< Facial muscles >
Facial muscles, which are found in the subcutaneous tissue, reach and pull the facial skin. However, in four legged animals, these kinds of muscles exist throughout the whole body.
Facial muscles of a cadaver should be preserved throughout the skinning process. To protect them, thin and careful skinning must be performed by keeping the subcutaneous papillae in mind (Fig. 3-3). When skinning face, students should invest enough time and care, as if they were plastic surgeons.
Fig. 4-1. Facial muscles (dotted arrows: deep muscles).
The frontalis makes horizontal wrinkles in the forehead. The corrugator supercilii makes vertical wrinkles between the eyebrows, while the procerus makes horizontal ones between the eyes. A facial muscle and the wrinkle formed by the muscle are in the right angle.
Another spelling bee time! Corrugator “superciLii” is right, while “superciLLi” is wrong. To remember this, look at the corresponding word “ciLia” having one L. In addition, the muscle corrugator “supercilII” ends with two Is, like the muscle biceps “brachII” (Fig. 2-25).
A surrounding muscle to narrow a passage is called a sphincter (Fig. 7-14) or a constrictor (Fig. 3-45). Another term is orbicularis, found around the eye and mouth.
The orbicularis oculi and orbicularis oris make wrinkles that make us look aged. On a positive note, we sometimes take advantage of the wrinkles. We contract the orbicularis oris and the mentalis when pretending to think.
There are numerous facial muscles to move the mouth. The levator labii superioris plays an opposite role from depressor labii inferioris; the levator anguli oris from depressor anguli oris. The zygomaticus minor and major are used for happy expressions and the platysma for sad expressions (Fig. 3-2).
A segmentalized part of “levator labii superioris” is “levator labii superioris alaeque nasi.” However, we require students to only remember the main muscle, “levator labii superioris.”
Major structures are generally placed inferior to minor structures. One hypothesis is that the major structures which are heavier have descended either due to the gravity or for the purpose of supporting the minor structures. The zygomaticus minor and major follow this rule.
When the zygomaticus major ends more laterally than the angle of mouth, a dimple is made. The dimple appears when smiling (contracting the zygomaticus major).
Fig. 4-2. Action of buccinator (solid arrow), divided into two (dotted arrows).
The buccinator which is located deep to the risorius (Fig. 4-1) originates from the deeply located raphe, which is also the origin of the superior constrictor (Fig. 3-45). Its insertion is superficial skin, so we can say that the muscle lies obliquely. The action of the buccinators can be explained by dividing its sum action into two vectors. One action is to abduct the angle of mouth like the risorius; the other is to pull the cheek toward oral cavity. The coordination between the latter action of buccinator and the tongue movement places food on the teeth for chewing.
< Parotid gland >
The parotid gland located near the ear: par (near) + otid (ear) gland. The parotid gland receives impulse from the otic ganglion (Figs. 3-21,22) which also etymologically originates from the ear.
Fig. 4-3. Branches of VII in parotid gland.
The “facial” muscles are the nomenclatural origin of the innervating VII, “facial” nerve. Inside the parotid gland, VII divides into five branches: Temporal, Zygomatic, Buccal, Marginal mandibular, and Cervical branches. For instance, the cervical branch is distributed in the platysma (Fig. 3-2). A helpful sentence for memorization is “Two Zebras Bite My Carrot.”
Fig. 4-4. Parotid gland, adjacent structures.
The parotid gland contains not only VII which exits the cranial cavity through the stylomastoid foramen (Fig. 4-27) but also the external carotid artery (Fig. 3-28) and the retromandibular vein which is a tributary of the external jugular vein (Fig. 3-29).
Just as the submandibular gland surrounds the posterior border of mylohyoid muscle (Fig. 3-33), the parotid gland surrounds the posterior border of ramus of mandible (Fig. 4-12).
However, unlike the submandibular duct which arises from the deep part of the gland (Fig. 3-34), the parotid duct arises from the superficial part of the gland. Therefore, the parotid duct can be easily injured by an external impact.
Saliva made from the parotid gland is released into the protruding oral mucosa lateral to the maxillary 2nd molar. With a mirror, the protruding oral mucosa of oneself can be seen little. Usually, opening of a duct of an exocrine gland is protruded; a representative example would be the nipple (Fig. 2-6).
In contrast to the submandibular and sublingual ducts opening beneath the tongue (Fig. 4-33), the parotid duct opens near the fauces (Fig. 3-43). While the secretion of the submandibular and sublingual glands is composed of mucous fluid, that of the parotid gland is composed of serous fluid only. The watery (serous) saliva secreted near the fauces helps you swallow.
< Scalp >
Fig. 4-5. Scalp, calvaria.
Five layers of SCALP are Skin, Connective tissue, epicranial Aponeurosis, Loose connective tissue, and Periosteum. However, it is only by coincidence that these first letters comprise the word SCALP.
The epicranial aponeurosis is the intermediate aponeurosis between the occipitalis and frontalis (Fig. 4-1). Because the occipitalis and frontalis are facial muscles, the epicranial aponeurosis also exists in the subcutaneous tissue (Fig. 3-1). It also serves the purpose of dividing subcutaneous tissue into the connective tissue and loose connective tissue.
Loose connective tissue enables the skin, connective tissue, and epicranial aponeurosis to slide above the periosteum. One can grab and move one’s hair to feel those three layers slide against the periosteum.
< Cranial meninges >
The calvaria is a portion of the skull that forms the roof of the cranial cavity. Inside calvaria, the cranial dura mater consists of periosteal and meningeal layers. The periosteal layer is a periosteum like the periosteum in scalp (Fig. 4-5); however, the periosteal layer oddly is included in the dura mater.
This contradiction is purposeful for the sake of convenience, as the two layers of the dura mater cannot be separated during dissection. Between two layers, there is no recognizable space whereas the spinal meninges have epidural space filled with fat (Fig. 1-16). If a middle meningeal artery (Fig. 4-20) between two layers is ruptured, the blood accumulates outside the periosteal layer (Fig. 4-7).
