and termed a nucleus. Some of the unicellular protozoa contain no nuclei but granular particles which, like true nuclei, stain with basic dyes. The other constituents of the ovum vis its limiting membrane and the denser spot contained in the nucleus called the nucleolus are not essential to the type cell and in fact many cells exist without them. Cytoplasm, brackets protoplasm, closed brackets, is a material probably of variable constitution during life but yielding on its disintegration bodies chiefly of proteid nature. Lessethin and cholesterin are constantly found in it as well as inorganic salts, chief among which are the phosphates and chlorides of potassium, sodium and calcium. It is of a semi-fluid, viscid, consistency and probably colloidal in nature. The living cytoplasm appears to consist of a homogeneous and structuralist ground substance in which are embedded granules of various types. The mitochondria are the most constant type of granule and vary in form from granules to rods and threads. Their function is unknown. Some of the granules are proteid in nature and probably essential constituents. Others are fat, glycogen or pigment granules and are regarded as adventitious material taken in from without and hence are styled cell inclusions or paraplasm. When however cells have been fixed by reagents a fibrile or granular appearance can often be made out under a high power of the microscope. The fibriles are usually arranged in a network or reticulum to which the term spondioplasm is applied the clear substance in the meshes being termed hyaloplasm. The size and shape of the meshes of the spondioplasm vary in different cells and in different parts of the same cell. The relative amounts of spondioplasm and hyaloplasm also vary in different cells the latter preponderating in the young cell and the former increasing at the expense of the hyaloplasm as the cell grows. Such appearances in fixed cells are no indication whatsoever of the existence of similar structures in the living although there must have been something in the living cell to give rise to the fixed structures. The peripheral layer of a cell is in all cases modified either by the formation of a definite cell membrane as in the ovum or more frequently in the case of animal cells by a transformation probably chemical in nature which is only recognizable by the fact that the surface of the cell behaves as a semi-permeable membrane. Nucleus The nucleus is a minute body embedded in the protoplasm and usually of a spherical or oval form its size having little relation to that of the cell. It is surrounded by a well-defined wall, the nuclear membrane. This encloses the nuclear substance brackets nuclear matrix close brackets which is composed of a homogeneous material in which is usually embedded one or two nucleoli. In fixed cells the nucleus seems to consist of a clear substance or carioplasm and a network or cariomytome. The former is probably of the same nature as the hyaloplasm of the cell but the latter which forms also the wall of the nucleus differs from the spondioplasm of the cell substance. It consists of fibres or filaments arranged in a reticular manner. These filaments are composed of a homogeneous material known as linin which stains with acid dyes and contains embedded in its substance particles which have a strong affinity for basic dyes. These basophil granules have been named chromatin or basicromatin and owe their staining properties to the presence of nucleic acid. Within the nuclear matrix are one or more highly refracting bodies termed nucleoli connected with the nuclear membrane by the nuclear filaments. They are regarded as being of two kinds. Some are mere local condensations brackets net knots close brackets of the chromatin. These are irregular in shape and are termed pseudonucleoli. Others are distinct bodies differing from the pseudonucleoli both in nature and chemical composition. They may be termed true nucleoli and are usually found in resting cells. The true nucleoli are oxyfill i.e. they stain with acid dyes. Most living cells contain in addition to their protoplasm and nucleus a small particle which usually lies near the nucleus and is termed the centrosome. In the middle of the centrosome is a minute body called the centriole and surrounding this is a clear spherical mass known as the centrosphere. The protoplasm surrounding the centrosphere is frequently arranged in radiating fibrile rows of granules forming what is termed the attraction sphere. Reproduction of cells Reproduction of cells is affected either by direct or by indirect division. In reproduction by direct division the nucleus becomes constricted in its centre assuming an hourglass shape and then divides into two. This is followed by a cleavage or division of the whole protoplasmic mass of the cell and thus two daughter cells are formed each containing a nucleus. These daughter cells are at first smaller than the original mother cell but they grow and the process may be repeated in them so that multiplication may take place rapidly. Indirect division or caryokinesis brackets caryomytosis close brackets has been observed in all the tissues generative cells epithelial tissue connective tissue muscular tissue and nerve tissue. It is possible that cell division may always take place by the indirect method. The process of indirect cell division is characterised by a series of complex changes in the nucleus leading to its subdivision. This is followed by cleavage of the cell protoplasm. Starting with the nucleus in the quiescent or resting stage these changes may be briefly grouped under the four following phases. 1. Prophase The nuclear network of chromatin filaments assumes the form of a twisted skein or spiron while the nuclear membrane and nucleolus disappear. The convoluted skein of chromatin divides into a definite number of V-shaped segments or chromosomes. The number of chromosomes varies in different animals but is constant for all the cells in an animal of any given species. In man the number is given by Fleming and Duisburg as 24. Coincidentally with or preceding these changes the centriole which usually lies by the side of the nucleus undergoes subdivision and the two resulting centrioles each surrounded by a centrosphere are seen to be connected by a spindle of delicate acromatic fibres the acromatic spindle. The centrioles move away from each other one toward either extremity of the nucleus and the fibrils of the acromatic spindle are correspondingly lengthened. A line encircling the spindle midway between its extremities or poles is named the equator and around this the V-shaped chromosomes arrange themselves in the form of a star thus constituting the mother star or monaster. 2. Metaphase Each V-shaped chromosome now undergoes longitudinal cleavage into two equal parts or daughter chromosomes the cleavage commencing at the apex of the V and extending along its divergent limbs. 3. Anaphase The daughter chromosomes thus separated travel in opposite directions along the fibrils of the acromatic spindle toward the centrioles around which they group themselves and thus two star-like figures are formed one at either pole of the acromatic spindle. This constitutes the diaster. The daughter chromosomes now arrange themselves into a skein or spiron and eventually form the network of chromatin which is characteristic of the resting nucleus. 4. Telophase The cell protoplasm begins to appear constricted around the equator of the acromatic spindle where double rows of granules are also sometimes seen. The constriction deepens and the original cell gradually becomes divided into two new cells each with its own nucleus and centrosome which assume the ordinary positions occupied by such structures in the resting stage. The nuclear membrane and nucleolus are also differentiated during this phase. End of Section 1 Recording by Ruth Golding Section 2 of Grey's Anatomy Part 1 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 Ruth Golding Anatomy of the Human Body Part 1 by Henry Gray Embryology 2. The Oven The ova are developed from the primitive germ cells which are embedded in the substance of the ovaries. Each primitive germ cell gives rise by repeated divisions to a number of smaller cells termed oagonia from which the ova or primary oocytes are developed. Human ova are extremely minute measuring about 0.2 millimetres in diameter and are enclosed within the egg follicles of the ovaries. As a rule each follicle contains a single ovum but sometimes two or more are present. Footnote Description of the ovary on a future page End of footnote By the enlargement and subsequent rupture of a follicle at the surface of the ovary an ovum is liberated and conveyed by the uterine tube to the cavity of the uterus. Unless it be fertilised it undergoes no further development and is discharged from the uterus. But if fertilisation take place it is retained within the uterus and is developed into a new being. In appearance and structure the ovum differs little from an ordinary cell but distinctive names have been applied to its several parts. Thus the cell substance is known as the yoke or ooplasm the nucleus as the germinal vesicle and the nucleolus as the germinal spot. The ovum is enclosed within a thick transparent envelope the zona striata or zona pollucida adhering to the outer surface of which are several layers of cells derived from those of the follicle and collectively constituting the corona radiata. Yoke The yoke comprises one the cytoplasm of the ordinary animal cell with its spondioplasm and hyaloplasm this is frequently termed the formative yoke. Two the nutritive yoke or dutoplasm which consists of numerous rounded granules of fatty and albuminoid substances embedded in the cytoplasm. In the mammalian ovum the nutritive yoke is extremely small in amount