< Muscles >
Fig. 3-1. Insertion of facial muscle at the skin.
Typical skeletal muscles end at the bone. However, facial muscles are exceptional because they end at the skin. Hence, the facial muscles enable us to make the facial expressions.
Fig. 3-2. Platysma.
The platysma is also a facial muscle, even though the muscle is mostly located in the neck. The muscle lowers the angle of mouth and the lower lip (Fig. 4-1). The platysma is very thin and situated even more superficially than the cutaneous veins (external jugular veins) (Fig. 3-29).
The more you use your platysma for sad facial expression (Fig. 4-1), the earlier you get the vertical neck wrinkles. These wrinkles are the anterior borders of platysma.
Fig. 3-3. Recommended thin skinning to identify subcutaneous papilla.
During the skinning process of the neck, it is recommended that students look for yellow subcutaneous papillae amid white dermis. This signifies that the skinning is done thinly enough. Remove the skin a little more, so as to find the platysma.
Fig. 3-4. Sternocleidomastoid muscle.
The sternocleidomastoid muscle is named after its two origins (the sternum and clavicle) and one insertion (the mastoid process). The other insertion, superior nuchal line is ignored in the nomenclature.
If the right sternocleidomastoid muscle contracts, the head is rotated to the left. This happens because the obliquely situated right sternocleidomastoid muscle becomes more vertical and shortens in length.
Koreans and Americans develop their right sternocleidomastoid muscle more than their left one. Since drivers occupy the right side of the road in the two countries, people who are waiting for buses or crossing the street need to look on their left more often. Do not be serious about this gossip.
The right sternocleidomastoid muscle also allows for the right (lateral) flexion of the head. When both the right and left muscles contract simultaneously, the head flexes in the forward direction. You can palpate the muscle contracting and thus thickening during these movements.
Fig. 3-5. Triangles of neck.
The sternocleidomastoid muscle is the most prominent muscle in the neck. It divides the neck region into anterior and posterior cervical triangles (Fig. 3-24).
The digastric muscle (Fig. 3-6), omohyoid muscle (Fig. 3-9), and their insertion (hyoid bone) are the boundaries that subdivide the posterior and anterior cervical triangles. These several triangles annoy the medical students who need to memorize them. But the triangles please medical doctors who need to describe the locations of neck lesions with more precision.
Do not confuse the occipital triangle with the suboccipital triangle (Fig 1-15).
Fig. 3-6. Suprahyoid muscles in lateral view (top) and sagittal plane (bottom).
In the figure above, the genioglossus (innervated by XII) does not belong to the category of suprahyoid muscles, but to that of tongue muscles (Fig. 4-32). The four suprahyoid muscles are the stylohyoid, digastric, mylohyoid, and geniohyoid muscles. Their common function is the elevation of the hyoid bone.
The digastric muscle has anterior and posterior bellies, as the muscle name (two bellies) implies.
Fig. 3-7. Mylohyoid muscle in lateral view and coronal plane.
The mylohyoid muscle has a free posterior border that is surrounded by the submandibular gland (Fig. 3-33). The anterior borders of the bilateral mylohyoid muscles meet each other to form a raphe. In daily life, a raphe means a sewing line of cloths; in anatomy, a raphe represents a conjoining line of muscle bellies.
The bilateral mylohyoid muscles form the floor of the oral cavity. If one touches the mylohyoid muscles below the mandible and swallow, one can feel the muscles contracting to elevate the hyoid bone (and larynx) and to narrow the oral cavity.
Fig. 3-8. 1st, 2nd, 3rd, 4th pharyngeal arches, related structures.
Gill-shaped pharyngeal arches are formed during the developmental stage of the head and neck. As their names imply, the inside of the pharyngeal arches is the pharynx. The oropharyngeal membrane (Fig. 6-16) ruptures to become the fauces between the oral cavity and pharynx (Figs. 3-43,44). The 1st, 2nd, 3rd, 4th pharyngeal arches are innervated by V, VII, IX, X, respectively.
Concerning the suprahyoid muscles, the anterior belly of digastric muscle and mylohyoid muscle (Fig. 3-5), that originate from the 1st pharyngeal arch, are governed by the mylohyoid nerve, a branch of V3 (Figs. 3-7, 4-25).
The stylohyoid muscle and the posterior belly of the digastric muscle, from the 2nd pharyngeal arch, are controlled by VII (Fig. 4-27). This innervation can be also explained by the fact that VII emerges from the stylomastoid foramen between the styloid and mastoid processes (Fig. 4-4) where the two muscles are attached (Fig. 3-6).
The CN1 to geniohyoid muscle will be explained later (Fig. 3-13).
Fig. 3-9. Infrahyoid muscles (dotted arrows: deep muscles).
Among the infrahyoid muscles, the sternothyroid and thyrohyoid muscles lie deep to the omohyoid and sternohyoid muscles. This is because short muscles are covered by long muscles. There are four infrahyoid muscles on one side, just as there are four suprahyoid muscles (Fig. 3-6).
Like the digastric muscle (Fig. 3-6), the omohyoid muscle has two bellies: superior and inferior bellies. The digastric and omohyoid muscles are the borders of cervical triangles (Fig. 3-5)
The infrahyoid muscles occupy the muscular triangle (Fig. 3-5). Their action is to depress the hyoid bone.
