Cranium.

The eight bones of the cranium surround the cranial cavity, which houses the brain. The single frontal bone forms the forehead and superior part of the orbit of the eye. It contains the frontal sinuses, which communicate with the nasal cavity. Two parietal bones form most of the top of the cranium. Two temporal bones form a portion of the sides and base, or floor, of the cranium. Each temporal bone has a thin, flat, squamous portion that forms the inferior-lateral part of the cranium. The posterior portion of the temporal bone is the mastoid process. The external auditory canal (meatus), the tympanic membrane, the middle ear, and the inner ear are located in the petrous portion, which extends medially to form part of the base of the cranium. The zygomatic process of the temporal bone extends anteriorly to join the zygomatic bone to form the zygomatic arch. The single ethmoid bone is located between the eyes and forms most of the medial wall of each orbit. The superior surface of the ethmoid bone forms a part of the base of the cranial cavity and the roof of the nasal cavities. A thin, perpendicular plate of the ethmoid bone extends inferiorly to form part of the nasal septum. The superior and middle conchae (turbinates) in the nasal cavity are part of the ethmoid bone. The single sphenoid bone lies at the base of the skull anterior to the temporal bones. Often described as bat shaped, the sphenoid bone has “wings” that form the anterior-lateral portion of the cranium and the lateral walls of the orbits. The sella turcica is found in the center of the bone and is the location of the pituitary gland. The single occipital bone forms the posterior portion and part of the base of the cranium. It has a large hole, the foramen magnum, for passage of the spinal cord and the vertebral vessels. The bones of the cranium are illustrated in Figs. 5-1 to 5-4.

The domelike superior portion of the cranium is the calvaria, or skullcap. It is composed of the superior portions of the frontal, parietal, and occipital bones. The calvaria is covered by the scalp, which extends from the eyebrows to the superior nuchal line on the occipital bone. Structurally, the scalp has five layers. The outer layer, the skin, covers the second layer, which is composed of highly vascularized subcutaneous connective tissue. The third layer, or epicranium, consists of two thin muscles connected by a broad, flat tendon, or aponeurosis. This musculoaponeurotic sheet has the frontalis muscle at the anterior end and the occipitalis muscle at the posterior end. The strong aponeurosis that connects the two muscles is known as the epicranial aponeurosis, or galea aponeurotica. These three layers of the scalp are bound tightly together and move as a unit. Collectively, they often are referred to as the scalp proper. A fourth layer that consists of loose connective tissue separates the scalp proper from the fifth layer, the periosteum, or pericranium. The loose connective tissue layer permits mobility of the scalp proper, but it is also considered a potentially dangerous area, because it allows scalp infections to spread easily. Lacerations of the scalp typically bleed profusely because of the extensive vascularization of the subcutaneous connective tissue. The acronym SCALP serves as a useful mnemonic tool for remembering the five layers of the scalp.

The cranial cavity, a large space formed by the eight cranial bones, contains the brain. The floor of the cranial cavity is subdivided into anterior, middle, and posterior cranial fossae (FIG. 5-5). The anterior cranial fossa is formed by portions of the ethmoid, sphenoid, and frontal bones. This fossa houses the frontal lobes of the brain. The floor of the middle cranial fossa is composed of the body and greater wings of the sphenoid and the squamosal and petrous portions of the temporal bones. This region contains the temporal lobes of the brain. The posterior cranial fossa comprises the remainder of the cranial cavity and is formed by parts of the sphenoid, temporal, and occipital bones. This region contains the cerebellum, the pons, and the medulla oblongata. The inferior-most portion of the posterior cranial fossa shows the large foramen magnum, through which the spinal cord passes.

Face.

The facial portion of the skull consists of 14 bones. Some of these bones are illustrated in Figs. 5-1, 5-2, and 5-4. The two maxillae (singular, maxilla) unite in the midline to form the upper jaw. A horizontal piece of each maxilla, the palatine process, forms the anterior portion of the roof of the mouth, which is the hard palate. If the palatine processes of the two maxillae fail to join during prenatal development, a cleft palate results. Each maxilla contains a maxillary sinus, which is a large air space that communicates with the nasal cavity. Two palatine bones form the posterior portion of the hard palate. Two zygomatic bones, one on each side, form the prominence of the cheek and the lateral margin of the orbit. A process of the zygomatic bone extends posteriorly and unites with the temporal bone to form the zygomatic arch. Two small, rectangular nasal bones join in the midline to form the bridge of the nose. Fractures of these bones are common facial injuries.

