Gross Anatomy: Anterior Triangle of Neck



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Dr. Zehren Gross Anatomy: Anterior Triangle of Neck Page of

SCM= Sternocleidomastoid, HB=Hyoid Bone, CCA=Common Cartoid Artery, ICA= Internal Carotid Artery, ECA=External Cartoid Artery, IJV= Internal Jugular Vein, CV=cervical vertebrae
Before lecture note: Brachial plexus (audio cut out )and trunks and rami or roots anesthetized superior to midpoint of the clavicle. Cervical plexus can be anesthetized as well at the posterior border of the SCM. Make a note of this.

  1. Anterior Triangle of the Neck[S1]:

  2. Boundaries [S2]

    1. [S3] The anterior triangle (green area), is bounded by the midline of the neck, inferior border of the mandible (superior boundary), and SCM (posterior boundary).

    2. [S4] There are number of osteological landmarks that you be useful to you for identifying certain things.

      1. The Hyoid bone is U-shaped and lies in front of the neck and below the mandible at the level of CV3. It can move up and down as we will explain later, but normally lies at C3 level. It is attached to the styloid process of the mandible with a ligament called the stylohyoid ligament, which you should find later on the course.

      2. The thyroid cartilage is the largest cartilage of the larynx, it’s responsible for the adam’s./eves apple or laryngeal prominence. This cartilage lies at CV4 and CV5 of the larynx.

      3. Just below is the cricoid cartilage. This is a complete ring of cartilage of the larynx, it’s the lowest cartilage of the larynx. The cricoid cartilage of the larynx lies at level CV6, important level because it marks the termination of the larynx.

      4. Below CV6 is the trachea.

        1. The tracheal rings are C-shaped rings that strengthen the wall of the trachea. You can palpate the structures on the cadaver or on yourself. The laryngeal prominence is just below that. Some of the rings are covered by the isthmus of the thyroid gland.

      5. Important: CV6 also marks the ending of the larynx and the pharynx, beginning of the trachea and beginning of esophagus (which is behind the trachea). Easily identified by palpating the cricoid cartilage.

    3. [S5] They hyoid bone has a body. The hyoid bone has the greater and a pair of lesser horns. The stylohyoid ligament attaches to the lesser horns. There are numerous muscles that attach to hyoid bone. The bone can be moved up and down in the neck during speaking and swallowing. It doesn’t have direct articulation with any other bone.

  3. Subdivisions [S6]

    1. The anterior triangle is subdivided into four smaller triangles as follows. (red boxes)

    2. Submental triangle (beneath the chin) is unpaired that crosses the midline is bounded by anterior bellies of digastric (on either side) and hyoid bone (below).

    3. Digastric or Submandibular triangle is bound by the two bellies: anterior and posterior bellies of digastric and the inferior border of mandible.

    4. The Muscular triangle, which is bounded by midline of neck, SCM and superior belly of omohyoid. The infrahyoid or strap muscles lie in this triangle.

    5. Carotid triangle is bounded by posterior belly of digastric, superior belly of omohyoid and the SCM. It is named because of the parts of the CCA, ICA, and ECA.

  4. Roof [S7,8]

    1. Roof of anterior triangle like the posterior triangle is platysma muscle and the underlying layer of investing layer of deep cervical fascia. Can see it in the posterior triangle.

    2. The investing layer is completely sleeve around the neck, and it is around the anterior triangle as well. Can’t see it here because the platysma is covering it.

  5. Submandibular Triangle [S10]

    1. Floor [S11,12]

      1. Floor of submandibular triangle formed by 2 muscles: mylohyoid (flat muscles that forms the anterior part) and posterior part and overlapping is the hypoglossus, which runs from the hyoid to and the tongue. These two muscles form the floor of the submandibular triangle.

    2. [S12 ] This pictures shows those muscles and the other the suprahyoid muscles (above the hyoid muscles) which include the two bellies of the digastric, mylohyoid, stylohyoid, and the geniohyoid (which can’t be seen).