With the exception of the digastric muscle, all suprahyoid and infrahyoid muscles are given individual names comprising the origins and the insertions (usually, hyoid bone). “Mylo” in the mylohyoid muscle means molar (Fig. 4-30), “genio” in the geniohyoid muscle means mental spine (Fig. 3-6), and “omo” in the omohyoid muscle means scapula.
The sternothyroid muscle’s nickname is the sword-shield muscle.
The innervation of the infrahyoid muscles will be described later (Figs. 3-13,14).
Fig. 3-10. Middle and posterior scalene muscles.
Three scalene muscles elevate R1 and R2 during forceful inhalation (Fig. 5-2). The middle and posterior scalene muscles can be distinguished by their insertions. The middle scalene muscle inserts at R1, while the posterior one at R2.
Fig. 3-11. Anterior scalene muscle, adjacent structures.
The anterior scalene muscle inserts at the scalene tubercle on R1. Anterior and posterior to the scalene tubercle, there exist grooves for the subclavian vein and artery, respectively. The artery with higher blood pressure is hidden behind the anterior scalene muscle for protection. The subclavian artery is subdivided on the basis of the anterior scalene muscle (Fig. 3-25).
The anterior scalene muscle can be distinguished from the middle scalene muscle rather easily. Both the subclavian artery (Figs. 2-16, 3-25) and the brachial plexus (Fig. 2-13) proceed between the anterior and middle scalene muscles. The subclavian artery and the brachial plexus pass the apex of axilla, which is situated lateral to R1 (Fig. 2-19).
Thus, it is natural that the inferior trunk of brachial plexus is in contact with R1 (Fig. 2-13). The three scalene muscles are innervated by nameless branches of the cervical plexus (Figs. 3-14,15,16) and brachial plexus.
Fig. 3-12. Deep cervical muscles.
Along with the superficial sternocleidomastoid muscle (Fig. 3-4), the deep cervical muscles also flex the head anteriorly and laterally. Longus colli and capitis are named with regards to their insertions. “Colli” is synonymous to the term “cervicis,” as they both indicate the cervical vertebrae (Fig. 1-13). The deepest cervical muscles are the rectus capitis anterior and rectus capitis lateralis that correspond to the rectus capitis posterior (major, minor) adjacent to the suboccipital triangle (Fig. 1-15). All deep cervical muscles are innervated by nameless branches of the cervical and brachial plexuses (Figs. 2-14, 3-14,15,16).
The atlantooccipital joint is an ellipsoid joint used for nodding. In order to nod, the occipital condyle and atlas (CV1), equipped with the anteroposteriorly long articular surfaces, slide along each other. Meanwhile, the atlantoaxial joint is a pivot joint used for shaking the head. The upper one is a “yes” joint, while the lower one is a “no” joint. Positive mind should be on the top of negative mind.
< Nerves >
Fig. 3-13. Anterior ramus of CN1 running alongside XII.
The first spinal nerve (CN1) is closely related to the last cranial nerve (XII). Neighbors are alike. Just as XII controls the tongue muscles (Figs. 3-34, 4-32) above the hyoid bone, the anterior ramus of CN1 innervates the geniohyoid muscle (Fig. 3-6) and the thyrohyoid muscle (Fig. 3-9) above and below the hyoid bone.
XII, which is thicker, and the anterior ramus of CN1 are enclosed by a common epineurium (connective tissue). Therefore, during dissection, CN1 that innervates the geniohyoid and thyrohyoid muscles may be mistaken as a branch of XII.
In contrast, the posterior ramus of CN1 (suboccipital nerve) controls the suboccipital triangle muscles (Fig. 1-15).
Fig. 3-14. Ansa cervicalis.
Moreover, CN1 participates in the cervical plexus which is constituted by the anterior rami of CN1–CN5. Just as is the case of the brachial plexus (Fig. 2-14), CN1–CN5 cooperate with each other to innervate muscles and skin.
CN1–CN3 form the ansa cervicalis which loops around the internal jugular vein. The Latin word “ansa” indicates a loop. In the case of the brachial plexus, trunks make loops by anterior and posterior divisions (Fig. 2-13). Branches from the ansa cervicalis supply mostly infrahyoid muscles (Fig. 3-9) with the exception of the thyrohyoid muscle, which is directly innervated by CN1 (Fig. 3-13).
Fig. 3-15. Cutaneous nerves of neck.
The somatic sensory nerves of CN2–CN4 become cutaneous nerves of the neck. They exit via a point on the posterior border of the sternocleidomastoid muscle. Therefore, to prepare for a superficial neck surgery, anesthetic drug is injected along the muscle’s posterior border.
Fig. 3-16. Phrenic nerve.
CN3–CN5 form the phrenic nerve and reach down to the diaphragm (Figs. 5-40,48). During embryonic development, the septum transversum, which later forms a portion of the diaphragm, is located cranial to the heart. As head fold takes place in later developmental stages, the septum transversum descends to be located caudal to the heart (Fig. 6-16). As it descends, it drags along the phrenic nerve with itself. As mentioned above, the nerve follows until the end.
Tips on how to memorize the cervical plexus are introduced in the comic strip.
The CN5 contributes to both the cervical and brachial plexuses (Figs. 2-13, 3-16). Such is common for spinal nerves located on the borderline.