Fig. 4-6. Cranial meninges, their related structures.
The dura mater includes dural venous sinuses. This coronal plane demonstrates two examples of the sinuses, the superior and inferior sagittal sinuses. The superior one is formed by the periosteal and meningeal layers, whereas the inferior one is formed only by the meningeal layer.
Two meningeal layers are bound together to hold the inferior sagittal sinus, just as a mesentery holds the intestine (Fig. 6-19). The two meningeal layers that intervene between the bilateral cerebral hemispheres are called the cerebral falx since it resembles a falx (sickle) (Fig. 4-8). Another sickle-like structure is the inguinal falx (Fig. 6-8).
The choroid plexus is the capillary in the ventricles of the brain. The plasma filtered from the choroid plexus is called the cerebrospinal fluid. The cerebrospinal fluid in the ventricles exits to the subarachnoid space to surround and protect the brain and the spinal cord (Fig. 1-16). Detailed stories of the ventricles are narrated in the neuroanatomy textbook.
Finally, the cerebrospinal fluid drains to the dural venous sinuses (including the superior sagittal sinus) through arachnoid granulation (Fig. 4-6). Granulation is an extension of the arachnoid mater into the dural venous sinuses (Fig. 4-10).
In sum, the cerebrospinal fluid flows from the capillary until it reaches the veins (dural venous sinus) just like lymph. We can say that the brain has the cerebrospinal fluid instead of the lymph.
Fig. 4-7. Epidural, subdural, and subarachnoid hemorrhages.
Similar to the cerebrospinal fluid, the blood in the cerebral vein flows to the dural venous sinuses.
Suppose that an external impact shifts the left cerebral hemisphere severely from the calvaria. Then, the arachnoid mater attached to the cerebral hemisphere would be shifted while the dura mater attached to the calvaria remains in place, and this causes the rupture of the connecting cerebral vein in the subdural space. This is the pathogenesis of subdural hemorrhage.
Even though the subdural space is drawn as if it had some volume in the figure above, it is actually a potential space and has no volume in normal state. The same goes for the spinal cord (Fig. 1-16).
The cerebral artery and vein are surrounded by the cerebrospinal fluid in the subarachnoid space. If a cerebral artery is ruptured, a subarachnoid hemorrhage occurs.
In summary, epidural, subdural, and subarachnoid hemorrhages are caused by the tear of the middle meningeal artery, cerebral vein, and cerebral artery, respectively.
The parietal foramen allows the passage of the emissary vein between the superior sagittal sinus and the vein in the scalp (Fig. 4-5). The direction of the blood flow in the emissary vein depends on the varying blood pressure on both sides. The word “emissary” means a “spy” that works for both sides.
Similarly, condylar canal in the occipital condyle allows for the passage of another emissary vein from the sigmoid sinus (Fig. 4-10). Clarify that the condylar canal has no association with the condylar process of mandible (Fig. 4-16).
Fig. 4-8. Cerebral falx, cerebellar tentorium.
The cerebral falx is continuous with the cerebellar falx, the cerebellar tentorium, and the sellar diaphragm. Except the sellar diaphragm, they collectively meet at the straight sinus.
The cerebellar tentorium is thought of as a property of the cerebellum.
Fig. 4-9. Sellar diaphragm, sella turcica.
One structure of the sphenoid bone is the sella turcica (Turkish saddle) which refers to dorsum sellae, tuberculum sellae, and hypophyseal fossa, collectively. The sellar diaphragm and sella turcica enclose the pituitary gland (hypophysis).
Fig. 4-10. Dural venous sinuses.
This figure shows the direction of the blood flow in the dural venous sinuses. Eventually, all the blood goes into the internal jugular vein (Fig. 3-30).
With the exception of the inferior sagittal sinus and the straight sinus composed of only the meningeal layer of dura mater, the dural venous sinuses are formed by both the periosteal and meningeal layers (Fig. 4-6). Therefore, the blood pressure in those sinuses builds grooves in the cranial cavity that are discernible with the dry skull. An evident example is the groove for sigmoid sinus that is continuous with the jugular foramen.
The confluence of sinuses is in contact with the internal occipital protuberance (Fig. 4-8) that is located close to the external occipital protuberance (Fig. 1-3).
Fig. 4-11. Cavernous sinus.
The cavernous sinus, a dural venous sinus, can be imagined as a paper box that is in contact with the pituitary gland (Fig. 4-9) on its medial surface. In the figure, the lateral wall is opened like a lid. The narrow cavernous sinus is very crowded with the cranial nerves and artery. III (Fig. 4-45), IV, V1 (Fig. 4-22), and V2 (Fig. 4-23) pass through the lateral wall. VI and internal carotid artery (Fig. 3-27) penetrate the cavernous sinus.
< Temporal and infratemporal fossae >
Fig. 4-12. Masseter, temporal muscle.
The temporal fossa and infratemporal fossa are distinguished by the palpable zygomatic arch. Since the lateral wall of infratemporal fossa is the ramus of mandible, the external masseter does not belong to the infratemporal fossa. The temporal and infratemporal fossae are filled with other masticatory muscles, arteries, and nerves.
Fig. 4-13. Floor of temporal fossa.
The floor of the temporal fossa is composed of the frontal, parietal, temporal, and sphenoid bones. These four bones collectively configurate an H-shaped suture known as the pterion. The pterion is the center of the floor of temporal fossa.
Fig. 4-14. Fontanelle becoming suture.
A suture is a kind of fibrous joint where fibrous tissue connects the bones. A suture with a slim fibrous tissue is immobile. However, during infancy, sutures have not yet completely formed. Instead, infant has fontanelles with more fibrous tissue, providing mobility. It is due to the mobility of the fontanelles that a newborn’s head can pass through the narrow birth canal of the mother. As the infant matures, fontanelles become sutures.
The pterion is the thinnest portion of the calvaria (Fig. 4-13); thus, it is protected by the temporal muscle that covers it (Figs. 4-12,15). However, the pterion can be fractured; a sharp fracture can tear the middle meningeal artery, causing an epidural hemorrhage (Fig. 4-7).