      1. Suprahyoid muscles

        1. Digastric muscle is similar to the omohyoid. It has two bellies united by an intervening tendon.

          1. Anterior belly

            1. ORIGIN: Originates from the digastric fossa, which is a depression in mandible on the lower posterior of the mandible.

            2. The bellies unite and insert via intermediate tendon into the hyoid. The intermediate tendon is held down to the hyoid bone by the fibrous loop.

          2. Digastric (posterior belly)

            1. ORIGIN: The mastoid notch of the temporal bone which is medial to the mastoid process.

            2. The bellies unite and insert via intermediate tendon to the hyoid. The tendon is held down to the hyoid bone by the fibrous loop.

        2. Stylohyoid is superior to the posterior belly of the digastric. The muscles are closely associated. It comes from the styloid process. If you trace it down, the muscle splits to allow the tendon of digastric to pass through.

            1. ORGIN: Styloid process inserts into the body of hyoid

        3. Mylohyoid

          1. Originates inside the mandible called the mylohyoid line and inserts to the midline raphe (raphe is when two muscles meet edge to edge and form a fibrous line or connection). Some fibers do insert directly into the hyoid.

          2. [S14] Superior view and can see the mylohyoid line, which is the oblique ridge on the mandible. See two muscles that pass medially and downward towards the insertion in the middle. The two muscles make the floor of the oral cavity along with two geniohyoid muscles.

        4. [S14] Geniohyoid muscles are paired muscles and elongated muscles.

          1. Geniohyoid muscle arise from either side from the inferior mental spine or inferior genio tubercle, there is a superior above and inferior below, and inferior of the mental spine and passes back to the hyoid bone (referring to slide). Won’t see the muscles until we do the floor of the mouth. They are superior to the mylohyoid muscles. They are classified as suprahyoid muscles, and functionally they act with the suprahyoid muscles.

    3. [S15] Functions of the muscles

      1. See the mandible, and the direction of pull of the suprahyoid muscle, they elevate the hyoid when the muscles contract. They larynx is also elevated because the hyoid and the larynx are connected by a membrane called thyrohyoid membrane.

      2. The suprahyoid muscle elevates the larynx in swallowing and in speaking.

    4. [S16] The innervations have to do with the embryological origin of these muscles. Some skeletal muscles in the head and neck come from the pharyngeal arches, this can be a problem when studying the head and neck.

      1. Any muscle from the pharyngeal arch is considered visceral. The pharyngeal arches are the in the pharynx, which is part of the gut tube

      2. The muscular derivatives are visceral, so the motor innervation to all pharyngeal arches is special visceral efferent.

      3. You just have to learn which muscles come from the pharyngeal arches. You can’t just tell by looking them histologically, because they are identical to muscles that come from myotone.

      4. Some skeletal muscles come from myotone, so the motor innervation is GSE (general somatic efferent). This would include the muscles of the body wall, the extremities, and some muscles in the head and neck.

      5. The anterior belly of the digastric and the mylohyoid are derived from pharyngeal arch 1. Therefore, the motor innervation is special visceral afferent (SVE) by the mylohyoid nerve (also innervates the anterior belly of the digastric), and that nerve is a branch of the trigeminal branch (CN V).

    5. [S17] Posterior Belly digastric and the stylohyoid

      1. The posterior belly of the digastric and the stylohyoid are innervated by the facial nerve.

      2. These two muscles are derived from pharyngeal arch 2, the nerve for this arch is the facial nerve. So the muscles are innervated (SVE) by CN VII.

      3. Digastric has different innervations because the anterior and posterior bellies come from different pharyngeal arches.

      4. SQ]: ant belly of the digastric muscle is innervated by what?

      5. [A]: It’s a branch of the V3, mandibular nerve. The mandibular nerve is a nerve of arch 1, so innervates all the muscles in arch 1.

      6. The geniohyoid muscle is derived from cervical myotomes, so not a pharyngeal arch. So it fibers of the first cervical nerves (C1) that travel with the hypoglossal nerve (CN XII) is the innervation of the geniohyoid.

