OVERVIEW OF HEAD

CRANIUM

Facial Aspect of Cranium

Lateral Aspect of Cranium

TABLE 7.1. Craniometric Points of Cranium

Occipital Aspect of Cranium

Superior Aspect of Cranium

External Surface of Cranial Base

Internal Surface of Cranial Base

TABLE 7.2. Foramina and Other Apertures of Cranial Fossae and Contents

Walls of Cranial Cavity

Regions of Head

BLUE BOX: Cranium. Head Injuries; Headaches and Facial Pain; Injury to Superciliary Arches; Malar Flush; Fractures of Maxillae and Associated Bones; Fractures of Mandible; Resorption of Alveolar Bone; Fractures of Calvaria; Surgical Access to Cranial Cavity: Bone Flaps; Development of Cranium; Age Changes in Face; Obliteration of Cranial Sutures; Age Changes in Cranium; Craniosynostosis and Cranial Malformations

FACE AND SCALP

Face

Scalp

Muscles of Face and Scalp

TABLE 7.3. Muscles of Scalp and Face

Nerves of Face and Scalp

TABLE 7.4. Cutaneous Nerves of Face and Scalp

Superficial Vasculature of Face and Scalp

TABLE 7.5. Superficial Arteries of Face and Scalp

TABLE 7.6. Veins of Face and Scalp

Surface Anatomy of Face

BLUE BOX: Face and Scalp. Facial Lacerations and Incisions; Scalp Injuries; Scalp Wounds; Scalp Infections; Sebaceous Cysts; Cephalohematoma; Flaring of Nostrils; Paralysis of Facial Muscles; Infra-Orbital Nerve Block; Mental and Incisive Nerve Blocks; Buccal Nerve Block; Trigeminal Neuralgia; Lesions of Trigeminal Nerve; Herpes Zoster Infection of Trigeminal Ganglion; Testing Sensory Function of CN V; Injuries to Facial Nerve; Compression of Facial Artery; Pulses of Arteries of Face and Scalp; Stenosis of Internal Carotid Artery; Scalp Lacerations; Squamous Cell Carcinoma of Lip

CRANIAL MENINGES

Dura Mater

Arachnoid Mater and Pia Mater

Meningeal Spaces

BLUE BOX: Cranial Cavity and Meninges. Fracture of Pterion; Thrombophlebitis of Facial Vein; Blunt Trauma to Head; Tentorial Herniation; Bulging of Diaphragma Sellae; Occlusion of Cerebral Veins and Dural Venous Sinuses; Metastasis of Tumor Cells to Dural Venous Sinuses; Fractures of Cranial Base; Dural Origin of Headaches; Leptomeningitis; Head Injuries and Intracranial Hemorrhage

BRAIN

Parts of Brain

Ventricular System of Brain

Arterial Blood Supply to Brain

Venous Drainage of Brain

TABLE 7.7. Arterial Blood Supply of Cerebral Hemispheres

BLUE BOX: Brain. Cerebral Injuries; Cisternal Puncture; Hydrocephalus; Leakage of Cerebrospinal Fluid; Anastomoses of Cerebral Arteries and Cerebral Embolism; Variations of Cerebral Arterial Circle; Strokes; Brain Infarction; Transient Ischemic Attacks

EYE, ORBIT, ORBITAL REGION, AND EYEBALL

Orbits

Eyelids and Lacrimal Apparatus

Eyeball

Extra-ocular Muscles of Orbit

TABLE 7.8. Extra-ocular Muscles of Orbit

Nerves of Orbit

Vasculature of Orbit

TABLE 7.9. Arteries of Orbit

Surface Anatomy of Eye and Lacrimal Apparatus

BLUE BOX: Orbital Region, Orbit, and Eyeball. Fractures of Orbit; Orbital Tumors; Injury to Nerves Supplying Eyelids; Inflammation of Palpebral Glands; Hyperemia of Conjunctiva; Subconjunctival Hemorrhages; Development of Retina; Retinal Detachment; Pupillary Light Reflex; Uveitis; Ophthalmoscopy; Papilledema; Presbyopia and Cataracts; Coloboma of Iris; Glaucoma; Hemorrhage into Anterior Chamber; Artificial Eye; Corneal Reflex; Corneal Abrasions and Lacerations; Corneal Ulcers and Transplants; Horner Syndrome; Paralysis of Extra-ocular Muscles/Palsies of Orbital Nerves; Oculomotor Nerve Palsy; Abducent Nerve Palsy; Blockage of Central Artery of Retina; Blockage of Central Vein of Retina