Fig. 4-15. Origins of temporal muscle.
The origins of the temporal muscle are the floor of temporal fossa (including pterion), the inferior temporal line, and the temporal fascia starting from superior temporal line.
The superior and inferior temporal lines are found not on the temporal bone but instead on the parietal and frontal bones (Fig. 4-13). The terms “temporal” lines originate from the “temporal” fascia and muscle.
Insertions of the masseter and temporal muscle are the ramus of mandible and coronoid process, respectively (Fig. 4-12). When one clenches one’s teeth, one can palpate the two muscles bulging out.
Fig. 4-16. Lateral and medial pterygoid muscles.
Origins of the lateral and medial pterygoid muscles are not the lateral and medial pterygoid plates, but the lateral and medial surfaces of lateral pterygoid plate. The small medial pterygoid plate, surrounded by the tensor veli palatini (Fig. 4-35), is too narrow for the pterygoid muscles to originate from.
Lateral pterygoid muscles reaching the condylar processes protracts the mandible forward (Fig. 4-16). The sentence in the comics may help students memorize this muscle function. The words “pterygoid” as well as “pterion” (Fig. 4-13) mean wing.
Medial pterygoid muscle is attached to the medial side of the ramus of the mandible (Fig. 4-16), while the masseter is attached to the lateral side of the ramus (Fig. 4-12). The two muscles that have similar insertion and direction collaborate to elevate the mandible. We may regard the medial pterygoid muscle as the medial masseter (Fig. 4-4).
The explained four muscles (temporal muscle, masseter, and lateral and medial pterygoid muscles) constitute the masticatory muscles. During a meal, we enjoy not only the taste but also the texture of food. The latter is felt as the proprioception by the masticatory muscles. Proprioception is a type of somatic sensory stimulus, conveyed by V3. For the simple delineation, the sensory nerve from the masticatory muscles is not drawn in Fig. 4-25.
The antagonists of most masticatory muscles are the suprahyoid and infrahyoid muscles which pull the mandible down (Figs. 3-6,9). When masticatory muscles are relaxed, the mandible is also depressed by gravity which is another invisible antagonist. This often takes place when we dose off in class.
Fig. 4-17. Articular disc that divides articular cavity.
In the temporomandibular joint (Fig. 4-18), articular cavity (Fig. 2-59) is divided by an articular disc, which relieves impacts on the joint. Articular disc comprises a supplementary synovial membrane, which facilitates joint movement.
Fig. 4-18. Excessive hinge movement of temporomandibular joint (left) that causes dislocation (right).
In the temporomandibular joint, the inferior articular cavity is a hinge joint used for dicing up food. When the mouth is opened excessively, it may cause dislocation of the temporomandibular joint.
On the other hand, superior articular cavity is a plane joint used for grinding food. In other words, protraction, retraction, and lateral movement of the mandible all happen in the superior articular cavity.
Fig. 4-19. Parts of maxillary artery.
Maxillary artery, a terminal division of the external carotid artery (Fig. 3-28), passes the infratemporal fossa (1st and 2nd parts), the pterygopalatine fossa (Fig. 4-24), and the orbit (3rd part) sequentially. In the 2nd part, maxillary artery is superficial enough to pass lateral to the lateral pterygoid muscle (Fig. 4-16).
Fig. 4-20. Branches of maxillary artery.
The 1st part of maxillary artery gives off the middle meningeal artery (Fig. 4-6) and the inferior alveolar artery, which enter the foramen spinosum and the mandibular canal (between the mandibular and mental foramina), respectively.
Branches from the 2nd part are for the masticatory muscles. It is notable that the lateral and medial pterygoid muscles exist in the same infratemporal fossa (Figs. 4-16,19). One of the branches is the deep temporal artery that supplies the temporal muscle in company with the superficial temporal artery (Fig. 3-28).
In the 3rd part of the maxillary artery (pterygopalatine fossa and orbit), the superior alveolar arteries branch off. The counterparts are the inferior alveolar arteries.
< V, VII >
Fig. 4-21. Three divisions of V.
All sensory nerves of V form the trigeminal ganglion, where V trifurcates into V1, V2, and V3. Among them, only V3 includes an extra motor nerve. The three branches are explained separately.
Fig. 4-22. V1.
V1 is drawn in the superior view because it is usually observed only once the superior wall of orbit has been opened (Fig. 4-47).
After passing the cavernous sinus (Fig. 4-11), V1 enters the orbit through the superior orbital fissure, where it trifurcates into three nerves. The main trunk among the three nerves is the frontal nerve which travels through the supraorbital notch or foramen in the frontal bone (Fig. 4-26). The lateral branch is the lacrimal nerve because the lacrimal gland is located laterally in the orbit (Fig. 4-42). The lacrimal nerve is responsible for the senses around the lacrimal gland, while the greater petrosal nerve is responsible for tear secretion of the lacrimal gland (Fig. 4-27). The medial branch is the “nasociliary” nerve because it goes toward the “nose” and is connected with the “ciliary” ganglion (Fig. 4-45).
Superior orbital fissure is located inside the orbit, whereas supraorbital foramen is located outside the orbit. It is crucial to note the difference between “superior” and “supra.” Unless one is able to palpate the supraorbital notch (Fig. 4-26), one has a supraorbital foramen. That is just a variation.
Fig. 4-23. V2.
V2 traverses the cavernous sinus (Fig. 4-11), the pterygopalatine fossa (Fig. 4-24), and the orbit. V2 gives off superior alveolar nerves in the pterygopalatine fossa and the orbit. This pattern is identical with the maxillary artery (3rd part) which gives off superior alveolar arteries (Fig. 4-20). Eventually, the main trunk of V2 (and maxillary artery) passes through the infraorbital foramen of the maxilla (Fig. 4-26).
Fig. 4-24. Right pterygopalatine fossa.
