The human skull consists of 28 bones (including the three middle-ear ossicles), most of which are very tightly articulated or fused together. This constitutes a reduction of the cranial arrangement characteristic of primitive mammals (e.g., several elements fuse to form the human temporal or occipital bones). Nevertheless, mammals as a rule have fewer skull bones than do reptiles. There are several excellent general accounts of the anatomy of the mammalian skull (e.g., Romer and Parsons, 1977; Barghusen and Hopson, 1979; Novacek, 1993) that supply detailed information. The summary presented here is based partly on these accounts.
The braincase of mammals is generally much larger than that of reptiles. Besides protecting the brain, it provides a surface of origin for the temporalis muscles (used in mastication) laterally, and for neck muscles on the posterior surface, or occiput. Additional attachment area for these muscles is provided by the midline sagittal crest and the transversely oriented lambdoidal (=nuchal or occipital) crest at the top of the occiput. Typically the braincase consists of paired frontals (fused in humans) and parietals dorsally, an occipital at the back, and paired squamosals laterally, below the frontals and parietals (Fig. 2.1). An interparietal may be present between the parietals and the occipital. Develop-mentally the occipital bone consists of several elements, including the supraoccipital, the basioccipital, and the paired exoccipitals (which include the occipital condyles and surround the foramen magnum, through which the spinal cord passes), but these are usually fused in adults. The squamosal bones contain the glenoid fossa posteriorly, for articulation with the condylar process of the dentary, and form the back of the zygomatic arch. Anterior to the basioccipital on the ventral surface are the basisphenoid and presphenoid; paired alisphenoids and orbitosphenoids extend laterally from the basisphenoid and presphenoid to form part of the lateral wall of the braincase. Commonly these bones fuse to form the complexly shaped sphenoid bone (as in humans). A delicate midline bone, the ethmoid, forms part of the floor of the braincase and extends into the upper nasal cavity.
Situated more or less between the squamosal and the basioccipital is the auditory region of the basicranium, which contains the tympanic or middle-ear cavity, with its three tiny auditory ossicles (malleus, incus, and stapes), and the inner ear, with its bony labyrinth enclosing the cochlea and semicircular canals. These are the organs of hearing and balance. The ossicular chain extends from the tympanic membrane (eardrum) laterally, to the fenestra ovalis (oval window) on the ventrolateral wall of the petrosal, which receives the footplate of the stapes. The eardrum is usually supported by a ringlike or tubular ectotympanic (=tympanic) bone. Within the dense petrosal bone, or otic capsule, is the inner ear. The cochlear canal in living mammals (except monotremes) is coiled, resembling a snail shell. There are always at least 1- turns (about 2- in humans). A coiled cochlea can accommodate a longer basilar membrane—which sup ports the spiral organ for hearing, within the cochlea—in a smaller space, and is therefore usually a good indicator of auditory acuity.
In many mammals, as in humans, the petrosal, ectotym-panic, and squamosal bones synostose to form the temporal bone. The tympanic cavity and otic capsule in mammals are typically surrounded and protected by a bubble-like bony structure, the auditory bulla, behind which the mastoid portion of the petrosal is often exposed. The bulla, which forms the floor of the tympanic region, is a mammalian innovation. When present in marsupials it usually forms from the al-isphenoid, whereas in placentals it is variously constructed of the ectotympanic, entotympanic, petrosal, or a combination of these or other elements. The bony anatomy of the auditory region, particularly bullar composition, and the pattern of vascular grooves on the ventral surface of the petrosal created by branches of the internal carotid artery (which usually flows through this region en route to the brain) are important considerations in mammalian systematics.
The basicranium is also important because of its numerous foramina that transmit the 12 pairs of cranial nerves and various vessels to and from the brain. The nerves and vessels do not actually penetrate the basicranium; instead, during development the bone ossifies around them. Cranial nerves (CNs) serve many vital functions: they are responsible for the special senses, control muscles and supply sensory innervation to the head and neck, and provide parasympa-thetic autonomic innervation to thoracic and abdominal viscera as well as glands and smooth muscle in the head. They are numbered from front to back as they emerge from the base of the brain. The configuration of the basicranial foramina through which the nerves enter or leave the cranial cavity also weighs heavily in phylogenetic interpretations. The foramina and nerves may be summarized as follows:
Cribriform plate of the ethmoid bone—A perforated bone at the anterior floor of the braincase, through which nerve bundles of CN I, the olfactory nerve (which provides the sense of smell), pass from the roof of the nasal cavity to reach the olfactory bulbs of the brain. Optic foramen—The opening that transmits CN II, the optic nerve (the nerve of vision), through the orbito-sphenoid to the orbit and the eyeball. Sphenorbital foramen (=anterior lacerate foramen, superior orbital fissure)—An opening between the orbito-sphenoid and alisphenoid through which CNs III, IV V1, and VI reach the orbit. CN III (oculomotor nerve), IV (trochlear nerve), and VI (abducent nerve) supply muscles that move the eye; V1 (the first division of the trigeminal nerve, called the ophthalmic nerve) is sensory to the eye, orbit, and forehead. Foramen rotundum—A hole in the alisphenoid that is the usual pathway of CN V2 (the second or maxillary division of the trigeminal nerve) to the floor of the orbit, where the nerve gives off sensory branches to the maxillary sinus and upper teeth. Its termination passes
Fig. 2.1. Anatomy of the skull of a primitive mammal (Monodelphis). The dental formula exhibited by this marsupial is 126.96.36.199/188.8.131.52. Key: an, angular process; as, alisphenoid; astp, alisphenoid tympanic process; bo, basioccipital; bs, basisphenoid; C, canines; coc, coronoid crest; con, mandibular condyle; cor, coronoid process; ec, ectotympanic; eo, exoccipital; fc, fenestra cochleae; fT, frontal; frp, frontal process of the jugal; ham, hamulus; I, incisors; inf, incisive foramen; iof, infraorbital foramen; ip, interparietal; ju, jugal; lac, lacrimal; lacf, lacrimal foramen; M, molars; maf, masseteric fossa; mapf, major palatine foramen; mf, mental foramina; mpf, minor palatine foramen; mx, maxilla; na, nasal; oc, occipital condyle; P, premolars; pa, parietal; pal, palatine; pcp, paracondylar process of the exoccipital; pe, petrosal; pmx, premaxilla; ppt, postpalatine torus; ps, presphenoid; pt, pterygoid; ptp, post-tympanic process; rtpp, rostral tympanic process of the petrosal; so, supraoccipital; sq, squamosal; tl, temporal line. (Modified from Wible, 2003.)