PAROTID AND TEMPORAL REGIONS, INFRATEMPORAL FOSSA, AND TEMPOROMANDIBULAR JOINT

Parotid Region

Temporal Region

Infratemporal Fossa

TABLE 7.10. Movements of Temporomandibular Joint

TABLE 7.11. Muscles Acting on Mandible/Temporomandibular Joint

TABLE 7.12. Parts and Branches of Maxillary Artery

BLUE BOX: Parotid and Temporal Regions, Infratemporal Fossa, and Temporomandibular Joint. Parotidectomy; Infection of Parotid Gland; Abscess in Parotid Gland; Sialography of Parotid Duct; Blockage of Parotid Duct; Accessory Parotid Gland; Mandibular Nerve Block; Inferior Alveolar Nerve Block; Dislocation of TMJ; Arthritis of TMJ

ORAL REGION

Oral Cavity

Lips, Cheeks, and Gingivae

Teeth

TABLE 7.13A. Deciduous Teeth

TABLE 7.13B. Permanent Teeth

Palate

TABLE 7.14. Muscles of Soft Palate

Tongue

TABLE 7.15. Muscles of Tongue

Salivary Glands

BLUE BOX: Oral Region. Cleft Lip; Cyanosis of Lips; Large Labial Frenulum; Gingivitis; Dental Caries, Pulpitis, and Tooth Abscesses; Supernumerary Teeth (Hyperdontia); Extraction of Teeth; Dental Implants; Nasopalatine Block; Greater Palatine Block; Cleft Palate; Gag Reflex; Paralysis of Genioglossus; Injury to Hypoglossal Nerve; Sublingual Absorption of Drugs; Lingual Carcinoma; Frenectomy; Excision of Submandibular Gland and Removal of a Calculus; Sialography of Submandibular Ducts

PTERYGOPALATINE FOSSA

Pterygopalatine Part of Maxillary Artery

Maxillary Nerve

BLUE BOX: Pterygopalatine Fossa. Transantral Approach to Pterygopalatine Fossa

NOSE

External Nose

Nasal Cavities

Vasculature and Innervation of Nose

Paranasal Sinuses

BLUE BOX: Nose. Nasal Fractures; Deviation of Nasal Septum; Rhinitis; Epistaxis; Sinusitis; Infection of Ethmoidal Cells; Infection of Maxillary Sinuses; Relationship of Teeth to Maxillary Sinus; Transillumination of Sinuses

EAR

External Ear

Middle Ear

Internal Ear

BLUE BOX: Ear. External Ear Injury; Otoscopic Examination; Acute Otitis Externa; Otitis Media; Perforation of Tympanic Membrane; Mastoiditis; Blockage of Pharyngotympanic Tube; Paralysis of Stapedius; Motion Sickness; Dizziness and Hearing Loss: Ménière Syndrome; High Tone Deafness; Otic Barotrauma

OVERVIEW OF HEAD

The head is the superior part of the body that is attached to the trunk by the neck. It is the control and communications center as well as the “loading dock” for the body. It houses the brain; therefore, it is the site of our consciousness: ideas, creativity, imagination, responses, decision making, and memory. It includes special sensory receivers (eyes, ears, mouth, and nose), broadcast devices for voice and expression, and portals for the intake of fuel (food), water, and oxygen and the exhaust of carbon dioxide.

The head consists of the brain and its protective coverings (cranial vault and meninges), the ears, and the face. The face includes openings and passageways, with lubricating glands and valves (seals) to close some of them, the masticatory (chewing) devices, and the orbits that house the visual apparatus. The face also provides our identity as individuals. Disease, malformation, or trauma of structures in the head form the bases of many specialties, including dentistry, maxillofacial surgery, neurology, neuroradiology, neurosurgery, ophthalmology, oral surgery, otology, rhinology, and psychiatry.

CRANIUM

The cranium (skull¹) is the skeleton of the head (Fig. 7.1A). A series of bones form its two parts, the neurocranium and viscerocranium (Fig. 7.1B). The neurocranium is the bony case of the brain and its membranous coverings, the cranial meninges. It also contains proximal parts of the cranial nerves and the vasculature of the brain. The neurocranium in adults is formed by a series of eight bones: four singular bones centered on the midline (frontal, ethmoidal, sphenoidal, and occipital), and two sets of bones occurring as bilateral pairs (temporal and parietal) (Figs. 7.1A, 7.2A, and 7.3).

FIGURE 7.1. Adult cranium I. A. In the anatomical position, the inferior margin of the orbit and the superior margin of the external acoustic meatus lie in the same horizontal orbitomeatal (Frankfort horizontal) plane. B. The neurocranium and viscerocranium are the two primary functional parts of the cranium. From the lateral aspect, it is apparent that the volume of the neurocranium, housing the brain, is approximately double that of the viscerocranium. C. The unpaired sphenoid and occipital bones make substantial contributions to the cranial base. The spinal cord is continuous with the brain through the foramen magnum, the large opening in the basal part of the occipital bone.