and is of service in nourishing the embryo in the early stages of its development only whereas in the egg of the bird there is sufficient to supply the chick with nutriment throughout the whole period of incubation. The nutritive yoke not only varies in amount but in its mode of distribution within the egg thus in some animals it is almost uniformly distributed throughout the cytoplasm in some it is centrally placed and is surrounded by the cytoplasm in others it is accumulated at the lower pole of the ovum while the cytoplasm occupies the upper pole a centrosome and centriole are present and lie in the immediate neighbourhood of the nucleus. Germinal vesicle The germinal vesicle or nucleus is the large spherical body which at first occupies a nearly central position but becomes eccentric as the growth of the ovum proceeds. Its structure is that of an ordinary cell nucleus this it consists of a reticulum or cariomytome the meshes of which are filled with carioplasm while connected with or embedded in the reticulum are a number of chromatin masses or chromosomes which may present the appearance of a skein or may assume the form of rods or loops. The nucleus is enclosed by a delicate nuclear membrane and contains in its interior a well-defined nucleolus or germinal spot. Coverings of the ovum The zona striator or zona pollucida is a thick membrane which under the higher powers of the microscope is seen to be radially striated. It persists for some time after fertilisation has occurred and may serve for protection during the earlier stages of segmentation. It is not yet determined whether the zona striator is a product of the cytoplasm of the ovum or of the cells of the corona radiator or both. The corona radiator consists of two or three strata of cells they are derived from the cells of the follicle and adhere to the outer surface of the zona striator when the ovum is set free from the follicle. The cells are radially arranged around the zona those of the innermost layer being columnar in shape. The cells of the corona radiator soon disappear in some animals they secrete or are replaced by a layer of adhesive protein which may assist in protecting and nourishing the ovum. The phenomena attending the discharge of the over from the follicles belong more to the ordinary functions of the ovary than to the general subject of embryology and are therefore described with the anatomy of the ovaries. Footnote See description of the ovary on a future page. End of footnote Maturation of the ovum Before an ovum can be fertilized it must undergo a process of maturation or ripening. This takes place previous to or immediately after its escape from the follicle and consists essentially of an unequal subdivision of the ovum first into two and then into four cells. Three of the four cells are small incapable of further development and are termed polar bodies or polar sites while the fourth is large and constitutes the mature ovum. The process of maturation has not been observed in the human ovum but has been carefully studied in the over of some of the lower animals to which the following description applies. It was pointed out on page 37 that the number of chromosomes found in the nucleus is constant for all the cells in an animal of any given species and that in man the number is probably 24. This applies not only to the somatic cells but to the primitive over and their descendants. For the purpose of illustrating the process of maturation a species may be taken in which the number of nuclear chromosomes is four. If an ovum from such be observed at the beginning of the maturation process it will be seen that the number of its chromosomes is apparently reduced to two. In reality however the number is doubled since each chromosome consists of four granules grouped to form a tetrad. During the metaphase, see page 37 each tetrad divides into two diads which are equally distributed between the nuclei of the two cells formed by the first division of the ovum. One of the cells is almost as large as the original ovum and is named the secondary oocyte. The other is small and is termed the first polar body. The secondary oocyte now undergoes subdivision during which each diad divides and contributes a single chromosome to the nucleus of each of the two resulting cells. This second division is also unequal producing a large cell which constitutes the mature ovum and a small cell, the second polar body. The first polar body frequently divides while the second is being formed and as a final result four cells are produced vis the mature ovum and three polar bodies each of which contains two chromosomes i.e. one half the number present in the nuclei of the somatic cells of members of the same species. The nucleus of the mature ovum is termed the female pro-nucleus. End of section 2 Recording by Ruth Golding Section 3 of Grey's Anatomy Part 1 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 Ruth Golding Anatomy of the Human Body Part 1 by Henry Gray Embryology 3. The Spermatozoan The Spermatozoa or male germ cells are developed in the testes and are present in enormous numbers in the seminal fluid. Each consists of a small but greatly modified cell. The human Spermatozoan possesses a head, a neck, a connecting piece or body, and a tail. The head is oval or elliptical but flattened so that when viewed in profile it is pear-shaped. Its anterior two-thirds are covered by layer modified protoplasm which is named the head cap. This in some animals e.g. the salamander is prolonged into a barbed, spear-like process or perforator which probably facilitates the entrance of the Spermatozoan into the ovum. The posterior part of the head exhibits an affinity for certain reagents and presents a transversely striated appearance being crossed by three or four dark bands. In some animals a central rod-like filament extends forward for about two-thirds of the length of the head while in others a rounded body is seen near its centre. The head contains a mass of chromatin and is generally regarded as the nucleus of the cell surrounded by a thin envelope. The neck is less constricted in the human Spermatozoan than in those of some of the lower animals. The anterior centriole, represented by two or three rounded particles is situated at the junction of the head and neck and behind it is a band of homogeneous substance. The connecting piece or body is rod-like and is limited behind by a terminal disc. The posterior centriole is placed at the junction of the body and neck and, like the anterior, consists of two or three rounded particles. From this centriole an axial filament surrounded by a sheath runs backward through the body and tail. In the body the sheath of the axial filament is encircled by a spiral thread around which is an envelope containing mitochondria granules and termed the mitochondria sheath. The tail is of great length and consists of the axial thread or filament surrounded by its sheath which may contain a spiral thread or may present a striated appearance. The terminal portion or end piece of the tail consists of the axial filament only. Krauser gives the length of the human Spermatozoan as between 52 mu and 62 mu, the head measuring 4 to 5 mu, the connecting piece 6 mu and the tail from 41 mu to 52 mu. By virtue of their tails which act as propellers the Spermatozoa are capable of free movement and if placed in favourable surroundings e.g. in the female passages will retain their vitality and power of fertilising for several days. In certain animals e.g. bats it has been proved that Spermatozoa retained in the female passages for several months are capable of fertilising. The Spermatozoa are developed from the primitive germ cells which have become embedded in the testes and the stages of their development are very similar to those of the maturation of the ovum. The primary germ cells undergo division and produce a number of cells termed Spermatogonia and from these the primary Spermatocytes are derived. Each primary Spermatocyte divides into two secondary Spermatocytes and each secondary Spermatocyte into two Spermatids or young Spermatozoa. From this it will be seen that a primary Spermatocyte gives rise to four Spermatozoa. On comparing this process with that of the maturation of the ovum it will be observed that the primary Spermatocyte gives rise to two cells the secondary Spermatocytes and the primary oocyte to two cells the secondary oocyte and the first polar body. Again the two secondary Spermatocytes by their subdivision give origin to four Spermatozoa and the secondary oocyte and first polar body to four cells the mature ovum and three polar bodies. In the development of the Spermatozoa as in the maturation of the ovum there is a reduction of the nuclear chromosomes to one half of those present in the primary Spermatocyte. But here the similarity ends for it must be noted that four Spermatozoa are of equal size and each is capable of fertilizing a mature ovum whereas the three polar bodies are not only very much smaller than the mature ovum but are incapable of further development and may be regarded as abortive over. 4. Fertilization of the ovum Fertilization consists in the union of the Spermatozoan with the mature ovum Nothing is known regarding the fertilization of the human ovum but the various stages of the process have been studied in other mammals and from the knowledge so obtained it is believed that fertilization of the human ovum takes place in the lateral or ampullary part of the uterine tube and the ovum is then conveyed along the tube to the cavity of the uterus a journey probably occupying seven or eight days and during which the ovum loses its corona radiator and zona striator and undergoes segmentation Sometimes the fertilized ovum is arrested in the uterine tube and there undergoes development giving rise to