through the infraorbital foramen in the maxilla, emerging onto the snout or face to provide sensory innervation to this area.
Foramen ovale—The opening that transmits CN V3 (third or mandibular division of the trigeminal nerve) through the alisphenoid to the mouth, where it supplies the masticatory muscles and is sensory to the cheek and most of the tongue. One branch enters the dentary posteromedially through the mandibular foramen to supply the lower teeth and gums, and emerges through one or more mental foramina (anterolaterally on the dentary) to provide sensation to the chin area.
Internal acoustic meatus—Visible only from inside the cranial cavity, this opening in the petrosal bone transmits CN VII (facial nerve) and CN VIII (vestibulocochlear nerve) into the ear region, where the latter runs to ganglia in the inner ear associated with hearing and balance. After giving off several branches within the pet-rosal (including two special sensory branches involved with taste), CN VII emerges through the stylomastoid foramen at the back of the basicranium to supply the muscles of the snout or face. Posterior lacerate foramen (=jugular foramen)—This large foramen between the basioccipital and the otic capsule transmits CNs IX (glossopharyngeal nerve), X (vagus nerve), and XI (accessory nerve), as well as the internal jugular vein. CN IX innervates aspects of the tongue, pharynx, and middle ear, and CN X innervates the pharynx, larynx, most palatal muscles, and thoracic and abdominal viscera. CN XI supplies two muscles of the neck and back (sternocleidomastoid and trapezius). Hypoglossal canal (=anterior condyloid foramen)—Paired or multiple openings within the foramen magnum and just anterior to the occipital condyles, which transmit CN XII (hypoglossal nerve) to the tongue muscles.
A few other cranial openings are of note. The carotid canal carries the internal carotid artery into the cranial cavity to supply the brain. Upon entering the basicranium, usually near the posterior lacerate foramen, the artery traverses the carotid canal in the alisphenoid and emerges into the cranial cavity immediately above the middle lacerate foramen (=lacerate foramen of human anatomy) at the front of the alisphenoid. An opening at the front of the auditory bulla joins the middle-ear cavity to the back of the throat via the cartilaginous part of the auditory (eustachian) tube.
The facial skeleton or snout includes the bones around the orbit (except the frontal), the nose, and the mouth. The paired maxillae hold most of the upper teeth and make up a large part of the secondary (hard) palate, a mammalian characteristic, which separates the oral and nasal cavities. They also usually form the front of the zygomatic arches and often contribute to the floor or anterior rim of the orbit. Enclosed within each maxilla is a large cavity, or sinus, which adjoins the nasal cavity. Similar sinuses are found in the sphenoid, ethmoid, and occasionally the frontal and various basicranial bones. The pneumatization created by these sinuses reduces the weight of the skull, contributes to vocal resonance, and at the same time provides advantageous muscle attachments. The premaxillae contain the incisors and form the front of the palate and the anterolateral wall of the nasal cavity. The back of the palate consists of the palatine bones, which also define the lower margin of the choanae, or internal nares. The bony palate has paired incisive foramina in front and palatine foramina at the back, which carry nerves and vessels. In some mammals, the incisive foramina lead to Jacobson's organ (=vomeronasal or accessory nasal organ). Marsupials often have additional openings in the palate, called vacuities. Behind and above the palatines, and anterior to the presphenoid, is a small mid-line element, the vomer, which, together with the ethmoid, divides the two nasal cavities. Attached to the lateral walls of the nasal cavity are the turbinals: delicate, scroll-like struc tures of cartilage or bone that expand the surface area of the nasal cavity. The lower one on each side is a separate element, the inferior turbinate (=maxilloturbinal or inferior nasal concha). Situated medial to the maxillae and pre-maxillae and behind the external nares are the nasals. At the anteromedial margin of the orbit are the lacrimal bones, pierced by the nasolacrimal (=lacrimal) canal, which contains a duct that drains lacrimal fluid from the eye into the nose. The paired zygomatic or jugal (=malar) bones are positioned between the maxilla and squamosal on each side. The zygomatic thus forms the middle of the zygomatic arch, a bony bar on the outside of the orbitotemporal fossa, which protects the eye and provides attachment area for the masseter muscle.
The mandible or lower jaw in mammals, which contains all the lower teeth, consists of a pair of dentaries, in contrast to the multi-element lower jaw of reptiles and non-mammalian therapsids. The two dentaries are either joined by ligaments or co-ossified at the front (the mandibular sym-physis). Behind the toothrow is the ascending ramus, with a coronoid process for attachment of the temporalis muscle, and a condyle that articulates with the squamosal. The medial side of the condylar neck is also the site of insertion of the lateral pterygoid muscle. At the posteroinferior margin of the jaw is the angular process, for attachment of the mas-seter (laterally) and medial pterygoid muscles (medially). These are all chewing muscles, supplied by CN V3.
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