FIGURE 7.2. Adult cranium II. A. The viscerocranium, housing the optical apparatus, nasal cavity, paranasal sinuses, and oral cavity, dominates the facial aspect of the cranium. B and C. The mandible is a major component of the viscerocranium, articulating with the remainder of the cranium via the temporomandibular joint. The broad ramus and coronoid process of the mandible provide attachment for powerful muscles capable of generating great force in relationship to biting and chewing (mastication).

FIGURE 7.3. Adult cranium III. The individual bones of the cranium are color coded. The supra-orbital notch, the infra-orbital foramen, and the mental foramen, giving passage to major sensory nerves of the face, are approximately in a vertical line.

The neurocranium has a dome-like roof, the calvaria (skullcap), and a floor or cranial base (basicranium). The bones forming the calvaria are primarily flat bones (frontal, parietal, and occipital; see Fig. 7.8A) formed by intramembranous ossification of head mesenchyme from the neural crest. The bones contributing to the cranial base are primarily irregular bones with substantial flat portions (sphenoidal and temporal) formed by endochondral ossification of cartilage (chondrocranium) or from more than one type of ossification. The ethmoid bone is an irregular bone that makes a relatively minor midline contribution to the neurocranium but is primarily part of the viscerocranium (see Fig. 7.7A). The so-called flat bones and flat portions of the bones forming the neurocranium are actually curved, with convex external and concave internal surfaces.

FIGURE 7.8. Adult cranium VI: Calvaria. A. The squamous parts of the frontal and occipital bones, and the paired parietal bones contribute to the calvaria. B. The external aspect of the anterior part of the calvaria demonstrates bregma, where the coronal and sagittal sutures meet, and vertex, the superior (topmost) point of the cranium. C. This external view demonstrates a prominent, unilateral parietal foramen. Although emissary foramina often occur in this general location, there is much variation.

FIGURE 7.7. Adult cranium V: Occipital aspect. A. The posterior aspect of the neurocranium, or occiput, is composed of parts of the parietal bones, the occipital bone, and the mastoid parts of the temporal bones. The sagittal and lambdoid sutures meet at the lambda, which can often be felt as a depression in living persons. B. The squamous part of the occipital bone has been removed to expose the anterior part of the posterior cranial fossa.

Most calvarial bones are united by fibrous interlocking sutures (Fig. 7.1A & B); however, during childhood, some bones (sphenoid and occipital) are united by hyaline cartilage (synchondroses). The spinal cord is continuous with the brain through the foramen magnum, a large opening in the cranial base (Fig. 7.1C).

The viscerocranium (facial skeleton) comprises the facial bones that mainly develop in the mesenchyme of the embryonic pharyngeal arches (Moore et al., 2012). The viscerocranium forms the anterior part of the cranium and consists of the bones surrounding the mouth (upper and lower jaws), nose/nasal cavity, and most of the orbits (eye sockets or orbital cavities) (Figs. 7.2 and 7.3).

The viscerocranium consists of 15 irregular bones: 3 singular bones centered on or lying in the midline (mandible, ethmoid, and vomer), and 6 bones occurring as bilateral pairs (maxillae; inferior nasal conchae; and zygomatic, palatine, nasal, and lacrimal bones) (Figs. 7.1A and 7.4A). The maxillae and mandible house the teeth—that is, they provide the sockets and supporting bone for the maxillary and mandibular teeth. The maxillae contribute the greatest part of the upper facial skeleton, forming the skeleton of the upper jaw, which is fixed to the cranial base. The mandible forms the skeleton of the lower jaw, which is movable because it articulates with the cranial base at the temporomandibular joints (Figs. 7.1A and 7.2).

FIGURE 7.4. Adult cranium IV. A. The individual bones of the cranium are color coded. Within the temporal fossa, the pterion is a craniometric point at the junction of the greater wing of the sphenoid, the squamous temporal bone, the frontal, and the parietal bones. B and C. Sutural bones occurring along the temporoparietal (B) and lambdoid (C) sutures are shown.

Several bones of the cranium (frontal, temporal, sphenoid, and ethmoid bones) are pneumatized bones, which contain air spaces (air cells or large sinuses), presumably to decrease their weight (Fig. 7.5). The total volume of the air spaces in these bones increases with age.

FIGURE 7.5. Radiograph of cranium. Pneumatized (air-filled) bones contain sinuses or cells that appear as radiolucencies (dark areas) and bear the name of the occupied bone. The right and left orbital parts of the frontal bone are not superimposed; thus the floor of the anterior cranial fossa appears as two lines (P). (Courtesy of Dr. E. Becker, Associate Professor of Medical Imaging, University of Toronto, Toronto, Ontario, Canada.)