The anterior part of the medial wall of each orbital cavity consists of a small, thin lacrimal bone. Each bone has a groove that helps to form the nasolacrimal canal, which allows the tears of the eye to drain into the nasal cavity. The single vomer is a thin bone shaped like the blade of a plow. It forms the inferior portion of the nasal septum. The two inferior nasal conchae are scroll-like bones that project horizontally from the lateral walls of the nasal cavities. The superior and middle conchae are part of the ethmoid bone, but the inferior nasal conchae are separate bones. The conchae are covered by a mucous membrane, which warms and moistens the air that enters the nasal cavities. The single mandible forms the lower jaw. The posterior ends of the mandible extend vertically to form the rami (singular, ramus). Each ramus has a knoblike condyle and a pointed coronoid process. The mandibular condyle articulates with the temporal bone to form the temporomandibular joint, which is the only movable joint in the skull.

Paranasal Sinuses.

The paranasal sinuses are air-filled spaces in some of the bones adjacent to the nasal cavity. The four sets of paranasal sinuses are named according to the bone in which they are located; they are the frontal sinus, ethmoid sinus, sphenoid sinus, and maxillary sinus. Usually developing after birth as outgrowths of the nasal cavity, the sinuses retain their original openings so that their secretions drain into the nasal cavity. The mucous membrane that lines the sinuses is continuous with the mucosa of the nasal cavity, but it is thinner and less vascular than the nasal mucosa. Fig. 5-6 illustrates the paranasal sinuses.

The frontal sinuses are located in the frontal bone near the midline. They develop in the child and are usually visible on radiographs by the time the child is 7 years of age; however, the sinuses continue to enlarge throughout adolescence. The frontal sinuses drain into the middle meatus of the nasal cavity, between the middle and inferior nasal conchae, by way of a frontonasal duct.

The ethmoid sinuses consist of numerous air spaces in the ethmoid bone between the orbit of the eye and the upper part of the nasal cavity. Although a few ethmoidal air cells are present in the neonate, they are not readily visible on radiographs until the infant is 2 years old, and they do not enlarge significantly until later in the childhood years, usually around 6 to 8 years of age. The ethmoid sinuses have numerous openings into the superior meatus between the superior and middle conchae and into the middle meatus between the middle and inferior conchae.

The sphenoid sinuses are located in the body of the sphenoid bone just posterior to the ethmoid sinuses and the nasal cavity. The sinuses occupy most of the volume of the sphenoid body, so that only a thin plate of bone separates the sinuses from the pituitary gland, the optic nerve, the optic chiasma, the internal carotid artery, and the cavernous sinus. Tiny sphenoid sinuses may be present in the newborn, but their development is more likely to occur in a child of about 2 years of age, with additional growth during late childhood and adolescence. The sphenoid sinuses drain into the sphenoethmoidal recess above the superior nasal conchae.

Small maxillary sinuses, which are located in the bodies of the maxillae, are present in the newborn and grow slowly until the child reaches puberty. The accelerated development of the maxillary sinuses during adolescence contributes to the apparent change in facial features that typically occurs during this period. When fully developed, the maxillary sinuses are the largest of the paranasal sinuses. The maxillary sinus drains into the nasal cavity by way of a relatively long hiatus semilunaris, which opens into the middle meatus. A couple of factors hinder the drainage of this sinus: the hiatus traverses a superior direction when the body is erect, thus necessitating drainage “against gravity,” and the opening from the sinus into the hiatus is in a superior location. The drainage problem is further complicated because the maxillary sinus is the most inferiorly located of the paranasal sinuses, and communicating channels allow the other sinuses to drain into the maxillary sinus. For these reasons, the maxillary sinus is the sinus most often involved in infections. The frontal, ethmoid, and sphenoid sinuses are innervated by branches of the ophthalmic division of the trigeminal nerve, which is the fifth cranial nerve. The maxillary sinus is innervated by branches of the maxillary division of the fifth cranial nerve. (The cranial nerves are discussed later in the chapter.) Sinuses and other features of the skeleton of the head are illustrated by the radiographs in Figs. 5-7 and 5-8.