Let’s review somatic nerves prior to examining autonomic nerves. Somatic motor nerves deliver signals to the skeletal muscles, while somatic sensory nerves deliver signals from receptors located near skeletal muscles (e.g., skin) (Figs. 2-10,12).
Fig. 3-17. Visceral motor nerves (autonomic nerve), visceral sensory nerve.
The counterparts to these nerves are visceral motor nerves (autonomic nerve) which deliver signals to smooth and cardiac muscles and the visceral sensory nerves which deliver signals from their neighbor. An example of the function of the visceral sensory nerve would be to deliver hunger signals from the gastrointestinal tract.
The visceral motor and sensory nerves are not discernible during dissection. Neither are the somatic motor and sensory nerves (Fig. 2-12).
Unlike the somatic motor nerve which consists of a single neuron (Fig. 2-10), the visceral motor nerve to smooth and cardiac muscles is composed of two neurons: preganglionic and postganglionic fibers. Note that the terms “neuron” and “fiber” are used interchangeably for the reason that a neuron’s axon is long like a fiber.
Among the visceral motor nerves, the sympathetic nerve has a longer postganglionic fiber, while the parasympathetic nerve has a shorter postganglionic fiber.
The Sympathetic nerve is for Stimulated state of the body, while the Parasympathetic nerve is for Peaceful state.
The sympathetic nerve is contained in TN1–LN2. (T1LTwo reminds us of a TILTed building in war state.) The parasympathetic nerve is contained in III, VII, IX, X, and SN2–SN4. (Do not confuse III, VII, IX, X of the parasympathetic nerve with V, VII, IX, X of the pharyngeal arches (Fig. 3-8).) Among the autonomic nerve, only the sympathetic nerve in TN1–LN2 will be described as a representative.
Fig. 3-18. Sympathetic nerves in the spinal nerves.
Let us examine TN1–LN2 from the spinal cord. A preganglionic fiber passes the anterior root and trunk of spinal nerve just as the somatic motor nerve (Fig. 2-12). This fiber soon travels through the white ramus communicans to reach a paravertebral ganglion. There the preganglionic fiber has three routes (A, B, and C in the figure above).
In the figure, the preganglionic fibers (solid lines) and the postganglionic fibers (dotted lines) are similar in lengths. However, one must keep in mind that postganglionic fibers are longer than preganglionic fibers (Fig. 3-17).
One exception in which the pre- and post- ganglionic fibers are similar in length is in the case where the synapse takes place in the prevertebral ganglion (Route B) (Fig. 6-49).
The three routes are explained as follows: (Route A: from TN1–TN4 to the thoracic cavity) The preganglionic fiber synapses at the paravertebral ganglion. The postganglionic fiber then proceeds in the splanchnic nerve (Fig. 3-19) to innervate the deep smooth and cardiac muscles (lung, heart...) in the thoracic cavity (Fig. 5-43).
Some postganglionic fiber passes the gray ramus communicans to rejoin the spinal nerve. The fiber, accompanying the somatic motor nerve (Fig. 2-12), travels to the thoracic and abdominal walls and innervates the superficial smooth muscles in the sweat glands, hair, and blood vessels.
(Route B: from TN5–LN2 to abdominal cavity.) Without synapsing at the paravertebral ganglion, the preganglionic fiber runs along the splanchnic nerve and synapses at the prevertebral ganglion. The splanchnic nerve from TN5–LN2 serves as the link between the paravertebral ganglia (Fig. 5-43) and prevertebral ganglia (Fig. 6-49). Postganglionic fiber innervates the deep smooth muscles in the abdominal cavity.
One must keep in mind that unlike in the schematic Fig. 3-18, the prevertebral ganglion is located literally in front of the vertebra. An example would be the celiac ganglion (Fig. 6-49).
(Route C: from the TN1–LN2 to the head, neck, pelvis, perineum, and limbs) Without synapsing at the paravertebral ganglion, the preganglionic fiber takes an elevator called the sympathetic trunk and synapses at the paravertebral ganglion of upper level (cervical ganglion) or lower level (lumbar or sacral ganglion). During dissection, one can find that the sympathetic trunk connects the serial paravertebral ganglia vertically (Figs. 3-19, 5-43, 7-18). The sympathetic trunk consists of preganglionic fibers, as one may have assumed.
Let us pay attention to CN8. The elevated preganglionic fiber synapses at the inferior cervical ganglion. The inferior cervical ganglion is a type of paravertebral ganglion (Fig. 3-19). The postganglionic fiber may proceed along arteries and innervate various smooth muscles in the head and neck region. A small portion of the postganglionic fiber passes through gray ramus communicans and joins CN8, then they together innervate smooth muscles in the upper limb. Recall that CN8 is a part of the brachial plexus (Fig. 2-14).
Fig. 3-19. Cervical ganglia, rami communicantes.
The superior cervical ganglion is the result of the fused 1st–4th cervical ganglia; middle cervical ganglion, of 5th–6th; and inferior cervical ganglion, of 7th–8th. Based on the number of ganglia that are fused, the superior cervical ganglion is the largest and thus responsible for the most number of functions.
Frequently, the inferior cervical ganglion merges with the 1st thoracic ganglion (paravertebral ganglion of TN1) to form the cervicothoracic ganglion (also known as the stellate ganglion) (Fig. 5-43).