In the anatomical position, the cranium is oriented so that the inferior margin of the orbit and the superior margin of the external acoustic opening of the external acoustic meatus of both sides lie in the same horizontal plane (Fig. 7.1A). This standard craniometric reference is the orbitomeatal plane (Frankfort horizontal plane).

Facial Aspect of Cranium

Features of the anterior or facial (frontal) aspect of the cranium are the frontal and zygomatic bones, orbits, nasal region, maxillae, and mandible (Figs. 7.2 and 7.3).

The frontal bone, specifically its squamous (flat) part, forms the skeleton of the forehead, articulating inferiorly with the nasal and zygomatic bones. In some adults a frontal suture persists; this remnant is called a metopic suture. It is in the middle of the glabella, the smooth, slightly depressed area between the superciliary arches. The frontal suture divides the frontal bones of the fetal cranium (see the blue box “Development of Cranium,” p. 839).

The intersection of the frontal and nasal bones is the nasion (L. nasus, nose), which in most people is related to a distinctly depressed area (bridge of nose) (Figs. 7.1A and 7.2A). The nasion is one of many craniometric points that are used radiographically in medicine (or on dry crania in physical anthropology) to make cranial measurements, compare and describe the topography of the cranium, and document abnormal variations (Fig. 7.6; Table 7.1). The frontal bone also articulates with the lacrimal, ethmoid, and sphenoids; a horizontal portion of bone (orbital part) forms both the roof of the orbit and part of the floor of the anterior part of the cranial cavity (Fig. 7.3).

FIGURE 7.6. Craniometric points.

TABLE 7.1. CRANIOMETRIC POINTS OF CRANIUM

The supra-orbital margin of the frontal bone, the angular boundary between the squamous and orbital parts, has a supra-orbital foramen (notch) in some crania for passage of the supra-orbital nerve and vessels. Just superior to the supra-orbital margin is a ridge, the superciliary arch, that extends laterally on each side from the glabella. The prominence of this ridge, deep to the eyebrows, is generally greater in males (Figs. 7.2A and 7.3).

The zygomatic bones (cheek bones, malar bones), forming the prominences of the cheeks, lie on the inferolateral sides of the orbits and rest on the maxillae. The anterolateral rims, walls, floor, and much of the infra-orbital margins of the orbits are formed by these quadrilateral bones. A small zygomaticofacial foramen pierces the lateral aspect of each bone (Fig. 7.3 and 7.4A). The zygomatic bones articulate with the frontal, sphenoid, and temporal bones and the maxillae.

Inferior to the nasal bones is the pear-shaped piriform aperture, the anterior nasal opening in the cranium (Figs. 7.1A and 7.2A). The bony nasal septum can be observed through this aperture, dividing the nasal cavity into right and left parts. On the lateral wall of each nasal cavity are curved bony plates, the nasal conchae (Figs. 7.2A and 7.3).

The maxillae form the upper jaw; their alveolar processes include the tooth sockets (alveoli) and constitute the supporting bone for the maxillary teeth. The two maxillae are united at the intermaxillary suture in the median plane (Fig. 7.2A). The maxillae surround most of the piriform aperture and form the infra-orbital margins medially. They have a broad connection with the zygomatic bones laterally and an infra-orbital foramen inferior to each orbit for passage of the infra-orbital nerve and vessels (Fig. 7.3).

The mandible is a U-shaped bone with an alveolar process that supports the mandibular teeth. It consists of a horizontal part, the body, and a vertical part, the ramus (Fig. 7.2B & C). Inferior to the second premolar teeth are the mental foramina for the mental nerves and vessels (Figs. 7.1A, 7.2A & B, and 7.3). The mental protuberance, forming the prominence of the chin, is a triangular bony elevation inferior to the mandibular symphysis (L. symphysis menti), the osseous union where the halves of the infantile mandible fuse (Fig. 7.2A & B).

Lateral Aspect of Cranium

The lateral aspect of the cranium is formed by both neurocranium and viscerocranium (Figs. 7.1A & B and 7.4A). The main features of the neurocranial part are the temporal fossa, the external acoustic meatus opening, and the mastoid process of the temporal bone. The main features of the viscerocranial part are the infratemporal fossa, zygomatic arch, and lateral aspects of the maxilla and mandible.

The temporal fossa is bounded superiorly and posteriorly by the superior and inferior temporal lines, anteriorly by the frontal and zygomatic bones, and inferiorly by the zygomatic arch (Figs. 7.1A and 7.4A). The superior border of this arch corresponds to the inferior limit of the cerebral hemisphere of the brain. The zygomatic arch is formed by the union of the temporal process of the zygomatic bone and the zygomatic process of the temporal bone.