  6. Contents of Submandibular triangle [S18,19]

    1. Contents of submandibular triangle include superficial part of submandibular gland, which is large and fills up the triangle and located close to it is submandibular lymph nodes.

    2. Parts of the facial arteries and veins can also seen in the triangle, the vein is superficial to gland and if you trace it down, toward the internal jugular you will see the vein. Branches of CN VII can lie in the submandibular triangle.

    3. [S20]

      1. Cervical branch of VII goes to platysma muscle is in the submandibular triangle. You need to take care when dissecting that you don’t cut that nerve, it enters the deep surface of the platysma.

      2. Another nerve, the marginal mandibular branch of CN VII. It runs usually along the inferior margin of mandible, and not actually in the triangle, but in can dip below the margin of the mandible. If it does, than it is in danger of being cut during surgical procedure. It’s important nerve because it innervates the muscles of lower lip and chin. So you can paralysis of those muscles.

      3. Cervical branch of VII or marginal mandibular branch may be there as well facial vessels and submanibular gland.

      4. Mylohyoid separates superficial and deep parts of the submandibular gland. (Deep part of gland and submandibular duct will be discussed in connection w/ oral cavity).

      5. Notice the submandibular gland is U-shaped, you will see the large superficial part, the deep part is on the other side of the mylohyoid and wraps around the posterior edge of the mylohyoid and you won’t see this part of the gland until we do the floor of the oral cavity. The submandibular duct arrives from the deep part of the gland.

  7. Submental Triangle Floor [S21,22]

    1. [S23] Mylohyoid muscle forms the floor of submental triangle and the boundaries of the two anterior bellies of the digastric and the hyoid bones

  8. Submental Triangle Contents [S24,25]

    1. [S25] Submental lymph nodes that drain draining tip of tongue, mandibular incisor teeth, median part of lower lip. .

    2. [S26] The upper parts of the anterior jugular veins also drain in this triangle.

      1. These are small veins which descend on either side of the midline of the beginning of the submental triangle and extend down to the muscular triangle and end in the external jugular vein by passing deep to the SCM.

      2. The veins are superficial in location, and usually can’t see them because they get removed with the platysma.

  9. Muscular Triangle Contents [S27,28]

    1. Named because of the strap muscles/infrahyoid muscles.

    2. [S29] The lower parts of the anterior jugular veins are in this triangle. These two veins are united in the midline above the manubrium by a jugular venous arch. The anterior jugular vein passes laterally, deep to the SCM, to end in the terminal part of the external jugular vein.

    3. [S30] Strap muscles (infrahyoid)

      1. Usually elongated (reason why they are named “strap muscles”).

      2. There are four muscles on each side that arranged into 2 groups: two deep and two superficial

      3. The two superficial muscles are the sternohyoid and omohyoid.

        1. Sternohyoid comes from the manubrium of the sternum and inserts in the hyoid.

        2. The omohyoid (omo- referring to the shoulder area), the inferior belly comes from the superior margin of the scapula. It then forms this intermediate tendon, the superior belly runs up and inserts into the hyoid. It’s a very elongate muscle.

      4. Deep to the superficial muscles are the sternothyroid and thyrohyoid.

        1. Sternothyroid inserts into the oblique line of the thyroid cartilage and continuing up as different muscle as thyrohyoid running from the oblique line of the cartilage of the thyroid to the hyoid.

      5. To see the deep muscles, you have to reflect these two superficial muscles.

      6. [S31] These are antagonist to the suprahyoid muscles. They depress the hyoid bone by pulling down on it or sternothyroid depresses the thyroid cartilage and indirectly pulls on the hyoid bone.

    4. [S32] There innervation is by ANSA cervicalis which is means a loop in the neck. Nerve loop made of superior and inferior roots which unite to make a loop. From this, muscular twigs come off to innervate the strap muscles.

    5. [S33] Seen this slide before.