Let us introduce the pterygopalatine fossa, an inverted pyramidal space. The “pterygopalatine fossa” is physically formed by the medial and lateral “pterygoid” plates (Figs. 4-16,35) and the “palatine” bone (Fig. 4-37).
The fossa is connected to other spaces in the following manner: laterally to the infratemporal fossa through the pterygomaxillary fissure, posteriorly to the cranial cavity through the foramen rotundum, and anteriorly to the orbit through the inferior orbital fissure. It is recommended that students insert a wire through these paths in the dry skull. The important structures passing the pterygopalatine fossa are the maxillary artery (Fig. 4-19) and maxillary nerve (V2) (Fig. 4-23).
Fig. 4-25. V3.
All the branches of V3 are given off in the infratemporal fossa. The lingual nerve is responsible for the general sense of the anterior two thirds of tongue. The main trunk of V3 is the inferior alveolar nerve, which passes through the mandibular canal. The inferior alveolar nerve shares its course with the inferior alveolar artery (Fig. 4-20).
V3 has a motor nerve to the 1st pharyngeal arch muscles (Fig. 3-8). Those muscles are the two tensors (tensor tympani (Fig. 4-51) and tensor veli palatini (Fig. 4-35)), the masticatory muscles (Figs. 4-12,16), the anterior belly of digastric muscle, and the mylohyoid muscle (Fig. 3-6). The branch for the last two muscles has a name, “mylohyoid nerve” (Fig. 3-7).
Fig. 4-26. Exits and skin areas of V1, V2, V3.
The main trunks of V1, V2, V3 exit through the supraorbital notch/foramen (supraorbital nerve) (Fig. 4-22), the infraorbital foramen (infraorbital nerve) (Fig. 4-23), and the mental foramen (mental nerve) (Fig. 4-25), sequentially. Roughly speaking, the three bony structures are located in a sagittal plane (Fig. 4-21).
Main trunks and other branches (e.g., nasociliary and lacrimal nerves) (Fig. 4-22) of V1, V2, V3 innervate the skin. The three skin areas of the ophthalmic nerve (main trunk: frontal nerve), maxillary nerve, and mandibular nerve are placed on the frontal bone, maxilla, and mandible. Eyes and nose serves as the boundary between V1 and V2; the mouth as the boundary between V2 and V3.
When repairing dental caries of mandibular teeth, a dentist anesthetizes one’s inferior alveolar nerve around the mandibular foramen. One thus feels numb on the mandibular teeth and the skin area of V3. As a collateral effect, the territory of the lingual nerve (anterior two thirds of tongue) also gets slightly anesthetized (Fig. 4-25).
Fig. 4-27. VII.
Shall we move on to VII? Through the internal acoustic meatus, VII enters the temporal bone where it divides in a complicated manner.
The greater “petrosal” nerve is the branch that passes through the “petrous” (stone-like) part of the temporal bone. The greater petrosal nerve synapses at the pterygopalatine ganglion (in pterygopalatine fossa) (Fig. 4-24) and eventually stimulates the lacrimal gland (Fig. 4-42) to secrete tears. It is crucial to recall that VII includes parasympathetic nerve that induces secretions in the head. Likewise, the lesser petrosal nerve of IX is the parasympathetic nerve for the parotid gland (Figs. 3-21,22).
In general, postganglionic fibers of autonomic nerves (not equipped with myelin sheath) (Fig. 3-17) are too thin to travel to a target by themselves. Therefore, a sympathetic postganglionic fiber usually travels along with an artery to reach the target organ. Such examples are the postganglionic fibers from the cervical ganglia (Figs. 3-18,19) and celiac ganglion (Fig. 6-49).
The parasympathetic postganglionic fiber often goes along with other nerves, enclosed by a common epineurium together. One example is the postganglionic fiber from the otic ganglion that travels along with a branch of V3 (Fig. 3-21).
In the case of the postganglionic fiber from the pterygopalatine ganglion, the fiber travels with V2. They pass through the inferior orbital fissure (Figs. 4-23,24). The postganglionic fiber then travels with the lacrimal nerve of V1 and arrives at the lacrimal gland (Fig. 4-22).
The chorda tympani traveling with a branch of V3 (lingual nerve) has already been discussed in the neck chapter (Fig. 3-35). Parasympathetic ganglion of the chorda tympani is the submandibular ganglion. Sensory ganglion of the chorda tympani is the “geniculate” ganglion, for it resembles a (flexed) knee which is called “genu” in Latin.
A somatic motor nerve of VII innervates the 2nd pharyngeal arch muscles (Fig. 3-8): stapedius (Fig. 4-51), stylohyoid muscle, posterior belly of digastric muscle (Fig. 3-6), and facial muscles (Fig. 4-1). As V3 innervates two Ts (Tensor tympani, Tensor veli palatini) (Fig. 4-25), VII innervates two Ss (Stapedius, Stylohyoid muscle).
IX has been explained in the neck chapter (Figs. 3-20,21,22), even though IX affects the tongue and parotid gland in the head. X influencing the palate has also been elucidated in the neck (Fig. 3-23).
< Oral cavity >
Fig. 4-28. Oral cavity.
Fig. 4-29. Tooth in dental alveolus.
Teeth are the foremost structures in the oral cavity. Teeth are firmly situated in the dental alveoli of the mandible and the maxilla by the periodontal ligament. This is an example of the fibrous joint.
Fig. 4-30. Number of permanent teeth (right mandibular teeth).
Teeth are symmetrical not only on the right and left sides, but also on the mandible and the maxilla. In each quadrant, two incisors, one canine, two premolars, and three molars are arranged from medial to lateral.
During childhood, 20 deciduous teeth are replaced by 32 permanent teeth. Among the deciduous teeth, it is the medial incisor that erupts first (approximately 6 months after birth). Among the permanent teeth, the 1st molar erupts first (approximately 6 years after birth).
Fig. 4-31. Dorsum of tongue.
On the dorsum of tongue, four kinds of papillae exist. These papillae are macroscopic structures that can be observed with the naked eye. With the exception of filiform papillae, those papillae include microscopic receptors which are called the taste buds.