In the anterior part of the temporal fossa, 3–4 cm superior to the midpoint of the zygomatic arch, is a clinically important area of bone junctions: the pterion (G. pteron, wing) (Figs. 7.4A and 7.6; Table 7.1). It is usually indicated by an H-shaped formation of sutures that unite the frontal, parietal, sphenoid (greater wing), and temporal bones. Less commonly, the frontal and temporal bones articulate; sometimes all four bones meet at a point.

The external acoustic meatus opening (pore) is the entrance to the external acoustic meatus (canal), which leads to the tympanic membrane (eardrum) (Fig. 7.4A). The mastoid process of the temporal bone is postero-inferior to the external acoustic meatus opening. Anteromedial to the mastoid process is the styloid process of the temporal bone, a slender needle-like, pointed projection. The infratemporal fossa is an irregular space inferior and deep to the zygomatic arch, and the mandible and posterior to the maxilla (see Fig. 7.67B).

FIGURE 7.67. Bony boundaries of temporal and infratemporal fossae. A. The lateral wall of the infratemporal fossa is formed by the ramus of the mandible. The space is deep to the zygomatic arch and is traversed by the temporal muscle and the deep temporal nerves and vessels. Through this interval, the temporal fossa communicates inferiorly with the infratemporal fossa. B. The roof and three walls of the infratemporal fossa are shown. The fossa is an irregularly shaped space posterior to the maxilla (anterior wall). The roof of the fossa is formed by the infratemporal surface of the greater wing of the sphenoid. The medial wall is formed by the lateral pterygoid plate; and the posterior wall is formed by the tympanic plate, styloid process, and mastoid process of the temporal bone. The infratemporal fossa communicates with the pterygopalatine fossa through the pterygomaxillary fissure.

Occipital Aspect of Cranium

The posterior or occipital aspect of the cranium is composed of the occiput (L. back of head, the convex posterior protuberance of the squamous part of the occipital bone), parts of the parietal bones, and mastoid parts of the temporal bones (Fig. 7.7A).

The external occipital protuberance, is usually easily palpable in the median plane; however, occasionally (especially in females) it may be inconspicuous. A craniometric point defined by the tip of the external protuberance is the inion (G. nape of neck) (Figs. 7.1A, 7.4A, and 7.6; Table 7.1). The external occipital crest descends from the protuberance toward the foramen magnum, the large opening in the basal part of the occipital bone (Figs. 7.1C, 7.7B, and 7.9).

FIGURE 7.9. Adult cranium VII. External cranial base. A. The contributing bones are color coded. B. The foramen magnum is located midway between and on a level with the mastoid processes. The hard palate forms both a part of the roof of the mouth and the floor of the nasal cavity. The large choanae on each side of the vomer make up the posterior entrance to the nasal cavities.

The superior nuchal line, marking the superior limit of the neck, extends laterally from each side of the protuberance; the inferior nuchal line is less distinct. In the center of the occiput, lambda indicates the junction of the sagittal and the lambdoid sutures (Figs. 7.1A, 7.6, and 7.7A; Table 7.1). Lambda can sometimes be felt as a depression. One or more sutural bones (accessory bones) may be located at lambda or near the mastoid process (Fig. 7.4B & C).

Superior Aspect of Cranium

The superior (vertical) aspect of the cranium, usually somewhat oval in form, broadens posterolaterally at the parietal eminences (Fig. 7.8A). In some people, frontal eminences are also visible, giving the calvaria an almost square appearance.

The coronal suture separates the frontal and parietal bones (Fig. 7.8A & B), the sagittal suture separates the parietal bones, and the lambdoid suture separates the parietal and temporal bones from the occipital bone (Fig. 7.8A & C). Bregma is the craniometric landmark formed by the intersection of the sagittal and coronal sutures (Figs. 7.6 and 7.8A; Table 7.1). Vertex, the most superior point of the calvaria, is near the midpoint of the sagittal suture (Figs. 7.6 and 7.7A).

The parietal foramen is a small, inconstant aperture located posteriorly in the parietal bone near the sagittal suture (Fig. 7.8A & C); paired parietal foramina may be present. Most irregular, highly variable foramina that occur in the neurocranium are emissary foramina that transmit emissary veins connecting scalp veins to the venous sinuses of the dura mater (see “Scalp,” p. 843).

External Surface of Cranial Base

The cranial base (basicranium) is the inferior portion of the neurocranium (floor of the cranial cavity) and viscerocranium minus the mandible (Fig. 7.9). The external surface of the cranial base features the alveolar arch of the maxillae (the free border of the alveolar processes surrounding and supporting the maxillary teeth); the palatine processes of the maxillae; and the palatine, sphenoid, vomer, temporal, and occipital bones.