a tubal pregnancy or it may fall into the abdominal cavity and produce an abdominal pregnancy Occasionally the ovum is not expelled from the follicle when the latter ruptures but is fertilized within the follicle and produces what is known as an ovarian pregnancy Under normal conditions only one Spermatozoan enters the yolk and takes part in the process of fertilization At the point where the Spermatozoan is about to pierce the yolk is drawn out into a conical elevation termed the cone of attraction As soon as the Spermatozoan has entered the yolk the peripheral portion of the latter is transformed into a membrane the Viteline membrane which prevents the passage of additional Spermatozoa Occasionally a second Spermatozoan may enter the yolk thus giving rise to a condition of polyspermy When this occurs the ovum usually develops in an abnormal manner and gives rise to a monstrosity Having pierced the yolk the Spermatozoan loses its tail while its head and connecting piece assume the form of a nucleus containing a cluster of chromosomes This constitutes the male pro-nucleus and associated with it there are a centriole and centrosome The male pro-nucleus passes more deeply into the yolk and coincidentally with this the granules of the cytoplasm surrounding it become radially arranged The male and female pro-nuclei migrate toward each other and, meeting near the centre of the yolk fuse to form a new nucleus the segmentation nucleus which therefore contains both male and female nuclear substance The former transmits the individualities of the male ancestors the latter, those of the female ancestors to the future embryo By the union of the male and female pro-nuclei the number of chromosomes is restored to that which is present in the nuclei of the somatic cells End of section 3 Recording by Ruth Golding Section 4 of Grey's Anatomy Part 1 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 Ruth Golding Anatomy of the Human Body Part 1 by Henry Grey Embryology 5. Segmentation of the Fertilised Oven The early segmentation of the human ovum has not yet been observed but judging from what is known to occur in other mammals it may be regarded as certain that the process starts immediately after the ovum has been fertilised i.e. while the ovum is in the uterine tube The segmentation nucleus exhibits the usual mitotic changes and these are succeeded by a division of the ovum into two cells of nearly equal size Footnote The nutritive yolk or deuterplasm is small in amount and uniformly distributed throughout the cytoplasm Such over undergo complete division during the process of segmentation and are therefore termed hollow-blastic In the ova of birds, reptiles and fishes where the nutritive yolk forms by far the larger portion of the egg the cleavage is limited to the formative yolk and is therefore only partial Such over are termed meroblastic Again it has been observed in some of the lower animals that the pronuclei do not fuse but merely lie in opposition At the commencement of the segmentation process the chromosomes of the two pronuclei group themselves around the equator of the nucleus spindle and then divide An equal number of male and female chromosomes travel to the opposite poles of the spindle and thus the male and female pronuclei contribute equal shares of chromatin to the nuclei of the two cells which result from the subdivision of the fertilized ovum End of footnote The process is repeated again and again so that the two cells are succeeded by 4, 8, 16, 32 and so on with the result that a mass of cells is found within the zona striata and to this mass the term moriola is applied The segmentation of the mammalian ovum may not take place in the regular sequence of 2, 4, 8, etc. since one of the two first formed cells may subdivide more rapidly than the other giving rise to a 3 or a 5 cell stage The cells of the moriola are at first closely aggregated but soon they become arranged into an outer or peripheral layer the trophoblast which does not contribute to the formation of the embryo proper and an inner cell mass from which the embryo is developed Fluid collects between the trophoblast and the greater part of the inner cell mass and thus the moriola is converted into a vesicle the blastodermic vesicle The inner cell mass remains in contact however with the trophoblast at one pole of the ovum This is named the embryonic pole since it indicates the situation where the future embryo will be developed The cells of the trophoblast become differentiated into two strata an outer termed the syncytium or syncytiotrophoblast so named because it consists of a layer of protoplasm studied with nuclei but showing no evidence of subdivision into cells and an inner layer the cytotrophoblast or layer of langhans in which the cell outlines are defined As already stated the cells of the trophoblast do not contribute to the formation of the embryo proper they form the ectoderm of the corium and play an important part in the development of the placenta On the deep surface of the inner cell mass a layer of flattened cells the entoderm is differentiated and quickly assumes the form of a small sac the yolk sac Spaces appear between the remaining cells of the mass and by the enlargement and coalescence of these spaces a cavity termed the amniotic cavity is gradually developed the floor of this cavity is formed by the embryonic disc composed of a layer of prismatic cells the embryonic ectoderm derived from the inner cell mass and lying in opposition with the entoderm the primitive streak formation of the mesoderm the embryonic disc becomes oval and then pear shaped the wider end being directed forward near the narrow posterior end an opaque streak the primitive streak makes its appearance and extends along the middle of the disc for about one half of its length at the anterior end of the streak there is a knob-like thickening and hensons knot a shallow groove the primitive groove appears on the surface of the streak and the anterior end of this groove communicates by means of an aperture the blastophore with the yolk sac the primitive streak is produced by a thickening of the axial part of the ectoderm the cells of which multiply grow downward and blend with those of the subjacent entoderm from the sides of the primitive streak a third layer of cells the mesoderm extends lateral wood between the ectoderm and entoderm the caudal end of the primitive streak forms the cloacal membrane the extension of the mesoderm takes place throughout the whole of the embryonic and extra embryonic areas of the ovum except in certain regions one of these is seen immediately in front of the neural tube here the mesoderm extends forward in the form of two crescentic masses which meet in the middle line so as to enclose behind them an area which is devoid of mesoderm over this area the ectoderm and entoderm come into direct contact with each other and constitute a thin membrane the buccofaryngeal membrane forms a septum between the primitive mouth and pharynx in front of the buccofaryngeal area where the lateral crescence of mesoderm fuse in the middle line the pericardium is afterward developed and this region is therefore designated the pericardial area a second region where the mesoderm is absent at least for a time is that immediately in front of the pericardial area this is termed the proamniotic area and is the region where the proamnion is developed in man however a proamnion is apparently never formed a third region is at the hind end of the embryo where the ectoderm and entoderm come into apposition and form the cloacal membrane the blastoderm now consists of three layers named from without inward ectoderm mesoderm and entoderm each has distinctive characteristics and gives rise to certain tissues of the body footnote the mode of formation of the germ layers in the human ovum has not yet been observed in the youngest known human ovum vis that described by Bryson teacher all three layers are already present the mesoderm is split into its two layers the extra embryonic selum is of considerable size and scattered mesodermal stands are seen stretching between the mesoderm of the yolk sac and that of the corian end of footnote ectoderm the ectoderm consists of columnar cells which are however somewhat flattened or cubicle toward the margin of the embryonic disk it forms the whole of the nervous system the epidermis of the skin the lining cells of the sebaceous, suderiferous and mammary glands the hairs and nails the epithelium of the nose and adjacent air sinuses and that of the cheeks and roof of the mouth from it also are derived the enamel of the teeth and the anterior lobe of the hypophysis cerebri the epithelium of the cornea conjunctiva and lacrimal glands and the neuroepithelium of the sense organs entoderm the entoderm consists at first of flattened cells which subsequently become columnar it forms the epithelial lining of the whole of the digestive tube accepting part of the mouth and pharynx and the terminal part of the rectum which are lined by involutions of the ectoderm the lining cells of all the glands which open into the digestive tube including those of the liver and pancreas the epithelium of the auditory tube and tympanic cavity of the trachea bronchi and air cells of the lungs of the urinary bladder and part of the urethra and that which lined the follicles of the thyroid glands and thymus mesoderm the mesoderm consists of loosely arranged branched cells surrounded by a considerable amount of intercellular fluid from it the remaining tissues of the body are developed the endothelial lining of the heart and blood vessels and the blood corpuscles are however regarded by some as being of entodermal origin as the mesoderm develops between the ectoderm and entoderm it is separated into lateral halves by the neural tube and notochord