It is recommended that students compare Fig. 3-18 and Fig. 3-19 to confirm their understanding. For instance, TN1 has both white and gray rami communicantes, but CN8 only has gray ramus communicans.
The cervical ganglia have been concretely described in this neck chapter. The lower paravertebral ganglia (lumbar, sacral ganglia) and their splanchnic nerves will be explained in other chapters (Figs. 6-49, 7-17,18).
The colors of white and gray rami communicantes (Figs. 3-18,19) are best explained by their histological differences. Usually, an axon is enclosed by the myelin sheaths composed of white fat; however, postganglionic fibers are an exception in that they are not covered by myelin sheaths. Otherwise put, a ramus communicans containing the postganglionic fibers is gray due to the absence of the myelin sheath. However, the color difference is not grossly perceived in cadavers.
Myelin sheath boosts the speed of neural conduction. Preganglionic fibers, which are enclosed in myelin sheaths, convey the impulse quickly. On the other hand, postganglionic fibers, which lacks the myelin sheath, conveys the impulse more slowly (Figs. 3-17,18).
Theoretically, the sympathetic nerve with the long postganglionic fiber should be slower than the parasympathetic nerve in conducting signals (Fig. 3-17). However, such difference does not matter for normal physiology.
Fig. 3-20. IX, X, XI passing through jugular foramen.
Among the cranial nerves, IX, X, XI are closely related with the neck. One easy way to remember this is the Roman numerals. A set of three nerves can be called triple X. The triple X exits the cranial cavity through the jugular foramen. Recall that the internal jugular vein is the biggest structure passing through jugular foramen (Fig. 4-10).
Notice that both IX and X have the superior and inferior ganglia individually. The four sensory ganglia (of pseudounipolar neurons) (Figs. 2-10,11) are found around the jugular foramen, which reminds us of the spinal ganglia around the intervertebral foramen (Fig. 1-19).
Fig. 3-21. IX.
Regarding the superior and inferior ganglia, IX receives sensory information from the “tongue” (posterior one third) and “pharynx.” Therefore, the full name of IX is the “glossopharyngeal” nerve.
If you still have trouble memorizing the glossopharyngeal nerve, the infantile mnemonics in the comics above will help you.
In terms of motor innervation, IX innervates the stylopharyngeus (Fig. 3-45) which originates from the 3rd pharyngeal arch (Fig. 3-8).
The parasympathetic nerve in IX (lesser petrosal nerve) descends through the foramen ovale and immediately synapses in the otic ganglion. (Recall that the visceral motor nerve consists of two neurons (Fig. 3-17).) The postganglionic fiber from the otic ganglion travels with a branch of V3 (Fig. 4-25) to reach the parotid gland.
It does not make sense that the lesser petrosal nerve sequentially passes the jugular foramen and the foramen ovale in Fig. 3-21. This contradiction can be solved by a more realistic portrayal below.
Fig. 3-22. Course of lesser petrosal nerve of IX.
The lesser “petrosal” nerve passes the jugular foramen (between occipital and temporal bones) only the halfway, then proceeds through the “petrous” part of temporal bone. Finally, the nerve goes back to the cranial cavity and exits through the foramen ovale of the sphenoid bone. The lesser petrosal nerve is like a French student “Les Peter” who drops out of the Jugular University, then enters the University of Ovale to graduate.
Fig. 3-23. X.
This realistic drawing of complex branches of X should be compared to the neuron drawing of X (Fig. 3-20). The superior and inferior ganglia of X are for sensory nerves; the Superior ganglion is Somatic (Fig. 2-10), while the Inferior ganglion is visceral (for Internal organs) (Fig. 3-17). For memorization, imagine that the visceral sensory nerve’s huge jurisdiction (cardiac and smooth muscles in thoracic and abdominal cavities) pulls the ganglion down to make it inferior (Figs. 5-44, 6-49).
The 1st branch of X innervates the muscles in the palate (Fig. 4-35) and the pharynx (Fig. 3-45). The 2nd branch is the superior laryngeal nerve that bifurcates into the internal laryngeal nerve that senses the superior part of larynx (Fig. 3-40) and the external laryngeal nerve that innervates the cricothyroid muscle (Figs. 3-41,42).
The 3rd branch forms the cardiac plexus to slow the heart beat down. (The cardiac plexus actually resides beside the heart (Fig. 5-27), but is drawn in the neck for simplification of figure.) The branching of this third branch is due to the relatively high location of the early heart (Fig. 6-16).
The parasympathetic nerve from X and sympathetic nerve from TN1–TN4 intermingle around and collectively innervate the thoracic organs. The heart, lung, and esophagus are innervated, and these mixed nerves are known as the cardiac, pulmonary, and esophageal plexuses (Figs. 5-27,43,44).
The 4th branch, the recurrent laryngeal nerves, hook around the subclavian artery (on the right side) and the aortic arch (on the left side) (Fig. 5-13). They become inferior laryngeal nerves, which sense the inferior part of larynx (Fig. 3-40) and control most of the intrinsic muscles of larynx (Fig. 3-41).