The hard palate (bony palate) is formed by the palatal processes of the maxillae anteriorly and the horizontal plates of the palatine bones posteriorly. The free posterior border of the hard palate projects posteriorly in the median plane as the posterior nasal spine. Posterior to the central incisor teeth is the incisive foramen, a depression in the midline of the bony palate into which the incisive canals open.

The right and left nasopalatine nerves pass from the nose through a variable number of incisive canals and foramina (they may be bilateral or merged into a single formation). Posterolaterally are the greater and lesser palatine foramina. Superior to the posterior edge of the palate are two large openings: the choanae (posterior nasal apertures), which are separated from each other by the vomer (L. plowshare), a flat unpaired bone of trapezoidal shape that forms a major part of the bony nasal septum (Fig. 7.9B).

Wedged between the frontal, temporal, and occipital bones is the sphenoid, an irregular unpaired bone that consists of a body and three pairs of processes: greater wings, lesser wings, and pterygoid processes (Fig. 7.10). The greater and lesser wings of the sphenoid spread laterally from the lateral aspects of the body of the bone. The greater wings have orbital, temporal, and infratemporal surfaces apparent in facial, lateral, and inferior views of the exterior of the cranium (Figs. 7.3. 7.4A, and 7.9A) and cerebral surfaces seen in internal views of the cranial base (Fig. 7.11). The pterygoid processes, consisting of lateral and medial pterygoid plates, extend inferiorly on each side of the sphenoid from the junction of the body and greater wings (Figs. 7.9A and 7.10A & B).

FIGURE 7.10. Sphenoid. The unpaired, irregular sphenoid is a pneumatic (air-filled) bone. A. Parts of the thin anterior wall of the body of the sphenoid have been chipped off revealing the interior of the sphenoid sinus, which typically is unevenly divided into separate right and left cavities. B. The superior orbital fissure is a gap between the lesser and greater wings of the sphenoid. The medial and lateral pterygoid plates are components of the pterygoid processes. C. Details of the sella turcica, the midline formation that surrounds the hypophysial fossa, are shown.

The groove for the cartilaginous part of the pharyngotympanic (auditory) tube lies medial to the spine of the sphenoid, inferior to the junction of the greater wing of the sphenoid and the petrous (L. rock-like) part of the temporal bone (Fig. 7.9B). Depressions in the squamous (L. flat) part of the temporal bone, called the mandibular fossae, accommodate the mandibular condyles when the mouth is closed. The cranial base is formed posteriorly by the occipital bone, which articulates with the sphenoid anteriorly.

The four parts of the occipital bone are arranged around the foramen magnum, the most conspicuous feature of the cranial base. The major structures passing through this large foramen are: the spinal cord (where it becomes continuous with the medulla oblongata of the brain); the meninges (coverings) of the brain and spinal cord: the vertebral arteries; the anterior and posterior spinal arteries; and the spinal accessory nerve (CN XI). On the lateral parts of the occipital bone are two large protuberances, the occipital condyles, by which the cranium articulates with the vertebral column.

The large opening between the occipital bone and the petrous part of the temporal bone is the jugular foramen, from which the internal jugular vein (IJV) and several cranial nerves (CN IX–CN XI) emerge from the cranium (Figs. 7.9A and 7.11; Table 7.2). The entrance to the carotid canal for the internal carotid artery is just anterior to the jugular foramen (Fig. 7.9B). The mastoid processes provide for muscle attachments. The stylomastoid foramen, transmitting the facial nerve (CN VII) and stylomastoid artery, lies posterior to the base of the styloid process.

FIGURE 7.11. Cranial foramina.

TABLE 7.2. FORAMINA AND OTHER APERTURES OF CRANIAL FOSSAE AND CONTENTS

^a The internal carotid artery and its accompanying sympathetic and venous plexuses actually pass horizontally across (rather than vertically through) the area of the foramen lacerum, an artifact of dry crania, which is closed by cartilage in life.

Internal Surface of Cranial Base

The internal surface of the cranial base (L. basis cranii interna) has three large depressions that lie at different levels: the anterior, middle, and posterior cranial fossae, which form the bowl-shaped floor of the cranial cavity (Fig. 7.12). The anterior cranial fossa is at the highest level, and the posterior cranial fossa is at the lowest level.

FIGURE 7.12. Adult cranium VII. Internal cranial base. A. The internal aspect demonstrates the contributing bones and features. B. The floor of the cranial cavity is divisible into three levels (steps): anterior, middle, and posterior cranial fossae.