presently to be described a longitudinal groove appears on the dorsal surface of either half and divides it into a medial column the paraxial mesoderm lying on the side of the neural tube and a lateral portion the lateral mesoderm the mesoderm in the floor of the groove connects the paraxial with the lateral mesoderm and is known as the intermediate cell mass in it the genitourinary organs are developed the lateral mesoderm splits into two layers an outer or somatic which becomes applied to the inner surface of the ectoderm and with it forms the somatoplour and an inner or splanchnic which adheres to the entoderm and with it forms the splanchnoplour the space between the two layers of the lateral mesoderm is termed the selum end of section 4 recording by Ruth Golding section 5 of Grey's Anatomy part 1 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 1 by Henry Gray part 6 the neural groove and tube in front of the primitive streak two longitudinal ridges caused by folding up of the ectoderm make their appearance one on either side of the middle line these are named the neural folds they commence some little distance behind the anterior end of the embryonic disk where they are continuous with each other and from there gradually extend backward one on either side of the anterior end of the primitive streak between these folds is a shallow median groove the neural groove the groove gradually deepens as the neural folds become elevated and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube the neural tube or canal the ectodermal wall of which forms the rudiment of the nervous system after the coalescence of the neural folds over the anterior end of the primitive streak the blastopore no longer opens on the surface but into the closed canal of the neural tube and thus a transitory communication the neuroenteric canal is established between the neural tube and the primitive digestive tube the coalescence of the neural folds occurs first in the region of the hind brain and from there extends forward and backward toward the end of the third week the front opening anterior neuropore of the tube finally closes at the anterior end of the future brain and forms a recess in contact for a time with the overlying ectoderm the hinder part of the neural groove presents for a time a rhomboidal shape and to this expanded portion the term sinus rhomboidalis has been applied before the neural groove is closed a ridge of ectodermal cells appears along the prominent margin of each neurofold this is termed the neural crest or ganglion ridge and from it the spinal and cranial nerve ganglia and the ganglia of the sympathetic nervous system are developed by the upward growth of the mesoderm the neural tube is ultimately separated from the overlying ectoderm the cephalic end of the neural groove exhibits several dilatations which when the tube is closed assume the form of three vesicles these constitute the three primary cerebral vesicles which correspond respectively to the future forebrain prosencephalon midbrain mesencephalon and hindbrain rhombencephalon the walls of the vesicles are developed into the nervous tissue and neuroglia of the brain and their cavities are modified to form its ventricles the remainder of the tube forms the medulla spinalis or spinal cord the normal wall the nervous and neuroglial elements of the medulla spinalis are developed while the cavity persists as the central canal part 7 the notochord the notochord consists of a rod of cells situated on the ventral aspect of the neural tube it constitutes the foundation of the axial skeleton since around it the segments of the vertebral column are formed its appearance synchronizes with the neural tube on the ventral aspect of the neural groove an axial thickening of the entoderm takes place this thickening assumes the appearance of a furrow the cordal furrow the margins of which come into contact and so convert it into a solid rod of cells the notochord which is then separated from the entoderm it extends throughout the entire length of the future vertebral column and reaches as far as the anterior end of the midbrain where it ends in a hook-like extremity in the region of the future dorsum celli of the spinoid bone it lies at first between the neural tube and the entoderm of the yolk sac but soon becomes separated from them by the mesoderm which grows medial word and surrounds it from the mesoderm surrounding the neural tube and notochord the skull and vertebral column and the membranes of the brain and medullospinalis are developed Part 8. The Primitive Segments Toward the end of the second week transverse segmentation of the paraxio-mesoderm begins and it is converted into a series of well-defined, more or less cubical masses, the Primitive Segments which occupy the entire length of the trunk on either side of the middle line from the occipital region of the head each segment contains a central cavity myosil which however is soon filled with angular and spindle shaped cells The Primitive Segments lie immediately under the ectoderm on the lateral aspect of the neural tube and notochord and are connected to the lateral mesoderm by the intermediate cell mass Those of the trunk may be arranged in the following groups these Cervical 8 Thoracic 12 Lumbar 5 Sacral 5 and Coxigial from 5 to 8 Those of the occipital region of the head are usually described as being 4 in number In mammals, Primitive Segments of the head can be recognized only in the occipital region but a study of the lower vertebrates leads to the belief that they are present also in the anterior part of the head Altogether 9 segments are represented in the cephalic region Part 9 Separation of the embryo The embryo increases rapidly in size but there's a conference of the embryonic disc or line of meeting of the embryonic and amniotic parts of the ectoderm is a relatively slow growth and gradually comes to form a constriction between the embryo and the greater part of the ulc sac By means of this constriction which corresponds to the future umbilicus a small part of the ulc sac is enclosed within the embryo and constitutes the primitive digestive tube The embryo increases more rapidly in length than in width and its cephalic and caudal ends soon extend beyond the corresponding parts of the circumference of the embryonic disc and are bent in a ventral direction to form the cephalic and caudal folds respectively The cephalic fold is first formed and as the pro-amniotic area lying immediately in front of the pericardial area forms the anterior limit of the circumference of the embryonic disc the forward growth of the head necessarily carries with it the posterior end of the pericardial area so that this area and the buccal pharyngeal membrane are folded back under the head of the embryo which now encloses a diverticulum of the ulc sac named the foregut The caudal end of the embryo is at first connected to the corian by a band of mesoderm called the body stalk but with the formation of the caudal fold the body stalk assumes a ventral position A diverticulum of the ulc sac extends into the tail fold and is termed the hindgut Between the foregut and the hindgut there exists for a time a wide opening into the ulc sac but the latter is gradually reduced to a small pear shaped sac sometimes termed the umbilical vesicle and the channel of communication is at the same time narrowed and elongated to form a tube called the Vitalin Duct Part 10 the Ulc sac The Ulc sac is situated on the ventral aspect of the embryo it is lined by intoderm outside of which is a layer of mesoderm it is filled with fluid the Vitalin fluid which possibly may be utilized for the nourishment of the embryo during the earlier stages of its existence blood is conveyed to the wall of the sac by the primitive aorti and after circulating through a wide meshed capillary plexus is returned by the Vitalin veins to the tubular heart of the embryo this constitutes the Vitalin circulation and by means of it, absorbed from the Ulc sac and conveyed to the embryo at the end of the fourth week the Ulc sac presents the appearance of a small pear shaped vesicle umbilical vesicle opening into the digestive tube by a long narrow tube the Vitalin Duct the vesicle can be seen in the after birth as a small somewhat oval shaped body whose diameter varies from one millimeter to five millimeters it is situated between the Amnion and the Corian and may lie on or at a varying distance from the placenta as a rule the Duct undergoes complete obliteration during the seventh week but in about three percent of cases its proximal part persists as a diverticulum from the small intestine Meckles diverticulum which is situated about three or four feet above the ileocolic junction and may be attached by fibrous cord to the abdominal wall at the umbilicus sometimes a narrowing of the lumen of the ileum is seen opposite the site of the attachment of the Duct end of section five recording by Leanne Howlett section six of Grey's Anatomy part one 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 one by Henry Gray part 11 development of the fetal membranes and placenta the Elantois arises as a tubular diverticulum of the posterior part of the yolk sac when the hind gut is developed the Elantois is carried backward with it and then opens into the cloaca or terminal part of the hind gut it grows out into the body stalk a mass of mesoderm which lies below and around the tail end of the embryo the diverticulum is lined by interderm and covered by mesoderm and in the latter are carried the Elantoic or umbilical vessels in reptiles, birds and many mammals the Elantois becomes expanded which projects into the extra embryonic selum if its further development be traced in the bird it is seen to project to the right side of the embryo and gradually expanding it spreads over its dorsal