The thyroid gland develops downward from the tongue by forming a duct (Fig. 3-37). After the development, the entrance of the duct is enclosed to become the foramen cecum (blind foramen). Thus, the foramen cecum is actually a depression like the continuous terminal sulcus.
There is terminal sulcus also in the right atrium of the heart (Fig. 5-15). The official term is sulcus terminalis cordis, but for the simplicity, it may be called “terminal sulcus.”
Fig. 4-32. Intrinsic and extrinsic tongue muscles.
Intrinsic muscles of the tongue in three directions give rise to a variety of tongue shapes, and extrinsic muscles pull the tongue up and down, back and forth. Intrinsic and extrinsic muscles of the larynx also have similar functions (Figs. 3-9,41). All the tongue muscles are innervated by XII (Fig. 3-34).
The styloid process is a common origin of the stylohyoid muscle (Fig 3-6), the stylopharyngeus (Fig. 3-45), and the styloglossus. This styloid process of the temporal bone (Fig. 4-4) cannot be palpable in one’s body unlike those of the radius (Fig. 2-34) and ulna.
XI is composed only of motor nerve (Fig. 3-20), so the trapezius and sternocleidomastoid muscle need additional sensory nerves (CN2–CN4) (Fig. 3-24). Likewise, XII only contains the motor nerve (Fig. 3-13), so the tongue should be innervated by other sensory nerves.
The terminal sulcus including the foramen cecum is the border between the anterior two thirds (general sense, V3 (Fig. 4-25); special sense, VII (Fig. 4-27)) and the posterior one third (both senses, IX (Fig. 3-21)). The innervation is best understood on the basis of embryology: The anterior 2/3 originate from the 1st pharyngeal arch (V) and the 2nd pharyngeal arch (VII), while the posterior 1/3 originate from the 3rd pharyngeal arch (IX) (Fig. 3-37).
A structure of exception is vallate papillae. (Circumvallate papilla is not the official term.) The vallate papillae that bear taste buds are anterior to the terminal sulcus, so one may assume it to be innervated by VII. However, they are innervated by IX.
Fig. 4-33. Frenulum of tongue, adjacent structures.
When one elevates the tongue strongly, one can feel stretch of the frenulum attached below the tongue. The frenulum of tongue decides the length of the free part of tongue.
Lateral to the frenulum of tongue, sublingual caruncle and sublingual folds are protruded, where submandibular duct (Fig. 3-34) and sublingual ducts (Fig. 4-32) open to release saliva, respectively.
Fig. 4-34. Arches from soft palate.
The palate comprises the hard palate and the soft palate (Fig. 4-28). When one opens the mouth widely, one can see not only the uvula but also the palatine tonsil with a mirror. The palatine tonsil is located between the palatoglossal and palatopharyngeal arches (Fig. 3-44). The palatine tonsil used to be the 2nd pharyngeal pouch (Fig. 3-37).
Fig. 4-35. Muscles of soft palate.
During cadaver dissection, students are suggested to remove the mucosa of the two arches to reveal the palatoglossus and palatopharyngeus. The two muscles are innervated by X, since they belong to palate muscles (Figs. 3-20,23). They elevate the tongue and pharynx which enables swallowing (Figs. 3-43,45).
Fig. 4-36. Depression of soft palate.
At a resting state, the palatoglossus depresses the soft palate (Fig. 4-35), which then becomes in contact with the tongue. Air is thus inhaled via the nasal cavity not via the oral cavity, in spite of the opened mouth.
Fig. 4-37. Core of hard and soft palates.
In order for the soft palate to firmly separate the nasopharynx and oropharynx when one swallows (Fig. 3-43), the soft palate should be “tense” by the “tensor” veli palatini. The palatine aponeurosis in the soft palate is regarded as the aponeurosis of the bilateral tensor veli palatini muscles. Simultaneously, the soft palate should be “elevated” by the “levator” veli palatini (Fig. 4-35).
The veli palatini, another name for the soft palate, looks like a bridal veil. Consider the veli and the veil as being identical.
The two muscles (tensor veli palatini and levator veli palatini) originate from the inferior wall of the auditory tube. Contraction of two muscles pulls the origin down, which eventually opens the collapsed auditory tube (Fig. 4-35). When one tries to swallow one’s own saliva, one can hear a loud pop of the auditory tube opening.
While most muscles of the palate are innervated by X (Fig. 3-23), the tensor veli palatini is innervated by V3 (Fig. 4-25).
Uvulae shortens and broadens the uvula to assist the partition between the nasopharynx and the oropharynx (Fig. 3-43). If the left uvulae does not work due to a left X damage, only the right part of the uvula will be shortened to result in a right-deviated uvula (Fig. 4-35).
< Nasal cavity >
Fig. 4-38. Nasal cavities.
The nasal septum separates two nasal cavities (Fig. 4-40). For each nasal cavity, there are three nasal conchae and four passages. Warm and moist mucosa that lines the entirely nasal structures warms and moistens the air before it is inhaled into the lungs.
The complexity of the nasal cavity is a double-edged sword. A disadvantage is that we easily get stuffy nasal cavity due to the inflammation. The two nasal cavities take their turns to widen for nasal breathing.
Fig. 4-39. Ethmoid bone.
Let us observe the coronal plane of the ethmoid bone. On either side of the crista galli, the cribriform plate supports the olfactory bulb, a part of I (olfactory nerve). The cribriform plate is perforated by foramina for the passage of I, which senses smell in the roof of the nasal cavity.
The ethmoid bone does not comprise the inferior nasal concha (Fig. 4-38) which is an independent bone.
Fig. 4-40. Constituents of nasal septum.
The ethmoid bone determines the shape of the superior part of the nasal cavity. For example, if its perpendicular plate is deviated, the nasal septum gets deviated, resulting in asymmetric nasal cavities.
Caves positioned around the nasal cavity are called paranasal sinuses. Paranasal sinuses are modified medullary cavities (Fig. 2-24) of the sphenoid, ethmoid, maxillary, and frontal bones. Therefore, the four bones hardly have the medullary cavity. Paranasal sinuses reduce the weight of one’s head and helps making good nasal voice.