ANTERIOR CRANIAL FOSSA

The inferior and anterior parts of the frontal lobes of the brain occupy the anterior cranial fossa, the shallowest of the three cranial fossae (Fig. 7.12B). The fossa is formed by the frontal bone anteriorly, the ethmoid bone in the middle, and the body and lesser wings of the sphenoid posteriorly. The greater part of the fossa is formed by the orbital parts of the frontal bone, which support the frontal lobes of the brain and form the roofs of the orbits. This surface shows sinuous impressions (brain markings) of the orbital gyri (ridges) of the frontal lobes (Fig. 7.11).

The frontal crest is a median bony extension of the frontal bone (Fig. 7.12A). At its base is the foramen cecum of the frontal bone, which gives passage to vessels during fetal development, but is insignificant postnatally. The crista galli (L. cock’s comb) is a thick, median ridge of bone posterior to the foramen cecum, which projects superiorly from the ethmoid. On each side of this ridge is the sieve-like cribriform plate of ethmoid bone. Its numerous tiny foramina transmit the olfactory nerves (CN I) from the olfactory areas of the nasal cavities to the olfactory bulbs of the brain, which lie on this plate (Fig. 7.12A; Table 7.2).

MIDDLE CRANIAL FOSSA

The butterfly-shaped middle cranial fossa has a central part composed of the sella turcica on the body of the sphenoid and large, depressed lateral parts on each side (Fig. 7.12). The middle cranial fossa is postero-inferior to the anterior cranial fossa, separated from it by the sharp sphenoidal crests laterally and the sphenoidal limbus centrally. The sphenoidal crests are formed mostly by the sharp posterior borders of the lesser wings of the sphenoid bones, which overhang the lateral parts of the fossae anteriorly. The sphenoidal crests end medially in two sharp bony projections, the anterior clinoid processes.

A variably prominent ridge, the limbus of the sphenoid forms the anterior boundary of the transversely oriented prechiasmatic sulcus extending between the right and the left optic canals. The bones forming the lateral parts of the fossa are the greater wings of the sphenoid, and squamous parts of the temporal bones laterally, and the petrous parts of the temporal bones posteriorly. The lateral parts of the middle cranial fossa support the temporal lobes of the brain. The boundary between the middle and the posterior cranial fossae is the superior border of the petrous part of the temporal bone laterally, and a flat plate of bone, the dorsum sellae of the sphenoid, medially.

The sella turcica (L. Turkish saddle) is the saddle-like bony formation on the upper surface of the body of the sphenoid, which is surrounded by the anterior and posterior clinoid processes (Figs. 7.10C and 7.12A). Clinoid means “bedpost,” and the four processes (two anterior and two posterior) surround the hypophysial fossa, the “bed” of the pituitary gland, like the posts of a four-poster bed. The sella turcica is composed of three parts:

1.  The tuberculum sellae (horn of saddle): a variable slight to prominent median elevation forming the posterior boundary of the prechiasmatic sulcus and the anterior boundary of the hypophysial fossa.

2.  The hypophysial fossa (pituitary fossa): a median depression (seat of saddle) in the body of the sphenoid that accommodates the pituitary gland (L. hypophysis).

3.  The dorsum sellae (back of saddle): a square plate of bone projecting superiorly from the body of the sphenoid. It forms the posterior boundary of the sella turcica, and its prominent superolateral angles make up the posterior clinoid processes.

On each side of the body of the sphenoid, a crescent of four foramina perforate the roots of the cerebral surfaces of the greater wings of the sphenoids (Figs. 7.10C, 7.11, and 7.12A); structures transmitted by the foramina are listed in Table 7.2:

1.  Superior orbital fissure: Located between the greater and the lesser wings, it opens anteriorly into the orbit (Fig. 7.2A).

2.  Foramen rotundum (round foramen): Located posterior to the medial end of the superior orbital fissure, it runs a horizontal course to an opening on the anterior aspect of the root of the greater wing of the sphenoid (Figs. 7.10A and 7.11A) into a bony formation between the sphenoid, the maxilla, and the palatine bones, the pterygopalatine fossa.

3.  Foramen ovale (oval foramen): A large foramen posterolateral to the foramen rotundum; it opens inferiorly into the infratemporal fossa (Fig. 7.9B).

4.  Foramen spinosum (spinous foramen): Located posterolateral to the foramen ovale and opens into the infratemporal fossa in relationship to the spine of the sphenoid (Fig. 7.11).

The foramen lacerum (lacerated or torn foramen) is not part of the crescent of foramina. This ragged foramen lies posterolateral to the hypophysial fossa, and is an artifact of a dried cranium (Fig. 7.12A). In life, it is closed by a cartilage plate. Only some meningeal arterial branches and small veins are transmitted vertically through the cartilage, completely traversing this foramen. The internal carotid artery and its accompanying sympathetic and venous plexuses pass across the superior aspect of the cartilage (i.e., pass over the foramen), and some nerves traverse it horizontally, passing to a foramen in its anterior boundary.