At an early stage of the development, recurrent laryngeal nerves took the short routes behind the arteries. In development, the nerves’ target (larynx) migrates superiorly while the arteries’ origin (heart) migrates inferiorly. So, the nerves’ routes finally have become detours; giraffes’ recurrent laryngeal nerves are longer than 4 meters.
The recurrent laryngeal nerve is a nerve whose name changes without dividing. Moreover, there is no distinct border between the recurrent and inferior laryngeal nerves.
To sum skeletal muscles up, X innervates the palate, pharynx, and larynx all of which have originated from the 4th pharyngeal arch (Fig. 3-8).
The 5th branch of X builds the pulmonary plexus to contract circular smooth muscles in the bronchi. Why? During rest (peace state), bronchi should be narrowed in order to force the inspired air to alveoli.
The last, the 6th branch of X forms the esophageal plexus which will be further discussed in the chapter on thorax (Fig. 5-44). X innervates a great amount of cardiac and smooth muscles throughout the thorax and abdomen (Fig. 6-49). This is why it is named the vagus nerve, which means wandering nerve like a vagabond.
Let us have a closer look at Fig. 3-20. The cranial root of XI follows a rather complicated route. Inside the jugular foramen, it is a part of XI; however, outside the jugular foramen, it is a part of X and supplies muscles in the palate, pharynx, and larynx (Fig. 3-23). We usually regard the cranial root of XI as a part of X, taking into account its function.
On the other hand, the genuine XI is the spinal root of XI that originates from the levels of CN1–CN5 level (Figs. 1-18,19). The spinal root enters the cranial cavity through the foramen magnum, and then exits the cranial cavity through the jugular foramen (Fig. 3-20).
Fig. 3-24. Spinal root of XI in posterior cervical triangle.
The posterior cervical triangle (Fig. 3-6) is filled with five muscles. The splenius capitis shares its superior insertion (superior nuchal line (Fig. 1-12)) with the sternocleidomastoid muscle (Fig. 3-4). However, the insertions of the other four muscles are inferior to their origins (Figs. 1-9, 3-10,11).
After the spinal root of XI exits the cranial cavity through the jugular foramen (Fig. 3-20), it is visible on the levator scapulae. XI innervates the sternocleidomastoid muscle and the trapezius for their motor function.
Additionally, CN2–CN4 join XI; CN2–CN3 account for the sensory impulse of the sternocleidomastoid muscle, while CN3–CN4 account for the sensory impulse of the trapezius.
< Arteries, veins,
lymph nodes >
From the aortic arch, the bilateral subclavian arteries and bilateral common carotid arteries branch off; those on the left are directly from the aortic arch, while those on the right are indirectly derived through the brachiocephalic trunk (Fig. 3-23). The subclavian and common carotid arteries supply not only the neck, but also other regions.
Fig. 3-25. Parts of subclavian artery.
The Subclavian Artery (SA) has three parts according to the anterior scalene muscle (Fig. 3-11). This muscle is Scalenus Anterior (SA) in Latin terminology. One can think of this as: SA is partitioned by SA.
Fig. 3-26. Branches of the subclavian artery.
Superior branches of the subclavian artery is shaped like “+1” when drawn out. The “1” is the vertebral artery passing through the transverse foramina of CV6–CV1. The artery is then found in the suboccipital triangle (Fig. 1-15) just before entering the foramen magnum. The “+” is the thyrocervical trunk dividing into the inferior thyroid artery, suprascapular artery (for supraspinatus and infraspinatus) (Fig. 2-2), and transverse cervical artery (for trapezius) (Fig. 1-3).
Inferior branches form the signal “+1” as well. The “1” is the internal thoracic artery (for anterior thoracic wall) (Figs. 5-4,7). The “+” is the costocervical trunk dividing into the deep cervical artery (for deep back muscle such as the semispinalis cervicis and capitis (Fig. 1-13)), the 1st and 2nd posterior intercostal arteries (Fig. 5-6).
Among the complicated branches of the subclavian artery, the two “+” can be kept in mind by saying “arTIST CD 1, 2.” The “arTIST” is the Thyrocervical trunk, Inferior thyroid artery, Suprascapular artery, Transverse cervical artery. The “CD 1, 2” is Costocervical trunk, Deep cervical artery, 1st and 2nd posterior intercostal arteries.
We do not like the terms, thyrocervical and costocervical “trunks” because they are neither as thick as the brachiocephalic “trunk” (Fig. 3-23) nor as complicated as the celiac “trunk” (Fig. 6-42). These four trunks are all of the arterial trunks in the body.
Like other arteries, the branches of subclavian artery have frequent variations. In identifying a variable artery, its origin matters less than its destination does.
Let us now study the carotid arteries. The internal carotid artery enters the cranial cavity through the carotid canal and the cavernous sinus (Fig. 4-11). Both the internal carotid artery and the vertebral artery (Figs. 1-15, 3-26) become the cerebral artery (Fig. 4-7) and the cerebellar artery. They supply the brain with 20% of the entire blood from the heart.
Fig. 3-27. Carotid sinus and carotid body.
Doctors frequently palpate the carotid arteries’ pulsation anterior to the sternocleidomastoid muscle to check the heartbeat. To be specific, the internal and external carotid arteries are located in the carotid triangle (Fig. 3-5).
Considering the round shape of the neck, the internal carotid artery is not only posterior but also lateral to the external carotid artery. The internal carotid artery ascends in the medial direction to approach the carotid canal.