Fig. 4-41. Lateral wall of nasal cavity with conchae removed.
Take note of the adjective change: the “sphenoid” bone has the “sphenoidal” sinus, while the “ethmoid” bone has “ethmoidal” cells.
The sphenoidal sinus opens into the sphenoethmoidal recess (Fig. 4-38), since two spaces are at the same latitudinal level. The sphenoethmoidal recess is not a normal airway leading to the nasopharynx (Fig. 3-41), thus it is not qualified to be called a “nasal meatus.”
Since the middle nasal concha is oblique, the posterior ethmoidal cells open into the superior nasal meatus, whereas the middle and anterior ethmoidal cells open into the middle nasal meatus (precisely, the ethmoidal bulla). It is suggested that students take a look at the schematic coronal plane (Fig. 4-38) where the ethmoidal cells open to the superior and middle nasal meatuses.
The semilunar hiatus exists just below the ethmoidal bulla, where maxillary and frontal sinuses open.
Among paranasal sinuses, the maxillary sinus is most likely to become inflamed. The rationale behind this claim is that since the upper portion of the maxillary sinus is connected to the nasal cavity, secretions easily accumulate in the lower portion of the sinus, which may result in an inflammation. Doctors commonly say, “Stagnant water is bound to rot.”
< Eye >
Fig. 4-42. Lacrimal apparatus.
The inferior nasal meatus receives the nasolacrimal duct (Fig. 4-38). Even when we are not crying, tears are constantly secreted from the lacrimal gland to clean and lubricate the eyeball. Tears pass through the nasolacrimal duct to arrive at the inferior nasal meatus, then get dried when air is inhaled (Fig. 4-41). Underneath the medial eyelids of a living person, one can discover two small holes, which are the openings to the lacrimal canaliculi.
In the dry bone, one can identify the nasolacrimal canal where the nasolacrimal duct used to be. The general rule is that canal is a bony structure, while duct is a structure of soft tissue (involving mucosa).
One way to remember the position of the lacrimal gland is as follows: LAcrimal gland is situated in the LAteral of the orbit. It is also like how LA (Los Angeles) is located at a LAteral side of the United States.
The wall of the eyeball is composed of three layers, of which the innermost layer is the retina (Fig. 4-43). The center of retina is the macula, and the center of macula is the fovea centralis that can sense the color very well. Cone cells placed in fovea Centralis are receptors to perceive Color; one can memorize it as CCC.
The fovea centralis is depressed since there are only a few neurons shading the cone cells.
Rod cells suRrounding the macula can perceive daRk environment fittingly; one can memorize it as RRR. When we walk in darkness, it is these rod cells to detect sudden attacks from every direction.
On the contrary, optic disc, which is the blind spot of the retina, has no receptors. The optic disc is attached to II, which is located medial to macula. Accordingly, a lateral object projected to the optic disc cannot be seen.
II as well as I (Fig. 4-39) are often regarded as the extended brain, namely the central nervous system. It is because the two nerves are enclosed by the pia mater (Fig. 4-6). As a result, the neural pathways of I and II are handled in detail in neuroanatomy textbook.
Fig. 4-43. Eyeball.
Black absorbs the light, while white reflects the light. If the inside of our eyeball were white, light entering the eyeball would be reflected over and over, and we would see the same object over and over. To prevent such from happening, color of the choroid covering the transparent retina is dark red. (The pigmented layer of retina is neglected to make it plain.) For the same reason, the inside of a camera is black.
The dark red choroid is responsible for the “red eye” reflection that occurs in flash photography. To avoid such red eye, the pupils must be made constricted by the small light that is flashed prior to the actual light for photography.
Fig. 4-44. Muscles around lens (left) and pupil (right).
Elastic lens has a tendency to get thick, while the ciliary body flattens the lens (Fig. 4-43) by stretching it with suspensory ligament of the lens. Ironically, the ciliary muscle inside the ciliary body performs an action opposite to the ciliary body. The ciliary muscle, a smooth muscle innervated by parasympathetic nerve of III, contracts to thicken the lens (Fig. 4-45).
In a state of Peace, Parasympathetic nerve is activated, and thus the ciliary muscle contracts (Fig. 4-45). Then the lens becomes thick, and one can peacefully look at a close object like a comic book.
Reversely, in a state of war, parasympathetic nerve is inactivated. Then the lens is flattened, and one can cautiously observe enemies from far away.
Even though the ciliary muscle is a smooth muscle, the muscle can be voluntarily controlled to some extent.
The pupil is the narrowest portion of light path to the retina (Fig. 4-43). The size of the pupil can be adjusted by the surrounding iris which contains the sphincter pupillae and dilator pupillae (Fig. 4-44).
In a state of war, sympathetic nerve from superior cervical ganglion is activated (Fig. 3-19), and thus the dilator pupillae shortens. Then the pupil is enlarged, and thus one can search for hidden enemies in the dark.
Fig. 4-45. III.
III, IV, VI, and V1 pass through the cavernous sinus and the superior orbital fissure (Fig. 4-11) to enter the orbit. Among them, III contains somatic and visceral motor nerves (Fig. 4-48); IV, VI contain somatic motor nerve; and V1 contains somatic sensory nerve (Fig. 4-22).
As the parasympathetic nerve, III innervates the two circular muscles (ciliary muscle and sphincter pupillae (Fig. 4-44)) by way of the ciliary ganglion. “Ciliary” ganglion is named after the “ciliary” muscle.
The outermost layer of the eyeball is the sclera and cornea. Since the white sclera is opaque, the ciliary body and choroid are not observable from the outside. In the meantime, since the cornea is transparent, the iris and pupil are observable from the outside (Figs. 4-42,43). The color of the iris depends on one’s race and ethnic group, but everyone’s pupil is black because the inside of everyone’s eyeball is dark (Fig. 4-43).