Extending posteriorly and laterally from the foramen lacerum is a narrow groove for the greater petrosal nerve on the anterosuperior surface of the petrous part of the temporal bone. There is also a small groove for the lesser petrosal nerve.

POSTERIOR CRANIAL FOSSA

The posterior cranial fossa, the largest and deepest of the three cranial fossae, lodges the cerebellum, pons, and medulla oblongata (Fig. 7.12B). The posterior cranial fossa is formed mostly by the occipital bone, but the dorsum sellae of the sphenoid marks its anterior boundary centrally (Fig. 7.12A), and the petrous and mastoid parts of the temporal bones contribute its anterolateral “walls.”

From the dorsum sellae there is a marked incline, the clivus, in the center of the anterior part of the fossa leading to the foramen magnum. Posterior to this large opening, the posterior cranial fossa is partly divided by the internal occipital crest into bilateral large concave impressions, the cerebellar fossae. The internal occipital crest ends in the internal occipital protuberance formed in relationship to the confluence of the sinuses, a merging of dural venous sinuses (discussed later on page 867).

Broad grooves show the horizontal course of the transverse sinus and the S-shaped sigmoid sinus. At the base of the petrous ridge of the temporal bone is the jugular foramen, which transmits several cranial nerves in addition to the sigmoid sinus that exits the cranium as the internal jugular vein (IJV) (Fig. 7.11; Table 7.2). Anterosuperior to the jugular foramen is the internal acoustic meatus for the facial (CN VII) and vestibulocochlear nerves (CN VIII), and the labyrinthine artery. The hypoglossal canal for the hypoglossal nerve (CN XII) is superior to the anterolateral margin of the foramen magnum.

Walls of Cranial Cavity

The walls of the cranial cavity vary in thickness in different regions. They are usually thinner in females than in males and are thinner in children and elderly people. The bones tend to be thinnest in areas that are well covered with muscles, such as the squamous part of the temporal bone (Fig. 7.11). Thin areas of bone can be seen radiographically (Fig. 7.5), or by holding a dried cranium up to a bright light.

Most bones of the calvaria consist of internal and external tables of compact bone, separated by diploë (Figs. 7.5 and 7.11). The diploë is cancellous bone containing red bone marrow during life, through which run canals formed by diploic veins. The diploë in a dried calvaria is not red because the protein was removed during preparation of the cranium. The internal table of bone is thinner than the external table, and in some areas there is only a thin plate of compact bone with no diploë.

The bony substance of the cranium is unequally distributed. Relatively thin (but mostly curved) flat bones provide the necessary strength to maintain cavities and protect their contents. However, in addition to housing the brain, the bones of the neurocranium (and processes from them) provide proximal attachment for the strong muscles of mastication that attach distally to the mandible; consequently, high traction forces occur across the nasal cavity and orbits that are sandwiched between. Thus thickened portions of the cranial bones form stronger pillars or buttresses that transmit forces, bypassing the orbits and nasal cavity (Fig. 7.13). The main buttresses are the frontonasal buttress, extending from the region of the canine teeth between the nasal and the orbital cavities to the central frontal bone, and the zygomatic arch–lateral orbital margin buttress from the region of the molars to the lateral frontal and temporal bones. Similarly, occipital buttresses transmit forces received lateral to the foramen magnum from the vertebral column. Perhaps to compensate for the denser bone required for these buttresses, some areas of the cranium not as mechanically stressed become pneumatized (air-filled).

FIGURE 7.13. Buttresses of cranium. The buttresses are thicker portions of cranial bone that transmit forces around weaker regions of the cranium.

Regions of Head

To allow clear communications regarding the location of structures, injuries, or pathologies, the head is divided into regions (Fig. 7.14). The large number of regions into which the relatively small area of the face is divided (eight) is a reflection of both its functional complexity and personal importance, as are annual expenditures for elective aesthetic surgery. With the exception of the auricular region, which includes the external ear, the names of the regions of the neurocranial portion of the head correspond to the underlying bones or bony features: frontal, parietal, occipital, temporal, and mastoid regions.

FIGURE 7.14. Regions of head.

The viscerocranial portion of the head includes the facial region, which is divided into five bilateral and three median regions related to superficial features (oral and buccal regions), to deeper soft tissue formations (parotid region), and to skeletal features (orbital, infra-orbital, nasal, zygomatic, and mental regions). The remainder of this chapter discusses several of these regions in detail, as well as some deep regions not represented on the surface (for example, the infratemporal region and pterygopalatine fossa). The surface anatomy of these regions will be discussed with the description of each region.

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