Foramina of the Skull.

Foramina are openings in bones that serve as passageways for nerves and blood vessels. The vessels that transport blood and the nerves that carry impulses must pass through the sutured, helmetlike bones of the skull on their way to and from the brain; therefore these bones have numerous foramina. Table 5-2 provides a summary of the foramina of the skull.

Additional Bones Associated With the Skull.

Six auditory ossicles (three pairs) and a single hyoid bone are associated with the head, in addition to the eight bones of the cranium and the 14 bones of the facial skeleton.

The ear has three chambers: the inner ear, the middle ear, and the external ear. The ear ossicles, the malleus, incus, and stapes, are located in the middle ear chamber in the petrous portion of the temporal bone. The ossicles transmit and amplify sound waves through the middle ear. The single hyoid bone is located in the neck just superior to the larynx. The hyoid is unique because it does not attach directly to any other bone; instead, it is suspended by ligaments. Muscles associated with the hyoid bone are described in Chapter 6.

Muscles of Facial Expression.

The muscles of facial expression are located in the subcutaneous tissue of the face. They originate in the fascia or on the underlying bone, and they insert on the skin of the face. All are innervated by cranial nerve VII, the facial nerve. Many of these muscles are small and thin and are difficult to dissect or to distinguish on sections. Actions of the facial muscles are easily observed, however, because they are used to express feelings. Five of the more prominent facial muscles are mentioned here. The frontalis muscle is part of the scalp. Originating from the aponeurosis, which is located on the top of the head, the frontalis inserts on the skin of the eyebrow and forehead. When it contracts, this muscle elevates the eyebrows and produces transverse wrinkles in the skin of the forehead. The orbicularis oris muscle is an important sphincter that encircles the mouth and forms the muscular bulk of the lips. This muscle’s function involves closing the mouth and puckering the lips, as in whistling. It plays an important role in the enunciation of words. A similar sphincter, the orbicularis oculi muscle, surrounds the eye. Contraction of these muscle fibers reduces the orbital opening, as in winking and blinking. The buccinator muscle is an accessory muscle in mastication and compresses the cheeks when blowing, as in playing a musical wind instrument. It inserts on the orbicularis oris at the angle of the mouth. The platysma muscle is a broad, flat muscle in the subcutaneous tissue of the neck. It inserts on the mandible, the skin of the neck, and the orbicularis oris muscle. When it contracts, it depresses the lower jaw and forms ridges in the skin of the neck.

Muscles of Mastication.

The four muscles of mastication provide chewing movements by acting on the temporomandibular joint to move the mandible (see Table 5-3). All insert on the mandible and are innervated by the mandibular division of the fifth cranial nerve. They are quite readily seen on transverse sections. The fan-shaped temporalis muscle covers the squamosal portion of the temporal bone and is a powerful muscle used to close the mouth by elevating the mandible. The masseter muscle, or chewing muscle, is located on the lateral aspect of the ramus of the mandible. Both the lateral and medial pterygoid muscles originate on the lateral pterygoid plate of the sphenoid bone and insert on the medial surface of the mandible. When these muscles are observed in transverse sections, the temporalis is seen in the more superior sections. In lower sections, starting at the lateral surface and in sequence, the masseter, the ramus of the mandible, the lateral pterygoid, and the medial pterygoid are seen.

Parotid Gland.

The largest of the glands is the parotid gland, wedged between the ramus of the mandible and the mastoid portion of the temporal bone. The parotid gland occupies the space just anterior and inferior to the auricle of the ear, and a portion of the gland overlies the masseter muscle in the cheek. The well-defined duct of the parotid gland, Stensen’s duct, extends across the masseter muscle, then turns to penetrate the buccinator muscle, and opens into the vestibule of the mouth near the upper second molar tooth.