A transparent fluid called the aqueous humor exists between the cornea and the lens (Fig. 4-43). In case of glaucoma, pressure of the aqueous humor is increased. Then the aqueous humor may mechanically exert excessive pressure on II behind the eyeball leading to blindness.
The space posterior to the lens is filled with another clear fluid called vitreous humor. This vitreous humor keeps the lens and retina in their places (Fig. 4-43).
Structures, through which light passes, should be made clear. These structures are cornea, aqueous humor, lens, and vitreous humor (Fig. 4-43). A frequent and serious problem, opaque lens, is a disease called cataract.
In contrast, dust particles in the vitreous humor are not a serious problem. If one constantly stares at a bright space, one will be able to see foggy objects, which are the dust particles.
The anterior sclera is covered by the bulbar conjunctiva. The continuous palpebral conjunctiva lines the eyelid (Fig. 4-43).
Fig. 4-46. Muscles of upper eyelid.
Muscles to open and close the eyelid are levator palpebrae superioris and orbicularis oculi (Fig. 4-1), respectively.
When levator palpebrae superioris is attached to the skin of superior eyelid rather than its end, a double eyelid is formed.
III innervates the levator palpebrae superioris which opens the upper eyelid (Fig. 4-45). VII innervates the orbicularis oculi which closes the upper and lower eyelids (Fig. 4-27). Such difference is because the orbicularis oculi is one of the facial muscles (Fig. 4-1).
The eyelid has the thinnest skin and subcutaneous tissue of all parts of our body. Therefore, the eyelid has helped mankind wake up when the sun rises since primitive age.
The filmy eyelid enables us to blink very rapidly like the shutter in a camera.
A neuron and its innervating muscle cells are defined as a motor unit. The fewer muscle cells are in a motor unit, the more finely the muscle contracts. Delicate eyelid muscles (Fig. 4-46) have very small motor units.
Additional to the levator palpebrae superioris, the superior tarsal muscle also contributes to the elevation of the upper eyelid. The superior tarsal muscle originates from the inferior side of the levator palpebrae superioris, and reaches the superior tarsus. Notice that the superior tarsus is not a muscle, despite of its muscle-like-name (Fig. 4-46).
While the levator palpebrae superioris is a skeletal muscle, the superior tarsal muscle is a smooth muscle innervated by the sympathetic nerve from the superior cervical ganglion (Fig. 3-19).
In a state of peace, the sympathetic nerve is inactivated and the superior tarsal muscle relaxes, causing sleepy (peaceful) eyes. However, in a state of war, sympathetic nerve is activated and the superior tarsal muscle contracts to roll the upper eyelid up. Then one can search for more enemies in a wide range.
Collectively, in a state of war, one can see enemies in far distance (with a thinner lens), enemies in the dark (with larger pupil) (Figs. 4-44,45), and enemies in a wide range (with goggle eye).
Fig. 4-47. Extraocular muscles.
There are six extraocular muscles that move the eyeball. Except for the inferior oblique muscle, all five muscles originate from the common tendinous ring surrounding the II (Fig. 4-46) that has passed through the optic canal.
Fig. 4-48. Actions of extraocular muscles (innervating nerves).
The medial rectus and the lateral rectus perform simple actions of adduction and abduction of the pupil. In contrast, the remaining four muscles have complicated actions.
The (superior, inferior) rectus and (superior, inferior) oblique muscles cause adduction and abduction, respectively. Recall that in the suboccipital triangle, the rectus muscles are medial to the oblique muscles (Fig. 1-15).
The superior rectus moves the pupil superiorly while the inferior rectus moves the pupil inferiorly; however, oblique muscles induce opposite actions. The Rectus has Righteous action, but the Oblique has Opposite action.
Intorsion and extorsion are defined as the internal and external rotations of the superior pole of eyeball with respect to the anteroposterior axis of the eyeball.
Fig. 4-49. Force of superior rectus, divided into 1, 2, and resultant actions.
As a representative, only the action mechanism of the superior rectus is explained in detail. Examine the insertion of the superior rectus closely (Fig. 4-47). The force of the superior rectus is divided into 1 and 2. Along the mediolateral axis, force 1 elevates the pupil (1A); along the superoinferior axis, force 1 adducts the pupil (1B); along the anteroposterior axis, force 2 intorts the eyeball (2). The three actions of the superior rectus are depicted with a curved arrow in Fig. 4-48.
The superior oblique muscle and lateral rectus are distanced from the rest of the extraocular muscles (Fig. 4-47). Therefore, the two muscles are not innervated by the central oculomotor nerve (III) (Figs. 4-45,48).
Because the right orbit looks like the capital letter “V” in the superior view (Fig. 4-47), the superior oblique muscle and lateral rectus are governed by IV and VI, respectively (Fig. 4-48). (Believe it or not.)
The superior oblique muscle hooks around the “trochlea” (Fig. 4-47), so the innervating IV is named the “trochlear” nerve. The lateral rectus results in the “abduction” of the pupil (Fig. 4-48), so the innervating VI is named the “abducens” nerve.
Not only IV and VI but also XI (Fig. 3-20) and XII (Fig. 3-13) contain only somatic motor nerves. These four cranial nerves are even simpler than the spinal nerves. What a relief!
< Ear >
The ear, dissected last in a cadaver, is usually taught last in head anatomy, but it is taught first in the ENT (Ear, Nose, Throat) class.
Fig. 4-50. External ear.
The ear consists of external, middle, and internal ears. The external ear can be divided into the auricle and the external acoustic meatus. The curved external acoustic meatus becomes straight when the auricle is pulled posteriorly.
Speaking about the whole shape of the external acoustic meatus, it leans anteromedially. Therefore, the earpiece of a stethoscope should face anteromedially when inserted inside an ear for examination.
Fig. 4-51. Movement of auditory ossicles, prohibited by two muscles.
Sound entering the ear vibrates the tympanic membrane, which mobilizes the auditory ossicles in the tympanic cavity to vibrate the oval window. As a result, the vibration of the oval window induces the flow of the perilymph inside the scala vestibuli (Fig. 4-55). Otherwise put, sound is the vibration of air in the external ear (Fig. 4-50), vibration of solid in the middle ear (tympanic membrane and cavity), and vibration of liquid in the internal ear.