The swollen junction between common and internal carotid arteries is the carotid sinus. Low blood pressure is perceived by the baroreceptor located inside the carotid sinus. Such is crucial because low blood supply to the brain can be deadly. To prevent this, signals from the baroreceptor are received by the IX (visceral sensory nerve) (Figs. 3-20,21) to make the heart beat faster via the sympathetic nerve (visceral motor nerve) (Figs. 5-27,43).
When a doctor wants to slow down the patient’s heartbeat in sympathetic state, he/she may press down above the carotid sinus to induce high blood pressure on the baroreceptor.
The carotid body beside the carotid sinus can only be identified by careful dissection. The chemoreceptor in the carotid body senses low oxygen level in blood and sends signals to the heart to pump faster through the identical sensory and motor nerves.
The common carotid artery has no branches; moreover, the internal carotid artery also has no branches before entering the carotid canal.
Fig. 3-28. Branches of external carotid artery.
There are, however, Eight branches of the External carotid artery that supply the neck and head outside the cranial cavity. Their initials are SAL-FO-PMS. Memorize it with “Some Anatomists Like Freaking Out Poor Medical Students.”
The terminal branch, superficial temporal artery, can be palpated on the temporal muscle (Figs. 4-12,15).
Only the anterior branches (lingual, facial, and maxillary arteries) will be concisely explained here and in the following head chapter (Figs. 4-19,20).
The facial artery proceeds inside the mandible running alongside the lingual artery, and exits the mandible to approach the medial angle of the eye. Once the facial artery reaches the medial angle of the eye, it is know as the angular artery.
Fig. 3-29. External jugular vein, its tributaries.
A representative cutaneous vein in the neck is the external jugular vein, which is formed by the unification of the posterior auricular vein and the retromandibular vein. The posterior auricular vein and the posterior auricular artery (Fig. 3-28) physically run alongside one another. The retromandibular vein is present lateral to the main trunk of external carotid artery (Figs. 3-28, 4-4).
The anterior jugular vein is a tributary of the external jugular vein. The external jugular vein is a tributary of the subclavian vein (Fig. 3-30).
Small arteries spreading from the big arteries are branches, while small veins flowing into big veins are tributaries.
Fig. 3-30. Tributaries of the superior vena cava.
On each side, the subclavian vein (Figs. 3-11,29) meets the internal jugular vein (Fig. 4-10) to form the brachiocephalic vein. The bilateral brachiocephalic veins join to become the superior vena cava. The superior vena cava is shifted to the right as will be clear during dissection (Fig. 5-13).
The external and internal jugular veins have different patterns from the external and internal carotid arteries (Fig. 3-27). If they had same pattern, the different terms (jugular, carotid) with the same meaning (neck) would not be used.
The junction of subclavian and internal jugular veins is the site into which right lymphatic duct and thoracic duct drain. The right lymphatic duct, which is very short, is formed by the jugular lymphatic trunk (from the head and the neck) (Fig. 3-31), the subclavian lymphatic trunk (from the upper limb) (Fig. 2-21), and the bronchomediastinal lymphatic trunk (from the thorax) (Fig. 5-38). On the left side, the three lymphatic trunks join the thoracic duct (Fig. 6-47).
Fig. 3-31. Lymph drainage of neck.
Lymph nodes in the neck are divided into superficial cervical nodes around external jugular vein and deep cervical nodes around internal jugular vein. The superficial ones send lymph to the deep ones. All lymph passes the jugular lymphatic trunk (Fig. 6-47).
Lymph nodes in the head and neck are highly simplified, despite their clinical significance. It is because their subgroups and connections are too complex and indistinguishable when performing routine dissection.
Fig. 3-32. Carotid sheath.
Throughout the body, it is very common to see a set of vein, artery, and nerve. In the neck, the internal jugular vein, common carotid artery (Fig. 3-27), and X (Fig. 3-23) proceed in group enclosed in the carotid sheath. They also follow the rule of “superficial vein and deep nerve.”
The sympathetic trunk is located posterior to the carotid sheath, for the reason that its cervical ganglia are connected with the cervical nerves (Fig. 3-19) close to the intervertebral foramina (Fig. 1-19). This topographic anatomy will help students find the sympathetic trunk which is the structure located in the deepest part of the neck.
< Submandibular and thyroid glands >
Fig. 3-33. Submandibular gland.
The submandibular gland is located in the submandibular triangle (Fig. 3-6). The gland surrounds the free posterior border of the mylohyoid muscle. The superficial part of the submandibular gland is not only lateral but also inferior to the deep part because mylohyoid muscle is oblique (Fig. 3-7).
Fig. 3-34. Submandibular duct, adjacent structures.
After removing the superficial part of submandibular gland and the mylohyoid muscle, its deep part and submandibular duct can be viewed. They are located external to the hyoglossus (innervated by XII) (Fig. 4-32), and superior to XII. The submandibular duct opens into the sublingual caruncle (Fig. 4-33).
The lingual nerve, a branch of V3 (Fig. 4-25), hooks around the submandibular duct, then enters the tongue. At a glance, the lingual nerve may appear to enter the submandibular ganglion to innervate the submandibular gland.
Fig. 3-35. Lingual nerve, chorda tympani.
