Chapter 6 The Skull
The skeleton of the head is a complex articulation of many bones and teeth, which are collectively referred to as the skull or cranium. On the basis of function the skull may be conveniently divided into two main areas: the (1) neurocranium and (2) the facial skeleton (Figure 6-1).
The neurocranium is the rounded vault, or braincase, that houses and protects the brain within. The neurocranium is made up of eight bones, most of which are curved and flat. Most of the bones are united by fibrous sutures; some are united by cartilaginous synchondroses, which ultimately fuse.
The bones of the neurocranium are the frontal bone, the paired parietal bones, the paired temporal bones, the occipital bone, the sphenoid bone, and the ethmoid bone. The floor of the neurocranium consists of the ethmoid, the sphenoid, and a portion of the occipital and temporal bones. Collectively the bones making up the floor of the neurocranium are called the cranial base.
The facial portion of the cranium is suspended from the cranial base, and the face consists of several important functional areas. Two orbits house and protect the organs of sight—the eyeballs. The nasal cavity is associated with respiration and the special sense of smell, and the oral cavity is associated with mastication, taste, and respiration.
The facial skeleton consists of several irregular bones: the paired maxillae, including the maxillary teeth; the paired nasal bones; the paired zygomatic bones; the paired palatine bones; the paired lacrimal bones; the paired inferior conchae; the single vomer; the single mandible, including the mandibular teeth; and the single hyoid bone.
The facial skeleton includes the upper and lower jaws. The upper jaw (right and left maxillae) is fixed to the cranial base; the lower jaw (mandible) is movable and hinged to the cranial base through the temporomandibular joint.
The skull is traditionally studied by rotating the skull to various views. By convention, however, the skull is described as oriented in the horizontal Frankfort plane, which is a plane that joins the uppermost points of the right and left external auditory meatuses (ear holes) and the lowermost points of the right and left orbits. The Frankfort plane is parallel to the floor or tabletop and approximates the anatomical position.
The regions of the skull seen in this view include the forehead; the zygomatic malar area formed by the cheekbones; the orbits, which house the eyeballs; the anterior nasal aperture, to which is affixed the external nose; the fixed upper jaw, which houses the upper teeth; and the lower movable jaw, which contains the lower teeth.
The bones evident in the anterior view (Figure 6-2, A) are the right and left maxillae, nasal bones, the right and left zygomatic bones, the right and left lacrimal bones, the right and left inferior conchae, the ethmoid bone, the vomer, the sphenoid bone, the frontal bone, and the mandible.
The forehead region is formed mainly by a portion of the frontal bone (Figure 6-2, B). The following features are found within the region of the forehead.
The superciliary arches (brow ridges) lie on either side of the glabella, just above the superior margins of the orbits. In men they are prominent, raised ridges of bone; in women they are less noticeable. They are absent in children, becoming prominent during adolescence when the frontal bone is ballooned out from within by the developing frontal air sinuses.
The zygomatic area of the cheekbones is arranged in an arch. Three bones support the zygomatic arch: (1) the maxilla anteriorly, (2) the frontal bone superiorly, and (3) the temporal bone posteriorly. Each bone provides a buttressing zygomatic process that helps support the zygomatic arch. The keystone of the arch is the diamond-shaped zygomatic bone.
The orbits contain the eyeballs and the extraocular muscles (see Figures 6-2 and 6-3). Several bones contribute to the margins (rims) and to the inner walls of the bony orbit.
There are four margins in the orbital area: the superior orbital, lateral, inferior, and medial. The superior orbital margin is formed by the frontal bone, the lateral margin is formed by the zygomatic bone, and the inferior margin is formed by the zygomatic bone laterally and the maxilla medially.
The medial margin is complicated in that it exhibits an anterior aspect and a posterior aspect, which spiral in a slight corkscrew fashion. The anterior crest is formed by the frontal process of the maxilla and a portion of the frontal bone. The posterior crest is formed by the lacrimal bone.
The roof is formed by the frontal bone, and the floor is formed by the maxilla. The lateral wall is formed by the zygomatic bone anteriorly and the greater wing of the sphenoid posteriorly. The medial wall is formed by the following bones from anterior to posterior: maxilla, lacrimal bone, ethmoid bone, and the lesser wing of the sphenoid.
The areas seen from the lateral view are the cranial vault region (including the forehead, scalp, and temporal regions); the facial region (including the zygomatic, orbital, and nasal regions studied in the anterior view); the infratemporal region, which is covered and obscured by the ramus of the mandible; and the mandible (lateral aspect).
The bones seen from the lateral view are the frontal bone (single), parietal bones (paired), temporal bones (paired), occipital bone (single), greater wings of the sphenoid bone (paired processes of a single bone), zygomatic bones (paired), maxillae (paired), nasal bones (paired), lacrimal bones (paired), and the mandible (single). (Figure 6-4, A).
The cranial vault is formed anteriorly by the single frontal bone, laterally by the paired parietal bones, and posteriorly by the single occipital bone. Inferolaterally the walls of the vault are formed by the paired temporal bones and the paired greater wings of the single sphenoid bone. The cranial vault region presents the following features.
The infratemporal region is obscured by the ramus of the mandible, which serves as the lateral wall of the region (Figure 6-5). With the mandible removed, the limits of the infratemporal region can be further delineated. The infratemporal region is separated from the temporal fossa above by an indistinct infratemporal crest.
The superior aspect of the skull presents a somewhat egg-shaped outline with the small end anteriorly (Figure 6-6). Only four bones are seen from this view: the frontal bone anteriorly, the right and left parietal bones laterally, and the occipital bone posteriorly.
The most prominent feature of the posterior view is the rounded posterior pole of the skull, called the occiput (see Figure 6-7). Hence, the area is often referred to as the occipital area.
The bones seen from the basal aspect (Figure 6-8, A) are the right and left maxillae (palatal processes), the right and left palatine bones (palatal processes), the sphenoid bone (body, pterygoid processes, and greater wings), the vomer, the right and left temporal bones, and the occipital bone.
The study of the rather complex basal view of the skull may be simplified by defining the following areas in relation to two imaginary lines. The anterior transverse line joins the right and left articular eminences. The posterior transverse line joins the anterior aspects of the right and left mastoid processes.
The pterygoid process consists of a lateral and a medial plate. The lateral plate provides attachment for both lateral and medial pterygoid muscles. The medial pterygoid plate forms the posterior limit of the lateral wall of the nasal cavity. The medial plate ends inferiorly as the hamulus, a small, slender hook.
The bones seen in this view are the frontal bone, the ethmoid bone, the sphenoid bone (including the body, lesser wings, and greater wings), the right and left temporal bones, and the occipital bone (Figure 6-10, A).