      1. The five parts of the cervical plexuses.

      2. The cervical plexus is the ventral rami of C1 – C4 of spinal nerves.

      3. The hypoglossal nerve going to the tongue, and the accessory nerve running to the SCM and trapezius.

      4. We have talked about the Cutaneous branches of cervical plexus are the posterior triangle like the supracavicular nerves and such. Talked about the twigs that go to the prevertebral and scalene muscles. We talked about the communication nerve that might supplement with the accessory nerve with the motor innervation.

      5. ANSA CERVICALIS is the 5th part of the cervical plexus. The C1 (ventral rami) fibers join CN XII high in the neck. They travel with CN XII and then they peel off to innervate various muscles. Some of these then branch off to form the superior root of the ANSA.

        1. Fibers from C2 and C3 join to form the inferior root of the ANSA.

        2. The twigs from the ANSA go to the strap muscles.

        3. Geniohyoid (not a strap muscles but it is a suprahyoid muscles) also gets innervations from C1. Some C1 fibers travel further along with XII before splitting off to serve this muscle.

        4. Good diagram view of the cervical plexus.

    6. [S34] This is what it looks like in the cadaver. When dissecting for the ANSA cervicalis, it lies right on the carotid sheath, you can destroy this part so be careful. To get to the common carotid, vagus, and IJV, you have to remove the sheath, so you can destroy the ANSA cervicalis. The superior root should meets with CN XII. The inferior root curves around the lateral aspect of the IJV, typically. It can sometimes come out medial to the IJV.

    7. When looking for the ANSA, it’s on the carotid sheath and it can be injured at that point.

  10. Carotid Triangle Floor [S35,36]

    1. [S37] The floor of carotid triangle formed by pieces thyrohyoid and hyoglossus form the anterior part, and middle and inferior constrictors of pharynx form the posterior part of the triangle floor.

  11. Carotid Triangle Contents [S38]

    1. Common Carotid Artery [S39]

      1. This is good schematic view of the common carotid.

      2. The CCA splits into anterior (ECA) and a more posterior (ICA). This division of the CCA happens at the CV4 level. The CV4 is around the superior border of the thyroid cartilage, which lies opposite of C4 and C5. Marks the division.

      3. The ICA has no branches in the neck and shoots straight up to the skull because it is going to the brain.

      4. The external carotid artery has 8 branches total--- 6 collateral and 2 terminal.

        1. The mnemonic for remembering these branches is S. A. L. F. O. P. S. M, in the order they arrive.

          1. Superior thyroid artery (the first branch) comes off and bends down and runs to the thyroid gland. Small twig off is called superior laryngeal artery which goes to the larynx.

          2. Ascending pharyngeal artery, above the superior thyroid, it is hard to find because it’s the only branch that arises from the deep or medial aspect of the ECA.

          3. Lingual artery runs to the tongue.

          4. Facial artery above the lingual. It appears at the lower part of the mandible and extends to the face

          5. Two arteries on the posterior side of the ECA.

            1. Occipital artery, which runs posterior and upward behind the ear and supplies the back of the head.

            2. Posterior auricular artery (above that) runs directly behind the ear and supplies the scalp.

          6. Two terminal branches : S and M

            1. Superficial temporal artery, which runs directly in front of the ear or auricle. You can feel the pulse through this one.

            2. Maxillary artery passes deep to the mandible, and this is important artery because it has many branches and supplies the deeper parts of the head.

v. Some of these arteries are outside of the anterior triangle, but you should look for the superior thyroid, ascending phyrangeal, lingual, facial, and occipital. This a good figure to identity those structures.

    1. Carotid Sinus [S40]

      1. The ICA is slightly expanded at the beginning, which is called carotid sinus.

      2. Its wall contains baroreceptors (barometer means it measures air pressure) which monitor blood pressure in the carotid sinus.

      3. The carotid body is a tiny clump of cells that lies in the bifurcation of the CCA. These cells monitor the oxygen content of the blood.

      4. Both of these structures are innervated carotid sinus nerve which is a branch of the CN IX. The functional component is general visceral afferent.