surface as a flattened sac between the amnion and the serosa and extending in all directions ultimately surrounds the yolk its outer wall becomes applied to and fuses with the serosa which lies immediately inside the shell membrane blood is carried to the Elantoic sac by the two Elantoic or umbilical arteries which are continuous with the primitive aorta and after circulating through the Elantoic capillaries is returned to the primitive heart by the two umbilical veins in this way the Elantoic circulation which is of the utmost importance and connection with the respiration and nutrition of the chick is established oxygen is taken from and carbonic acid is given up to the atmosphere through the egg shell while nutritive materials are at the same time absorbed by the blood from the yolk in man and other primates the nature of the Elantois is entirely different from that just described here it exists merely as a narrow tubular diverticulum of the hindgut and never assumes the form of a vesicle outside the embryo the formation of the amnion the embryo is in most animals entirely separated from the corian and is only again united to it when the Elantoic mesoderm spreads over and becomes applied to its inner surface the human embryo on the other hand as was pointed out by his is never wholly separated from the corian its tail end being from the first connected with the corian by means of a thick band of mesoderm named the body stock into this stock the tube of the Elantois extends the amnion the amnion is a membranous sac which surrounds and protects the embryo it is developed in reptiles, birds and mammals which are hence called amniota but not in amphibia and fishes which are consequently termed an amnia in the human embryo the earliest stages of the formation have been observed in the youngest embryo which has been studied the amnion was already present as a closed sac and is indicated on page 46 appears in the inner cell mass as a cavity this cavity is roofed in by a single stratum of flattened ectodermal cells the amniotic ectoderm and its floor consists of the prismatic ectoderm of the embryonic disc the continuity between the roof and floor is established at the margin of the embryonic disc outside the amniotic ectoderm is a thin layer of mesoderm which is continuous with that of the soma deplore and is connected by the body stock with the mesodermal lining of the corian when first formed the amnion is in contact with the body of the embryo but about the fourth or fifth week fluid liquor amnii begins to accumulate within it this fluid increases in quantity and causes the amnion to expand and ultimately to adhere to the inner surface of the corian so that the extra embryonic part of the selum is obliterated the liquor amnii increases in quantity up to the sixth or seventh month of pregnancy after which it diminishes somewhat at the end of pregnancy it amounts to about one liter it allows of the free movements of the fetus during the later stages of pregnancy and also protects it by diminishing the risk of injury from without it contains less than 2% of solids consisting of urea and other extractives in organic salts a small amount of protein and frequently a trace of sugar that some of the liquor amnii is swallowed by the fetus is proved by the fact that epidermal debris and hairs have been found among the contents of the fetal alimentary canal in reptiles, birds and many mammals the amnion is developed in the following manner at the point of constriction where the primitive digestive tube of the embryo joins the yolk sac a reflection or folding upward of the soma to pleur takes place this the amniotic fold first makes its appearance at the cephalic extremity and subsequently at the caudal end and sides of the embryo and gradually rising more and more parts meet and fuse over the dorsal aspect of the embryo and enclose a cavity the amniotic cavity after the fusion of the edges of the amniotic fold the two layers of the fold become completely separated the inner forming the amnion the outer the false amnion or serosa the space between the amnion and the serosa constitutes the extra embryonic selen the embilical cord and body stock the embilical cord attaches the fetus to the placenta its length at full time as a rule is about equal to the length of the fetus that is about 50 centimeters but it may be greatly diminished or increased the rudiment of the embilical cord is represented by the tissue which connects the rapidly growing embryo with the extra embryonic area of the ovum included in this tissue are the body stock and the vitilinduct the former containing the allantoic diverticulum and the embilical vessels the latter forming the communication between the digestive tube and the yolk sac the body stock is the posterior segment of the embryonic area and is attached to the corian it consists of a plate of mesoderm covered by thickened ectoderm on which a trace of neural groove can be seen indicating its continuity with the embryo running through its mesoderm are the two embilical arteries and the two embilical veins together with the canal of the elantois the last being aligned by entoderm its dorsal surface is covered by the amnion while its ventral surface is bounded by the extra embryonic selum and is in contact with the elongation of the embryo and the formation of the tail fold the body stock comes to lie on the ventral surface of the embryo where its mesoderm blends with that of the yolk sac and the vitilinduct the lateral leaves of soma to pleur then grow round on each side and meeting on the ventral aspect of the elantois enclose the vitilinduct and vessels together with a part of the extra embryonic selum the latter is ultimately obliterated the cord is covered by a layer of ectoderm which is continuous with that of the amnion and its various constituents are enveloped by embryonic gelatinous tissue jelly of worton the vitilind vessels and duct together with the right umbilical vein undergo atrophy and disappear and thus the cord at birth contains a pair of umbilical arteries and one the left umbilical vein implantation or embedding of the ovum as described fertilization of the ovum occurs in the lateral or ampillary end of the uterine tube and is immediately followed by segmentation on reaching the cavity of the uterus the segmented ovum adheres like a parasite to the uterine mucous membrane destroys the epithelium over the area of contact and excavates for itself a cavity in the mucous membrane in which it becomes embedded in the ovum described by brice and teacher the point of entrance was visible as a small gap closed by a mass of fibrin and leukocytes in the ovum described by peters the opening was covered by a mushroom shaped mass of fibrin and blood clot the narrow stock of which plugged the aperture in the mucous membrane soon however all trace of the opening is lost and the ovum is then completely surrounded by the fibrin and mucous membrane the structure actively concerned in the process of excavation is the trophoblast of the ovum which possesses the power of dissolving and absorbing the uterine tissues the trophoblast proliferates rapidly and forms a network of branching processes which cover the entire ovum and invade and destroy the maternal tissues and open into the maternal blood vessels with the result that the spaces in the trophoblastic network form the maternal blood these spaces communicate freely with one another and become greatly distended and form the intervillus space the decidua before the fertilized ovum reaches the uterus the mucous membrane of the body of the uterus undergoes important changes and is then known as the decidua the thickness and vascularity of the mucous membrane are greatly increased its glands are elongated and open on its free surface by funnel shaped orifices while their deeper portions are tortuous and dilated into irregular spaces the inter glandular tissue is also increased in quantity and is crowded with large round oval or polygonal cells termed decidual cells these changes are well advanced by the second month of pregnancy when the mucous membrane consists of the following strata one stratum compactum next the free surface in this the uterine glands are only slightly expanded and are lined by columnar cells two stratum spongiosum in which the gland tubes are greatly dilated and very tortuous and are ultimately separated from one another by only a small amount of inter glandular tissue while their lining cells are flattened or cubicle three altered or boundary layer next the uterine muscular fibers containing the deepest parts of the uterine glands which are not dilated and are lined with columnar epithelium it is from this epithelium that the epithelial lining of the uterus is regenerated after pregnancy distinctive names are applied to different portions of the decidua the part which covers in the ovum is named the decidua capsularis the portion which intervenes between the ovum and the uterine wall is named the decidua basalis or decidua placentalis it is here that the placenta is subsequently developed the part of the decidua which lines the remainder of the body of the uterus is known as the decidua vera or decidua parietalis coincidentally with the growth of the embryo the decidua capsularis is thinned and extended and the space between it and the decidua vera is gradually obliterated so that by the third month of pregnancy the two are in contact by the fifth month of pregnancy the decidua capsularis has practically disappeared while during the succeeding months the decidua vera also undergoes atrophy owing to the increased pressure the glands of the stratum compactum are obliterated and their epithelium is lost in the stratum spongiosum the glands are compressed and appear as slit like fissures while their epithelium undergoes degeneration