In fact, the submandibular ganglion is approached by the chorda tympani, a branch of VII (Fig. 4-27). From the submandibular ganglion, postganglionic fibers enter the submandibular and sublingual glands (Fig. 4-32). It is challenging to observe the connection between the ganglion and sublingual gland in cadavers.
In the figure above and Fig. 3-18, the two postganglionic fibers are shown to share a common nerve cell body. However, no such neuron exists in reality; every neuron has one nerve cell body and one axon (Fig. 2-10). Two neurons with a common nerve cell body is shown as a means of simplification. Anyhow, the postganglionic fibers of parasympathetic nerve are short (Fig. 3-17).
Another important function of the chorda tympani (VII) is to receive special sense (taste) from the anterior two thirds of tongue (Fig. 4-27). From the same territory of the tongue, the lingual nerve (V3) receives general sense like the pain felt when the tongue is bitten (Fig. 4-25).
Fig. 3-36. Thyroid gland.
The isthmus connects the bilateral lobes of the thyroid gland. The isthmus overlies the 2nd to 4th tracheal cartilages (Fig. 5-36), while the inferior margin of the gland is located at the level of the 6th tracheal cartilage. These numbers are identical to the levels of the breast and nipple (R2, R4, R6) (Fig. 2-7).
It is a prevalent misunderstanding that the thyroid gland is located anterior to the thyroid cartilage. The thyroid gland is anterior to the trachea and cricoid cartilage, which are situated lower than the thyroid cartilage (Fig. 3-38).
Fig. 3-37. Development of pharyngeal arches, related structures.
The pyramidal lobe of thyroid gland is located above the isthmus. During development, the thyroid gland (initially, a duct) descends from foramen cecum that is between anterior 2/3 (V, VII) and posterior 1/3 (IX) of the tongue (Fig. 4-31). In other words, the thyroid gland originates from between the 2nd pharyngeal arch (VII) and the 3rd pharyngeal arch (IX). The pyramidal lobe above the isthmus is the part that has not descended completely during development. Thus, it is not abnormal for the pyramidal lobe to be absent.
The parathyroid glands on the posterior surface of the thyroid gland may not be clearly seen (Fig. 3-36). Nonetheless, students should attempt to distinguish the parathyroid glands, as they play a crucial role as endocrine glands.
The superior and inferior parathyroid glands originate from the 4th and 3rd pharyngeal pouches, respectively. Another tissue of the 3rd pharyngeal pouch descends into the thoracic cavity and becomes the thymus (Fig. 5-41).
< Larynx >
Fig. 3-38. Cartilages of larynx.
The larynx is comprised of the epiglottic, thyroid, arytenoid, and cricoid cartilages, followed by the C-shaped tracheal cartilages (Fig. 5-36). Only the cricoid cartilage is circular (Figs. 3-39,41), and contributes primarily to maintaining airway. The hyoid bone also helps keep the airway open.
Fig. 3-39. Vestibular and vocal ligaments.
Vestibular ligament and vocal ligament can be thought of as strings between the thyroid and arytenoid cartilages. Since the arytenoid cartilages are bilateral (Figs. 3-38,41), the ligaments are bilateral also (Fig. 3-40).
Fig. 3-40. False vocal cord, vocal cord.
The vestibular ligament is the inferior thick border of the quadrangular membrane, whereas the vocal ligament is the superior thick border of the conus elasticus.
The term “conus elasticus” is similar to the terms “conus medullaris” (Fig. 1-17) and “conus arteriosus” (Fig. 5-17). Bilateral coni elastici constitute a part of an imaginary cone.
The vestibular ligament covered by mucosa is the false vocal cord, while the vocal ligament covered by mucosa is the vocal cord. The false vocal cord, less protruded than the vocal cord, does not play a part in phonation. As an exception, death metal vocalists employ the false vocal cord in making a monster-like growling voice.
The narrowness of the gap (rima glottidis) between two vocal cords can be beneficial. For example, food particles that are accidentally swallowed into the larynx (Fig. 3-43), are usually caught by the vocal cords and are coughed up. If the food particles move further down, it will damage the lungs, especially the right lung (Fig. 5-36). Thus vocal cords are known to protect the lungs.
Reversely, having a slender rima glottidis can be harmful. For instance, a candy might get wedged between the vocal cords and cause airway obstruction. In the case of fire, inhaled hot air causes swelling of the bilateral vocal cords, easily obstructing the rima glottidis. In such emergent situation, the trachea should be incised open (tracheotomy).
When at rest, the vocal cords are located apart from each other. This is why we do not make any sound when breathing. However, when the vocal cords stick together, and one breathes out, the exhaled air vibrates the vocal cords for phonation.
Functionally, the larynx is where we make sound, while the mouth is where we mold the sound to make words.
The arytenoid cartilage which holds the vocal ligament (Figs. 3-39,42) is the key to determine one’s voice.
Fig. 3-41. Cartilages, intrinsic muscles of larynx.
The intrinsic muscles of the larynx can be summarized by a schematic figure drawn above. However, such depiction may not reflect the reality; we advise students to utilize other resources such as the atlas to understand those structures more fully (Fig. 3-42).