Submandibular Gland.

The submandibular (submaxillary) gland is located medial to the body and angle of the mandible. The gland can be felt as a small lump along the inferior border of the posterior half of the mandible. The secretions of this gland reach the oral cavity by means of the submandibular (Wharton’s) duct, which opens near the midline beneath the tongue.

Sublingual Gland.

The third salivary gland is the sublingual gland. It is the smallest and most deeply situated of the salivary glands. Located under the mucous membrane in the floor of the mouth, the two sublingual glands unite anteriorly to form a glandular mass around the lingual frenulum. These glands open into the floor of the mouth by means of a major sublingual duct (Bartholin’s duct) and numerous small sublingual ducts (ducts of Rivinus) along the midline. Sometimes ducts from the sublingual glands may open into the submandibular duct.

Regions of the Brain.

For descriptive purposes, the brain may be divided into the cerebrum, diencephalon, brainstem, and cerebellum.

Cerebrum.

The largest portion of the brain is the cerebrum, which consists of two cerebral hemispheres connected by a mass of white matter called the corpus callosum. The anterior end of the corpus callosum is called the genu, and the posterior end is called the splenium. The deep cleft between the two cerebral hemispheres is the longitudinal fissure. The surface of the cerebrum has numerous convolutions, which greatly increase the surface area of the cerebral cortex. The ridges are called gyri (singular, gyrus), and the furrows between them are called sulci (singular, sulcus).

Superficially, the cerebrum is divided into lobes. A central sulcus separates the frontal lobe from the parietal lobe. Posteriorly, the parieto–occipital sulcus separates the parietal lobe from the occipital lobe. Laterally, the temporal lobe is situated below the lateral fissure (sulcus). A fifth lobe, called the insula, or island of Reil, is located deep within the lateral fissure. The lobes and fissures of the brain are illustrated in Fig. 5-9.

The surface layer of the cerebrum is gray matter, which consists of nerve cell bodies and unmyelinated fibers; this is called the cerebral cortex. It is 2 to 4 mm thick. Lying beneath the cerebral cortex is the lighter colored white matter, which is made up of myelinated nerve fibers.

Scattered throughout the white matter are distinct regions of gray matter called basal ganglia. Two of the larger basal ganglia are the caudate nucleus and the lentiform nucleus, or lenticular nucleus. In sectional anatomy, the caudate nucleus is usually visualized in association with the lateral ventricle. The lentiform nucleus is rather centrally located in each cerebral hemisphere. It is subdivided into the lateral, or external, putamen, and the medial, or internal, globus pallidus. The claustrum, another of the basal ganglia, is a thin layer of gray matter just lateral to the lentiform nucleus and deep to the cortex of the insula. A band of white matter that is medial to the lentiform nucleus is called the internal capsule. The white matter that is present between the lentiform nucleus and the claustrum is the external capsule. The claustrum is separated from the insula by the extreme capsule. Because of their appearance, the caudate nucleus, the internal capsule, and the lentiform nucleus are sometimes referred to as the corpus striatum. Fig. 5-10 illustrates the arrangement of the basal ganglia in a transverse section. The axial computed tomography (CT) image in Fig. 5-11 shows some of the features of the cerebrum and basal ganglia.

Brainstem.

The brainstem is subdivided into the midbrain, the pons, and the medulla oblongata. The smallest division is the midbrain, which is located between the diencephalon and the pons. The midbrain surrounds the cerebral aqueduct, a long, slender channel for the passage of cerebrospinal fluid from the third to the fourth ventricle. Four rounded protuberances, the corpora quadrigemina, are visible on the dorsal aspect of the midbrain. The upper pair, superior colliculi, functions in the visual pathway, whereas the lower pair, the inferior colliculi, functions in the auditory pathway. Just above the corpora quadrigemina is a small glandular structure projecting from the diencephalon. This is the pineal body, or pineal gland. On the ventral aspect of the midbrain are two ropelike bundles called cerebral peduncles. These are composed of motor fibers that extend from the cerebral cortex to the spinal cord. A narrow band of darkly pigmented cells crosses each cerebral peduncle; this is the substantia nigra.

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