 The lateral cervical region in surgical parlance is also referred to as the posterior triangle of the neck. First, let's trace the boundaries of the lateral cervical region. This is the anterior boundary of the lateral cervical region which is formed by the posterior border of the sternocleidomastoid muscle. This is the sternocleidomastoid muscle, extending from this marrow rim sternum, clavicle, and extending at the mastoid process and supineucleid. The posterior boundary is formed by this line which is being traced by my finger here. This is the descending fibers of the trapezius. The inferior boundary is the middle one-third of the clavicle where the trapezius ends and the sternocleidomastoid ends. The apex of the triangle is where my finger is located. This is the place where the sternocleidomastoid insertion ends and the origin of the trapezius starts. That's on the supineucleid. The roof is bridged over by a deep fascia of the neck, it's called the investing layer of deep cervical fascia. The investing layer of deep cervical fascia, it splits to enclose this sternocleomastoid and it also splits to enclose the trapezius and it bridges over and that forms the roof. Now let's come to the floor of the, that's the cervical region. In order to see the floor property, I have reflected the sternocleomastoid, I have reflected part of the descending fibers of the trapezius and therefore what we see in front of us, this is the floor. So let's start from top. This muscle that we can see here, this is the spleenus capitis. Under that we have this muscle here. This is the levator scapulae. Under that we have this muscle here. This is the scalyness posterior. Then we have the next muscle, that is the scalyness medius and finally we can see a little bit of this muscle also, this is the scalyness anterior. So all these together constitute the floor of the lateral cervical region. During life this was covered by another layer of deep fascia and that is known as the pre-vertebral layer of deep cervical fascia which we have removed. In the floor, in the depths of the floor, we have another triangle and that is this triangle which is being traced by my instrument right now. This one we had mentioned was the scalyness medius and this one we had mentioned was the scalyness anterior. So this triangular space between the two is referred to as the scalyne triangle and the inferior border of the scalyne triangle is formed by the first rib which is located deep to the clavicle here because both the scalyness medius and the scalyness anterior, both of them get inserted onto the first rib. The point to be remembered is that this scalyne triangle is not within the lateral cervical region. It is deep to the lateral cervical region. It is in the floor of the lateral cervical region. The lateral cervical region is subdivided into two triangles and the demarcating structure which separates them is this one which I have held up here. This is the inferior belly of the omohide muscle. The omohide muscle is a double-bellied muscle. The inferior belly starts on the scapula and goes obliquely up like this. Here there is an intermediate tendon which has been removed when they were trying to embalm this cadaver and from the intermediate tendon we have the next set of fibers. This is the superior belly which we can see here. This change in direction of the omohide is by virtue of a facial sling part of which is visible here which extends from the intermediate tendon and gets attached to the clavicle which most of it we have removed here. This inferior belly of the omohide subdivides the lateral cervical region into this portion here that is referred to as the omoclavicular or the subclavian triangle. This is called the subclavian triangle because the subclavian artery is present here and the larger upper portion here which is referred to as the occipital triangle because this is where we have a branch of the external carotid artery namely the occipital artery at the apex of the triangle. So let's come to the contents of the lateral cervical region. Let's take a look at this structure here which is running on the surface of the sternocleum mastoid. This is the retromandibular vein. The retromandibular vein strictly speaking is not in the triangle it is running on the surface of the sternocleum mastoid however as it comes down here in just in the lower part it pierces the investing layer of deep cervical fascia and then it becomes a content of the triangle and then it opens into the subclavian vein which we can see here. So this is partly in the lateral cervical region. The more important contents are once we reflect the sternocleum mastoid then we can see that this is the cervical plexus. This is the C1 root. This is the C2 root. This is the C3 root. This is the C4 root and this is the C5 root and C6 roots and there will be other roots also which are coming through the scalyne triangle. On each of these roots they form connections with each other which are referred to as primary loops and we can see the primary loops here and from there the branches of the cervical plexus will originate. We have these branches, great auricular, lesser occipital. The transverse cervical has been removed because it runs like this towards the anterior cervical region and the largest other branch is this one. This is the supraclavical branch dividing into lateral, intermediate and medial branches which runs over the clavicle and supplies the skin here and last but definitely not the least we have this nerve here. This is the phrenic nerve which is actually a mixed nerve both motor and sensory which runs on the surface of the scalyne anterior and then goes behind the subclavian vein. So these are the other contents and the branches that we can see in the lateral cervical region. Now let's take a look at the subclavian artery itself. This is the subclavian artery. That's the reason why we said this was called the subclavian triangle. The subclavian artery is divided into three parts by the scalyne anterior muscle. The portion of the subclavian artery behind the scalyne anterior muscle that is the second part of the subclavian artery. The portion of the subclavian artery proximal to that is the first part of the subclavian artery. So it is not in the lateral cervical region and the portion of the subclavian artery distilled to the scalyne anterior till the outer border of the first strip. This is the third part of the subclavian artery and this is the one which which is truly in the lateral cervical region. The next content is this structure which I have lifted up here. This is the spinal axis renom, CN11. We can see it is coming high up through the jugular foremen where menstrual mat is disappeared along with the internal jugular vein and then it runs on the surface of the internal jugular vein and then it enters into the sternocleomastoid from its t-baspect and then it runs on the slivator scapulae muscle. So this is also an important content of the lateral cervical region. In this place the spinal axis renom is liable to injury either during surgery for lymph node dissections for metastatic cancer or traumatic knife slash injury in which case it will produce paralysis of the trapezius muscle because after supplying the sternocleomastoid it gives a branch and we can see that branch here it goes and supplies the trapezius. So therefore it will lead to paralysis of the trapezius and drooping of the shoulder. Another important clinical correlation pertains to this external jugular vein. This is because it is running superficial to the sternocleomastoid it can also be injured in a knife slash injury and this vein is notorious for forming air embolism because there is a relatively low negative pressure inside this vein caused by Bernoulli's principle as it pierces the investing layer deep cervical fascia that the vein is adhering to the margins of the investing layer therefore it cannot retract. So combination of negative pressure as well as non-rectile margin causes air to be sucked in and so therefore injury to this vein is liable to produce air embolism. In surgery the lateral cervical region is considered to be the relatively safe area as compared to the anterior cervical region. The reason being anterior cervical region contains these important structures namely the internal jugular vein and the carotid artery however the word safe is a relative term because it does contain many other important structures the most important being accessory nerve the cervical plexus and the external jugular vein. So that's all for now. Thank you very much for watching Dr Sanjay Sanyal, please like and subscribe if you have any questions or comments please put them in the comment section below have a nice day.