In order to narrow the rima glottidis, the lateral cricoarytenoid muscle contracts, since arytenoid cartilage can rotate on the cricoid cartilage (Figs. 3-38,39). To narrow the rima glottidis even further, the arytenoid muscle contracts, since the arytenoid cartilage can slide on the cricoid cartilage. The joint between the two cartilages is both pivot and plane joints.
When we are not making any sound, the posterior cricoarytenoid muscle contracts to maintain the airway in the larynx. Thanks to the posterior cricoarytenoid muscle, we can breathe.
How about memorizing the functions of three muscles with the pneumonics introduced in the comics?
Fig. 3-42. Action of cricoarytenoid muscle.
Contraction of the cricothyroid muscle (insertion: thyroid cartilage) induces the vocal ligament to become tense, allowing for high pitch. To understand the axis for movement of the thyroid cartilage, refer to Fig. 3-38 and other book.
Reversely, contraction of the thyroarytenoid muscle (insertion: arytenoid cartilage) induces the vocal ligament to become loose, allowing for low pitch (Fig. 3-41). Within the vocal cord is the vocalis, which is a part of the thyroarytenoid muscle.
If one pushes down on one’s thyroid cartilage while saying ‘Ah-’, the voice tone will get lower.
A woman’s larynx is not protruded like a man’s. The short vocal ligament of woman, like short string of a violin, produces high pitch sound. On the other hand, a man’s long vocal ligament is like long string of a contrabass.
X is responsible for the innervation of the larynx (Fig. 3-23).
< Pharynx >
Fig. 3-43. Three parts of pharynx.
The pharynx consists of the nasopharynx (posterior to nasal cavity), the oropharynx (posterior to oral cavity), and the laryngopharynx (posterior to larynx).
Food in the oral cavity should be prevented from entering the nasal cavity. Therefore, when we swallow, the soft palate elevates, blocking the passage between the oropharynx and the nasopharynx (Fig. 4-35). The elevation of one’s own soft palate can be observed with a mirror. Try to swallow your saliva with your mouth open.
A common cold may cause inflammation of the nasopharynx. Then swallowing may become painful since the soft palate irritates the already inflamed nasopharynx when elevating.
The muscles for elevation of the soft palate will be elucidated in the soft palate section (Figs. 4-35,37).
Food must not enter the larynx either. As larynx moves upward, the epiglottis (involving epiglottic cartilage (Fig. 3-39)) blocks the laryngeal inlet (Fig. 3-44) between the laryngopharynx and the larynx. The elevation of your own larynx during swallowing can be easily seen and palpated over the skin.
The elevation of larynx is caused by the contraction of the suprahyoid muscles (Fig. 3-6) and the thyrohyoid muscle (Fig. 3-9). Moreover, palatopharyngeus, salpingopharyngeus, stylopharyngeus raise the larynx as well as the pharynx (Fig. 3-45).
Food thus passes through the oral cavity, the oropharynx, the laryngopharynx, and then the esophagus. Meanwhile, air flows through the nasal cavity, the nasopharynx, the oropharynx, the laryngopharynx, the larynx, and then the trachea. As you can see, the pharynx belongs to both digestive and respiratory systems.
The valley between tongue and epiglottis is called the epiglottic vallecula. The vallecula and calvaria (Fig. 4-5) seem to be plural, just like the cilia. But they are actually singular. Plural forms of them are valleculae and calvariae.
Fig. 3-44. Posterior view of opened pharynx.
Once the posterior wall of the pharynx is removed, the posterior view of choanae, nasal septum, and soft palate can be seen in the nasopharynx (Fig. 3-43). Torus tubarius surrounding the pharyngeal opening of auditory tube (Fig. 4-53) can also be seen. Inferior extension of the torus tubarius is the salpingopharyngeal fold containing salpingopharyngeus (Fig. 3-45).
In the laryngopharynx, we can see the epiglottis, laryngeal inlet (Fig. 3-43), aryepiglottic fold, and piriform fossa. The piriform fossa is the site where foreign material, like a piece of meat, gets stuck.
In the nasopharynx and oropharynx, the tonsillar ring composed of the pharyngeal, palatine, lingual, and other tonsils is located. The tonsils are discriminable by their crypts during gross dissection. The palatine tonsils can be viewed from the oral cavity as well (Fig. 4-34). Lingual tonsils are located at the posterior one third of tongue (Fig. 4-31).
Fig. 3-45. Muscles of pharynx.
The term “constrictor” is used only in the pharynx. This is why the term “constrictor” as well as “pharyngeal constrictor” are acceptable.
The origin of the superior constrictor is a raphe formed with the buccinator (Fig. 4-2). The origins of the middle and inferior constrictors are the hyoid bone and the larynx, respectively.
The superior constrictors is for constricting the oropharynx while the inferior constrictor is for constricting the laryngopharynx (Fig. 3-43); the middle constrictor intervenes. When swallowing, constrictors contract sequentially to let the food down the oropharynx and the laryngopharynx. Peristalsis in the pharynx is followed by that in esophagus (by the internal circular muscle).
When swallowing, the pharynx as well as larynx is elevated (Fig. 3-43) by the palatopharyngeus (Fig. 4-35), the salpingopharyngeus (Fig. 3-44), and the stylopharyngeus.
All muscles of the pharynx are innervated by X (Figs. 3-20,23) with an exception of styLOpharyngeus by gLOssopharyngeal nerve (IX) (Figs. 3-20,21).