      5. Sensory innervation to blood vessels is visceral no matter where they are. These are GVA fiber and important. Important because if the blood pressure rises too high or the oxygen content becomes too low, afferent impulses goes to the medulla and efferent reflex is sent to increase the respiratory rate if the oxygen content is low, or lower the heart rate if the blood pressure is too high.

      6. [SQ]: Clarification about SVA and CN. [A]: SVA is taste fibers. These are GVA fibers, this would include hollow organs like the when your stomach and bladder are full, or pain from visceral organ will be GVA as well.

      7. Carotid body is hard to find, and not likely not see. The carotid sinus is usually obvious.

    2. Internal Jugular Vein [S41]

      1. The internal jugular vein (IJV) is the largest vein in the neck. It descends vertically here and it emerges at the jugular formen which is a continuation of the sigmoid venous sinus inside the skull. It terminates at the root of the neck by uniting with the subclavian vein (which drains the upper limp) to form the brachiocephalic vein. Brachio-drains the upper arm cephalic-draining the head.

      2. The brachiocephalic veins unite to form the vena cava.

      3. The IJV receives numerous tributaries as it descends. These include not only the inferior petrosal sinus and the sigmoid sinus which drains the brain, but also the facial vein, lingual vein drain into the IJV, and superior and middle thyroid veins, pharyngeal vein (drain the pharynx) and sometimes the occipital vein.

    3. Carotid Sheath [S42]

      1. Important: remember the contents of the carotid sheath and the relationship. The IJV is the most lateral of the contents of the carotid sheath. Medial to it is the CCA or ICA depending on what vertebral level you are at. Posterior is the vagus nerve. These are all surrounded by the carotid sheath. Note relationships of the three contents within the carotid sheath, and that the carotid sheath lies posterior to the lobe of the thyroid gland.

    4. Puncture of IJV [S43]

      1. We have mentioned puncturing the subclavian vein and catheterizing it, you can also do a similar procedure to the IJV.

      2. The CCA can be palpated and inject the vein which is lateral to it because vein is lateral to IJV.

      3. Right IJV is preferred because it is larger and in line with the right brachiocephalic vein and the vena cava. This is what you want to do put a catheter to the heart, this more a direct line and less angle compared to the left IJV.

      4. IJV directly lateral to the CCA.

    5. Lymph Vessels [S44]

      1. A chain of lymph nodes along the IJV. You can see these while we dissect the carotid sheath. Collectively called the deep cervical lymph nodes. They can be divided into superior and inferior groups by the presence of the omohyoid.

      2. Those nodes above the omohyoid muscle are superior deep cervical nodes; those below are inferior deep cervical nodes.

      3. The jugulodigastric and juguloomohyoid nodes are named because of the muscles cross where they are located.

      4. All lymph is from the head and neck is drained in the deep cervical chain indirectly or directly. It might go first to the submandibular first, but eventually to the deep cervical chain.

      5. Efferent vessels from the deep cervical nodes will form the jugular lymph trunk on either side will terminate on the left side thoracic duct or on the right side lymphatic duct, and then into the venous system.

    6. Branches of Vagus Nerve [S45]

      1. The Vagus Nerve is in the anterior triangle.

      2. Branches of the vagus nerves are in the anterior triangle and carotid sheath.

      3. The internal laryngeal nerve pierces the thyrohyoid membrane to get into the larynx. Between the upper border of the thyroid cartilage and hyoid bone, there is this fibrous tissue. It pierces the membrane to innervate the upper part of the larynx. This is a sensory nerve, its GVA, it innervates the mucosa of the upper part above the vocal folds. If you have an irritation in the upper part of the larynx, like a foreign object in the airway, it automatically sets up the cough reflex and start coughing. The sensory information telling your brain there is something in the larynx, travels back to the brain via the internal laryngeal nerve which is GVA.

      4. The other branch is the external laryngeal nerve. This is a smaller nerve and it descends here in close company with the superior thyroid artery and innervates the small cricothyroid muscle. Its SVE and comes from the pharyngeal arch. It tenses the vocal folds and raises the pitch of the voice.