in the unaltered or boundary layer however the glandular epithelium retains a columnar or cubical form the corian the corian consists of two layers an outer formed by the primitive ectoderm or trophoblast and an inner by the somatic mesoderm with this latter the amnion contact the trophoblast is made up of an internal layer of cubical or prismatic cells the cytotrophoblast or layer of langans and an external layer of richly nucleated protoplasm devoid of cell boundaries the sensishio trophoblast it undergoes rapid proliferation and forms numerous processes the corionic villi which invade and destroy the uterine decidua and at the same time absorb from it nutritive materials for the growth of the embryo the corionic villi are at first small and non-vascular and consist of trophoblast only but they increase in size and ramify while the mesoderm carrying branches of the umbilical vessels grows into them and in this way they are vascularized blood is carried to the villi by the branches of the umbilical arteries and after circulating through the capillaries the villi is returned to the embryo by the umbilical veins until about the end of the second month of pregnancy the villi cover the entire corion and are almost uniform in size but after this they develop unequally the greater part of the corion is in contact with the decidua capsularis and over this portion of the villi with their contained vessels undergo atrophy so that by the fourth month scarcely a trace of them is left and hence this part of the corion becomes smooth and is named the corion live as it takes no share in the formation of the placenta it is also named the non-placental part of the corion on the other hand the villi on that part of the corion which is in contact with the decidua placentalis increase greatly in size and complexity and hence this part is named the corion frondosum the placenta the placenta connects the fetus to the uterine wall and is the organ by means of which the nutritive, respiratory and excretory functions of the fetus are carried out it is composed of fetal and maternal portions fetal portion the fetal portion of the placenta consists of the villi of the corion frondosum these branch repeatedly and increase enormously in size these greatly ramified villi are indented in the intervillus space and are bathed in maternal blood which is conveyed to the space by the uterine arteries and carried away by the uterine veins a branch of an umbilical artery enters each villus and ends in a capillary plexus from which the blood is drained by a tributary of the umbilical vein the vessels of the villus are surrounded by a thin layer of mesoderm consisting of gelatinous connective tissue which is covered by two normal cells derived from the trophoblast the deeper stratum next the mesodermic tissue represents the cytotrophoblast or layer of langans the superficial in contact with the maternal blood the syncytiotrophoblast after the fifth month the two strata of cells are replaced by a single layer of somewhat flattened cells maternal portion the maternal portion of the placenta by the deciduo-placentalis containing the intravillus space as already explained this space is produced by the enlargement and intercommunication of the spaces and the trophoblastic network the changes involve the disappearance of the greater portion of the stratum compactum but the deeper part of this layer persists and is condensed to form what is known as the basal plate between this plate and the uterine muscular fibers are the stratum gongeosum and the boundary layer through these and the basal plate the uterine arteries and veins pass to and from the intervillus space the endothelial lining of the uterine vessels ceases at the point where they terminate in the intervillus space which is lined by the syncytiotrophoblast portions of the stratum compactum persist and are condensed to form a series of septa which extend from the basal plate to the thickness of the placenta and subdivide it into the lobules or cotyledons seen on the uterine surface of the detached placenta the fetal and maternal blood currents traverse the placenta the former passing through the blood vessels of the placental villi and the latter through the intervillus space the two currents do not intermingle being separated from each other by the delicate walls of the villi nevertheless the fetal blood will absorb through the walls of the villi oxygen and nutritive materials from the maternal blood and give up to the latter its waste products the blood so purified is carried back to the fetus by the umbilical vein it will thus be seen that the placenta not only establishes a mechanical connection between the mother and the fetus but subserves for the latter the purposes of nutrition respiration and excretion in favor of the view that the placenta possesses certain selective powers may be mentioned the fact that glucose is more plentiful in the maternal than in the fetal blood it is interesting to note also that the proportion of iron and of lime and potash in the fetus is increased during the last months of pregnancy further there is evidence that the maternal leukocytes may migrate into the fetal blood since leukocytes are much more numerous in the blood of the umbilical vein that of the umbilical arteries the placenta is usually attached near the fundus uteri and more frequently on the posterior than on the anterior wall of the uterus it may however occupy a lower position and in rare cases its site is close to the orphyseum interneum uteri which it may occlude thus giving rise to the condition known as placenta previa separation of the placenta after the child is born the placenta and membranes are expelled from the uterus as the afterbirth the separation of the placenta from the uterine wall takes place through the stratum spongiosum and necessarily causes rupture of the uterine vessels the orifices of the torn vessels are however closed by the firm contraction of the uterine muscular fibers and thus postpartum hemorrhage is controlled the epithelial lining of the uterus is regenerated by the proliferation and extension of the epithelium which lines the persistent portions of the uterine glands in the unaltered layer of the decidua the expelled placenta appears as a discoid mass which weighs about 450 grams and as a diameter of from 15 to 20 centimeters its average thickness is about 3 centimeters but this diminishes rapidly toward the circumference of the disc which is continuous with the membranes its uterine surface is divided by a series of fissures into lobules or cotyledons the fissures containing the remains of the septa which extended between the maternal and fetal portions most of these septa end in irregular or pointed processes others especially those near the edge of the placenta pass through its thickness and are attached to the corian in the early months these septa convey branches of the uterine arteries which open into the intravella space on the surfaces of the septa the fetal surface of the placenta is smooth being closely invested by the amnion seen through the ladder the corian presents a modeled appearance consisting of gray purple or yellowish areas the umbilical cord is usually attached near the center of the placenta but may be inserted anywhere between the center and the margin in some cases it is inserted into the membranes that is the volamentous insertion from the attachment of the cord the longer branches of the umbilical vessels radiate under the amnion the veins being deeper and larger than the arteries the remains of the vidal induct and yoke sac may be sometimes observed beneath the amnion close to the cord the former as an attenuated thread the ladder is a minute sac on section the placenta presents a soft spongy appearance caused by the greatly branched villi surrounding them is a varying amount of maternal blood giving the dark red color to the placenta many of the larger villi extend from the corionic to the decidual surface while others are attached to the septa which separate the cotyledons but the great majority of the villi hang free in the intravella space end of section 6 by Leanne Howlett section 7 of Grey's Anatomy Part 1 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 1 by Henry Gray Part 12 the Branchial Region the Branchial or Visceral Arches and Pharyngeal Pouches in the lateral walls of the interior part of the foregut 5 Pharyngeal Pouches appear each of the upper 4 pouches is prolonged into a dorsal and ventral diverticulum over these pouches corresponding indentations of the ectoderm occur forming what are known as the Branchial or Outer Pharyngeal grooves the intervening mesoderm is pressed aside and the ectoderm comes for a time into contact with the interdermal lining of the foregut and the two layers unite along the floors of the grooves to form thin closing membranes between the foregut and the exterior later the mesoderm again penetrates between the interderm and the ectoderm in gill bearing animals the closing membranes disappear the grooves become complete clefs the gill clefs opening from the pharynx onto the exterior perforation however does not occur in birds or mammals the grooves separate a series of rounded bars or arches the branchial or visceral arches in which thickening of the mesoderm takes place the dorsal ends of these arches are attached to the sides of the head while the ventral extremities in the middle line of the neck in all six arches make their appearance but of these only the first four are visible externally the first arch is named the mandibular and the second the hyoid the others have no distinctive names in each arch a cartilaginous bar consisting of right and left halves is developed and with each of these there is one of the primitive aortic arches the mandibular arch lies between the first