The auditory ossicles are MAlleus, INCus, STApes in the order of sound conduction. An old and good way to remember them is “MAil INCludes STAmp.” Mail delivery reminds us of sound conduction.
The auditory ossicles play the role of a lever. Simply put, the shift of stapes is shorter but stronger than that of malleus. A stronger shift is needed to push the perilymph inside the oval window (Fig. 4-55). The liquid in the internal ear is not as mobile as the air of the external ear.
Extremely loud noise may damage the internal ear. Therefore, the transmission of loud noise should be reduced in the middle ear. The tensor tympani and stapedius reduce the movements of the auditory ossicles. The name of the muscle, tensor tympani, comes from the fact that it pulls the malleus attached to the tympanic membrane (Fig. 4-51).
All the muscles that end at a bone are skeletal muscles, including the tensor tympani and stapedius. These muscles are innervated not by autonomic nerves, but by somatic motor nerves. Nevertheless, they are involuntary muscles which contract by reflex. Do not listen to the Taoist in the above comics.
Embryologically, the malleus originates from the 1st pharyngeal arch (Fig. 3-8), so the tensor tympani, which ends at the malleus (Fig. 4-51), is innervated by V3 (Fig. 4-25). On the other hand, the stapes originates from the 2nd pharyngeal arch; therefore, the stapedius is innervated by VII (Fig. 4-27).
The external acoustic meatus, tympanic membrane, and tympanic cavity (together with auditory tube) originate from the 1st pharyngeal groove, 1st pharyngeal membrane, and 1st pharyngeal pouch, in that order. Even though the tympanic membrane is very thin, it is made of all three of the germ layers: the ectoderm, mesoderm, and endoderm (Figs. 3-8,37).
Fig. 4-52. Tympanic cavity.
The tympanic cavity can be thought of as a square box. In the figure above, its lateral wall including the tympanic membrane is opened like a lid to expose the inside. This representation is similar to that of the cavernous sinus (Fig. 4-11).
Even though it’s not depicted in the Fig. 4-52, both the tympanic membrane and promontory bulge into the tympanic cavity. As a result, the tympanic cavity is actually a biconcave disc like a red blood cell.
The promontory of the tympanic cavity is like the tip of a dull mountain, formed by the cochlea in the internal ear (Fig. 4-55).
Fig. 4-53. Tympanic cavity, its connections.
The anterior and posterior walls of the tympanic cavity have openings of the auditory tube (Fig. 3-44) and mastoid antrum, respectively (Fig. 4-52). The mastoid antrum is continuous with the mastoid cells. Like the ethmoidal cells (a kind of paranasal sinus) (Fig. 4-38), mastoid cells reduce the weight of the head.
Strangely, a branch of VII enters and exits the tympanic cavity. Since this nerve is the only “cord”-like structure in the “tympanic” cavity, it is called “chorda tympani” (Figs. 3-35, 4-27). It runs medial to the malleus (Fig. 4-52).
Fig. 4-54. General form of bony and membranous labyrinths.
The bony labyrinth is a complex canal found in the temporal bone. The bony labyrinth surrounds membranous labyrinth like a tunnel encircling an oil pipe. Bony labyrinth contains perilymph, and membranous labyrinth contains endolymph as shown in the figure.
Fig. 4-55. Bony and membranous labyrinths of cochlea.
Cochlea (a bony labyrinth) is divided into the scala vestibuli and scala tympani by the cochlear duct (a membranous labyrinth). Unlike the above simplified figure, the actual cochlea is spiral in form, like a snail’s shell. The two scalas containing perilymph are separated unlike the general form (Fig. 4-54).
An easy way to remember two scalas is introduced in this comics, showing capital letters V (scala Vestibuli) and T (scala Tympani).
The cochlea makes two and a half turns, which presses the tympanic cavity to form the promontory (Fig. 4-52).
In Fig. 4-55, the oval window pushed by the stapes creates the flow of perilymph in the scala vestibuli and scala tympani, which finally produces the flow of endolymph in the intervening cochlear duct. In the cochlear duct, the flow of endolymph stimulates receptor; the stimulus then proceeds to the brain through the cochlear nerve, a large part of VIII (vestibulocochlear nerve).
The end of the scala tympani is the round window of the tympanic cavity (Figs. 4-52,55). The round window is flexible. If the round window were rigid, the stapes would push the oval window and perilymph at a lesser degree, lowering the hearing ability. Perilymph, which is liquid, is not as compressible as air.
Fig. 4-56. Bony and membranous labyrinths of vestibule.
Another bony labyrinth is the vestibule, which envelops two membranous labyrinths, the globular saccule and the oval utricle.
The “vestibule” is the origin of the term scala “vestibuli” which is connected to it. The scala “tympani” originates from the adjacent “tympanic” cavity.
When a car accelerates forward, the endolymph inside the saccule and utricle of the rider flows backward. The flow stimulates a receptor; the stimulus proceeds to the brain by way of the vestibular nerve, a small part of VIII. Consequently, the rider notices the acceleration even with his/her eyes closed.
The saccule and utricle also sense the inclination of the head. For sensing the inclination, gravity matters rather than inertia. Even with one’s eyes closed, one knows the inclination status of one’s head.
The remaining bony labyrinth (semicircular canal) resembles its membranous labyrinth (semicircular duct) just like the general form (Fig. 4-54). The semicircular canal and duct are structures of the bone and soft tissue, respectively, similar to the nasolacrimal canal and duct (Fig. 4-42).
When the body spins, endolymph in the semicircular duct flows in the opposite direction due to inertia. The endolymph stimulates receptor; we then feel the body spinning. The vestibular nerve is also responsible for the sensation.
There are three semicircular ducts and each of them is mutually right-angled, so we can sense the body rotation in all directions.
The sense from the saccule, utricle, and semicircular ducts are essential for the body equilibrium.