      5. Internal and external nerves, one motor and one sensory, are branches of the superior laryngeal nerve, which comes of the vagus nerve. Might have trouble finding this superior laryngeal nerve, easier in the posterior aspect. Appreciate the internal and external branches.

      6. Recurrent laryngeal nerve, on the right side, branches off the vagus and loops around the subclavian artery. On the left side it loops around the aortic arch. But, both sides, the nerve ascends between the trachea and esophagus. It enters the larynx from below.

        1. It is a mixed nerve, it has GVA, which sensory to the mucosa of the larynx below the vocal folds and SVE, which is motor to all the muscles of the larynx except the cricothyroid.

    7. Recurrent Laryngeal Arteries [S46]

      1. Anterior view showing you the different relationship between the recurrent laryngeal nerves.

      2. The right nerve loops around the right subclavian artery and descends into the neck between the trachea and esophagus.

      3. The left nerve (can’t see it branching of the Vagus nerve because that happens in the chest) loops around the aortic arch and runs up and enters the neck and ascends in the same position as in the right (tracheal esophageal groove). If you can remember the relationship, then it is easy to find.

    8. Posterior Belly of Digastric [S47]

      1. Important relationships of the posterior belly of digastric, it is a key muscle.

      2. Numerous structures pass deep to the posterior belly of digastric, in lab cut the tendon and reflect the posterior belly of the digastric muscle to get a better view of the structures. Deep to this muscle:

        1. Both ICA & ECA (because part of the carotid triangle)

        2. Lingual and facial artery as they pass upward and pass deep to the digastric

        3. IJV descending in the carotid sheath

        4. Last 3 cranial nerves CN X, XI (crossing the IJV on the way to SCM and trapezius)

        5. Note that XII cross deep to posterior belly two times , enters the carotid triangle, and leaves it and enters the submandibular triangle by passing deep to the digastrics.

        6. Sympathetic trunk behind the carotid sheath, its deep to the post belly of the digastric muscle.

    9. Tip of Great Horn of hyoid bone [S48]

      1. The tip of the greater horn of the hyoid (can feel it) is another important landmark for many structures:

        1. The thyrohyoid membrane runs to the hyoid just above the tip is the hypoglossal nerve.

        2. Just superior to of the greater horn is CN XII, which crosses the lingual artery. If the lingual artery has to be ligated here, it may damage XII. This is important to know.

        3. The twig that goes to the thyrohyoid muscle comes off superior to the tip. These are C1 fibers part of the ANSA

        4. Just below the tip of greater horn is the internal laryngeal nerve, piercing the thyrohyoid membrane and accompanied by superior laryngeal artery.

        5. The external laryngeal nerve runs with the superior thyroid artery.

        6. Keep these landmarks in the mind!

    10. Cervical Parts of Sympathetic Trunk [S49]

      1. Review this slide on your own and the notes that go with it. Dr. Salter talked about the cervical part of the sympathetic trunk. The gray rami communicates, internal carotid and external carotid nerves that travel along with their arteries. Review it on your own.

  1. Horner’s Syndrome [S50]

    1. What happens if the cervical part of the sympathetic trunk is interrupted? Or if its damaged and you cut off the sympathetic supply to the head? It could be damage due to trauma or a tumor.

    2. You are interrupting the presynaptic fibers that are ascending to the superior cervical ganglion. De-innervate the head of the sympathetic nerve supply, leading to Horner’s syndrome.

    3. These symptoms include:

      1. Miosis- pupillary constriction because the dilator is sympathetically innervated. So the dilator is paralyzed.

      2. Ptosis -drooping upper eyelid because superior tarsal muscle, that is sympathetically innervated, is paralyzed.

      3. Anhidrosis -no sweating, sweat glands are de-innervated.

      4. Vasodilatation – redness. Sympathetic fibers constrict peripheral blood vessels. The skin becomes red because of the blood vessels dilate. It is also warm to the touch.

      5. These signs occur on the same side as the lesion.

End [44:38]


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