branchial groove and the stymodium from it are developed the lower lip the mandible the muscles of mastication and the anterior part of the tongue its cartilaginous bar is formed by what are known as mechels cartilages above this the incus is developed the dorsal end of each cartilage is connected with the ear capsule and is ossified to form the malleus the ventral ends meet each other in the region of the synthesis menti and are usually regarded as undergoing ossification to form that portion of the mandible which contains the incisor teeth the intervening part of the cartilage disappears the portion immediately adjacent to the malleus is replaced by fibrous membrane which constitutes the sphenomendibular ligament while from the connective tissue covering the remainder of the cartilage the greater part of the mandible is ossified from the dorsal ends of the mandibular arch a triangular process the maxillary process grows forward on either side and forms the cheek and lateral part of the upper lip the second or hyoid arch assists in forming the side in front of the neck from its cartilage are developed the styloid process stylohyoid ligament and lesser corneaux of the hyoid bone the stages probably arises in the upper part of this arch the cartilage of the third arch gives origin to the greater corneaux of the hyoid bone the ventral ends of the second and third arches unite with those of the opposite side and form a transverse band from which the body of the hyoid bone and the posterior part of the tongue are developed the ventral portions of the cartilages of the fourth and fifth arches unite to form the thyroid cartilage from the cartilages of the sixth arch the cricoid and aerotinoid cartilages and the cartilages of the trachea are developed the mandibular and hyoid arches grow more rapidly than those behind them would the result that the latter become to a certain extent telescoped within the former and a deep depression the sinus cervicalis is formed on either side of the neck this sinus is bounded in front by the hyoid arch and behind by the thoracic wall it is ultimately obliterated by the fusion of its walls from the first brachial groove the concha auriculae and external acoustic metis are developed while around the groove there appear on the mandibular and hyoid arches a number of swellings from which the auricula or pina is formed the first pharyngeal pouch is prolonged dorsally to form the auditory tube and the tympanic cavity the closing membrane between the mandibular and hyoid arches is invaded by mesoderm and forms the tympanic membrane no traces of the second, third and fourth brachial grooves persist the inner part of the second pharyngeal pouch is named the sinus tonsillaris in it the tonsil is developed above which a trace of the sinus persists as the super tonsillar fossa the fossa of Rosenmuller or lateral recess of the pharynx is by some regarded as a persistent part of the second pharyngeal pouch but it is probably developed as a secondary formation from the third pharyngeal pouch the thymus arises as an entodermal diverticulum on either side and from the fourth pouch's small diverticula project and become incorporated with the thymus but in man these diverticula probably never form true thymus tissue the parathyroids also arise as diverticula from the third and fourth pouches from the fifth pouches the ultimobranchio bodies originate and are enveloped by the lateral prolongations of the median thyroid rudiment they do not however form true thyroid tissue but they do trace traces of them found in the human adult the nose and face during the third week two areas of thickened ectoderm the olfactory areas appear immediately under the forebrain in the anterior wall of the stymodium one on either side of a region termed the frontonasal process by the upgrowth of the surrounding parts these areas are converted into pits the olfactory pits indent the frontonasal process and divide it into a medial and two lateral nasal processes the rounded lateral angles of the medial process constitute the globular processes of his the olfactory pits form the rudiments of the nasal cavities and from their ectodermal lining the epithelium of the nasal cavities with the exception of that of the inferior medices is derived the inferior processes are prolonged backward as plates termed the nasal laminae these laminae are at first some distance apart but gradually approaching they ultimately fuse and form the nasal septum the processes themselves meet in the middle line and form the primaxole and the filtrum or central part of the upper lip the depressed part of the medial nasal process between the globular processes is the part of the nasal septum or kalumala while above this is seen a prominent angle which becomes the future apex and still higher a flat area the future bridge of the nose the lateral nasal processes form the alie of the nose continues with the dorsal end of the mandibular arch and growing forward from its sepholic border is a triangular process the maxillary process the ventral extremity of which is separated from the mandibular arch by a shaped notch the maxillary process forms the lateral wall and floor of the orbit and in it are ossified the zygomatic bone and the greater part of the maxilla it meets with the lateral nasal process from which however it is separated for a time by a groove the naso-optic furrow that extends from the furrow and circling the eyeball to the factory pit the maxillary processes ultimately fuse with the lateral nasal and globular processes and form the lateral parts of the upper lip and the posterior boundaries of the nares from the third to the fifth month the nares are filled by masses of epithelium on the breaking down a disappearance of which the permanent openings are produced the maxillary process also gives rise to the lower portion of the lateral wall of the nasal cavity the roof of the nose and the remaining parts of the lateral wall is the ethmoidal labyrinth the inferior nasal concha the lateral cartilage and the lateral crust of the allar cartilage are developed in the lateral nasal process by the fusion of the maxillary and nasal processes in the roof of the stymodium the primitive pallet is formed and the olfactory pits extend backward above it the posterior end of each pit is closed by an epithelial membrane the buccal nasal membrane formed by the apposition of the nasal and stymodial epithelium by the rupture of these membranes the primitive coane or openings between the olfactory pits and the stymodium are established the floor of the nasal cavity is completed by the development of a pair of shelf-like palatine processes which extend medial word between the maxillary processes these coalesce with each other in the middle line and constitute the entire pallet except a small part in front which is formed by the premaxillary bones two apertures persist for a time between the palatine processes and the premaxillary and represent the permanent channels which in the lower animals connect the nose and mouth the union of the parts the premaxillary and palatine processes joining in the eighth week while the region of the future hard pallet is completed by the ninth and that of the soft pallet by the eleventh week by the completion of the pallet the permanent coane are formed and are situated a considerable distance behind the primitive coane the deformity known as cleft pallet results from a non-union of the palatine processes with an air lip through a non-union of the maxillary and globular processes see page 199 the nasal cavity becomes divided by a vertical septum which extends downward and backward from the medial nasal process and nasal laminy and unites below with the palatine processes into this septum a plate of cartilage extends from the under aspect of the ethmoid plate of the chondrocranium the anterior part of this cartilaginous plate persists as the septal cartilage of the nose and the medial crust of the allar cartilage but the posterior and upper parts are replaced by the vomer and perpendicular plate of the ethmoid on either side of the nasal septum at its lower and anterior part the ectoderm is invaginated to form a blind pouch or diverticulum which extends backward and upward into the nasal septum and is supported by a curved plate of cartilage these pouches form the rudiments of the vomeronasal organs of Jacobson which open below close to the junction of the premaxillary and maxillary bones the limbs the limbs begin to make their appearance in the third week as small elevations or buds at the side of the trunk prolongations from the muscle and cutis plates of several primitive segments extend into each bud and carry with them the anterior divisions of the corresponding spinal nerves the nerves supplying the limbs indicate the number of primitive segments which contribute to their formation the upper limb being derived from 7 v's 4th cervical to 2nd thoracic inclusive and the lower limb from 10 v's 12th thoracic to 4th sacral inclusive the axial part of the mesoderm of the limb bud becomes condensed and converted into its cartilaginous skeleton and by the ossification of this the bones of the limbs are formed by the 6th week the three cheap divisions of the limbs are marked off by furrows the upper into arm forearm and hand the lower into thigh, leg and foot the limbs are at first directed backward nearly parallel to the long axis of the trunk and each presents two surfaces of the borders of the surfaces, one the future flexor surface of the limb is directed ventrally the other, the extensor surface dorsally one border, the preaxial looks forward toward the cephalic end of the embryo and the other the postaxial backward toward the caudal end the lateral epicondyle of the humerus the radius and the thumb the radial border of the upper limb and the medial epicondyle of the femur, the tibia and the great toe along the corresponding border of the lower limb the preaxial part is derived from the anterior segments the postaxial from the posterior segments of the limb bud and this explains to a large extent the innervation of the adult limb the nerves of the more anterior segments being distributed along the preaxial or tibial and those of the more posterior along the postaxial ulnar or fibular border of the limb the limbs next undergo a rotation or torsion through an angle of 90 degrees around their long axes the rotation being affected almost entirely at the limb girdles in the upper limb the rotation is outward and forward in the lower limb inward and backward as a consequence of this rotation the preaxial radial border of the fore limb is directed lateral word and the preaxial tibial border of the hind limb is directed medial word thus the flexor surface of the fore limb is turned forward and that of the hind limb backward end of section 7 recording by Leanne Howlett for more information or to volunteer please visit in the human embryo described by Peters the mesoderm outside the embryonic disc is split into two layers and closing an extra embryonic column there is no trace of an intra embryonic column at a later stage four cavities are formed within the embryo these one on either side within the mesoderm of the pericardial area and one in either lateral mass of the general mesoderm all these are at first independent of each other and of the extra embryonic selum but later they become continuous the two cavities in the general mesoderm unite on the ventral aspect of the gut and form the pleuroperitoneal cavity which becomes continuous with the remains of the extra embryonic selum around the embolicus the two cavities in the pericardial area rapidly join to form a single pericardial cavity and this from two lateral diverticula extend caulderward to open into the pleuroperitoneal cavity between the two latter diverticula is a mass of mesoderm containing the ducts of cuvier and this is continuous eventually with the mesoderm in which the umbilical veins are passing to the sinus venousis a septum of mesoderm thus extends across the body of the embryo it is attached in front to the body wall between the pericardium and umbilicus behind the body wall at the level of the second cervical segment laterally it is deficient with the pericardial pleuroperitoneal cavities communicate perforated in the middle line by the foregut this partition is termed septum transversum and is at first a bulky plate of tissue as development proceeds the dorsal end of the septum is carried gradually caulderward and when it reaches the fifth cervical segment muscular tissue with the phrenic nerve grows into it it continues to recede however until it reaches the position of the adult diaphragm on the bodies the upper lumbar vertebrae the liver buds grow into the septum transversum and undergo development there the lung buds meantime have grown out from the foregut and project laterally into the four part of the pleuroperitoneal cavity the developing stomach and liver are embedded in the septum transversum caulderward to this the intestines project into the back part of the pleuroperitoneal cavity owing to the descent of the dorsal end of the septum transversum the lung buds come to lie above the septum and thus pleuro and peritoneal portions of the pleuroperitoneal cavity still however in free communication with one another may be recognized the pericardial cavity opens into the pleuro part the ultimate separation of the permanent cavities from one another is effected by the growth of a ridge of tissue on either side of the mesoderm surrounding the duct of Cuvier the front part of this ridge grows across and obliterates the pleuropericardial opening the hindre part grows across the pleuroperitoneal opening with the continued growth of the lungs the pleural cavities are pushed forward in the body wall towards the ventral median line thus separating the pericardium from the lateral thoracic walls the further development of the peritoneal cavity has been described with the development of the digestive tube the form of the embryo at different stages of its growth first week during this period the ovum is in the uterine tube having been fertilized in the upper part of the tube it slowly passes down undergoing segmentation and reaches the uterus Peters describes a specimen the age of which he reckoned as from three to four days footnote Bryce and teacher early development and embedding of the human ovum 1908 have described an ovum which they regard as 13 to 14 days old in it the two vesicles the amnion and yolk sac were present but there was no trace of a layer of embryonic ectoderm there of opinion that the age of Peters ovum has been understated and estimated as between 13 and 1 half and 14 and 1 half days and footnote it was embedded in the decidua on the posterior wall of the uterus and enveloped by a decidua capsularis the central part of which however consisted merely of a layer of fibrin the ovum was in the form of a sac the outer wall of which consisted of a layer of trophoblast inside this was a thin layer of mesoderm composed of round oval and spindle shaped cells numerous villus processes some consisting of trophoblast only others possessing a core of mesoderm projected from the surface of the ovum into the surrounding decidua inside this sac the rudiment of the embryo was found in the form of a patch of ectoderm covered by a small but completely closed amnion it possessed a minute yolk sac and was surrounded by mesoderm which was connected by a band with the trophoblast second week by the end of this week the ovum has increased considerably in size and the majority of its villi are vascularized the embryo has assumed a definite form and its cephalic and caudal extremities are easily distinguished the neural folds are partly united the embryo is more completely separated from the yolk sac and the paraxial mesoderm and the primitive segments third week by the end of the third week the embryo is strongly curved and the primitive segments number about 30 the primary divisions of the brain are visible and the optic and auditory vesicles are formed for brachial grooves are present the stomadium is well marked and the bucopharyngeal membrane has disappeared the rudiments of the limbs are seen as hollow buds and the wolfian bodies are visible fourth week the embryo is markedly curved on itself and when viewed in profile is almost circular in outline the cerebral hemispheres appear as hollow buds and the elevations which form the rudiments of the auricular are visible the limbs now appear as oval flattened projections fifth week the embryo is less curved and the head is relatively of large size differentiation of the limbs into their segments occurs the nose forms a short flattened projection the collocal tubersal is evident sixth week the curvature of the embryo is further diminished the brachial grooves except the first have disappeared and the rudiments of the fingers and toes can be recognized in a few weeks the flexure of the head is gradually reduced and the neck is somewhat lengthened the upper lip is completed and the nose is more prominent the nostrils are directed forward and the palate is not completely developed the eyelids are present in the shape of folds above and below the eye and the different parts of the auricular are distinguishable by the end of the second month the fetus measures from 28 meters in length third month the head is extended and the neck is lengthened the eyelids meet and fuse remaining closed until the end of the sixth month the limbs are well developed and nails appear on the digits the external generative organs are so far differentiated that it is possible to distinguish the sex by the end of this month the length of the fetus is about seven centimeters but if the legs be included it is from nine to ten centimeters fourth month the loop of gut which projected into the umbilical cord is withdrawn within the fetus the hairs begin to make their appearance there is a general increase in size so that by the end of the fourth month the fetus is from 12 to 13 centimeters in length but if the legs be included it is from 16 to 20 centimeters fifth month this month that the first movements of the fetus are usually observed the eruption of hair on the head commences and the vernex caciosa begins to be deposited by the end of this month the total length of the fetus including the legs is from 25 to 27 centimeters sixth month the body is covered by fine hairs lanugo and the deposit of vernex caciosa is considerable the papillae of the skin are developed and the free border of the nail projects from the corium of the dermis measured from vernex to heels the total length of the fetus at the end of this month is from 30 to 32 centimeters seventh month the pupillary membrane atrophies and the eyelids are open the testes descends with the vaginal sac of the peritoneum from vernex to heels the total length at the end of the seventh month is from 35 to 36 centimeters the weight is a little over three pounds eighth month the skin assumes a pink color and is now entirely coated with vernex caciosa and the langua begins to disappear subcutaneous fat has been developed to a considerable extent and the fetus presents a plump appearance the total length that is, from head to heels at the end of the eighth month is about 40 centimeters and the weight varies between four and one-half and five and one-half pounds ninth month the langua has largely disappeared from the trunk the umbilicus is almost in the middle of the body and the testes are in the scrotum at full time the fetus weighs from six and one-half to eight pounds which measures from head to heels about 50 centimeters