Figure 6.4 Major bones of the skeleton. a. Anterior view. b. Posterior view. The bones of the axial skeleton are shown in orange, and those of the appendicular skeleton are shown in yellow.

Axial Skeleton

The skeleton is divided into the axial skeleton and the appendicular skeleton. The tissues of the axial and appendicular skeletons are bone (both compact and spongy), cartilage (hyaline, fibrocartilage, and elastic cartilage), and dense connective tissue, a type of fibrous connective tissue. In Figure 6.4, the bones of the axial skeleton are colored orange, and the bones of the appendicular skeleton are colored yellow for easy distinction. Notice that the axial skeleton lies in the midline of the body and contains the bones of the skull, the hyoid bone, the vertebral column, and the thoracic cage. Six tiny middle ear bones (three in each ear) are also in the axial skeleton.

Skull
The skull is formed by the cranium and the facial bones. These bones contain sinuses (Fig. 6.5), air spaces lined by mucous membranes, that reduce the weight of the skull and give the voice a resonant sound. The paranasal sinuses empty into the nose and are named for their locations. They include the maxillary, frontal, sphenoidal, and ethmoidal sinuses. The two mastoid sinuses drain into the middle ear. Mastoiditis, a condition that can lead to deafness, is an inflammation of these sinuses.

Bones of the Cranium
The cranium protects the brain and is composed of eight bones. These bones are separated from each other by immovable joints called sutures. Newborns have membranous regions called fontanels, where more than two bones meet. The largest of these is the anterior fontanel, which is located where the two parietal bones meet the two parts of the frontal bone. The fontanels permit the bones of the skull to shift during birth as the head passes through the birth canal. The anterior fontanel (often called the “soft spot”) usually closes by the age of two years. Besides the frontal bone, the cranium is composed of two parietal bones, one occipital bone, two temporal bones, one sphenoid bone, and one ethmoid bone (Figs. 6.6 and 6.7).
     Frontal Bone One frontal bone forms the forehead, a portion of the nose, and the superior portions of the orbits (bony sockets of the eyes).
     Parietal Bones Two parietal bones are just posterior to the frontal bone. They form the roof of the cranium and also help form its sides.
     Occipital Bone One occipital bone forms the most posterior part of the skull and the base of the cranium. The spinal cord joins the brain by passing through a large opening in the occipital bone called the foramen magnum. The occipital condyles (Fig. 6.7a) are rounded processes on either side of the foramen magnum that articulate with the first vertebra of the spinal column.

Figure 6.5 Sagittal section of the skull.

Figure 6.6 Skull anatomy. a. Anterior view.

Figure 6.6 Skull anatomy. b. Lateral view.

     Temporal Bones Two temporal bones are just inferior to the parietal bones on the sides of the cranium. They also help form the base of the cranium (Figs. 6.6b and 6.7a). Each temporal bone has the following: external auditory meatus, a canal that leads to the middle ear; mandibular fossa, which articulates with the mandible; mastoid process, which provides a place of attachment for certain neck muscles; styloid process, which provides a place of attachment for muscles associated with the tongue and larynx; zygomatic process, which projects anteriorly and helps form the cheekbone.
     Sphenoid Bone The sphenoid bone helps form the sides and floor of the cranium and the rear wall of the orbits. The sphenoid bone has the shape of a bat and this shape means that it articulates with and holds together the other cranial bones (Fig. 6.7). Within the cranial cavity, the sphenoid bone has a saddle-shaped midportion called the sella turcica (Fig. 6.7b), which houses the pituitary gland in a depression.
     Ethmoid Bone The ethmoid bone is anterior to the sphenoid bone and helps form the floor of the cranium. It contributes to the medial sides of the orbits and forms the roof and sides of the nasal cavity (Figs. 6.6 and 6.7b). The ethmoid bone contains the following: crista galli (cock’s comb), a triangular process that serves as an attachment for membranes that enclose the brain; cribriform plate with tiny holes that serve as passageways for nerve fibers from the olfactory receptors; perpendicular plate (Fig. 6.5), which projects downward to form the nasal septum; superior and middle nasal conchae, which project toward
the perpendicular plate. These projections support mucous membranes that line the nasal cavity.

Bones of the Face
     Maxillae The two maxillae form the upper jaw. Aside from contributing to the floors of the orbits and to the sides of the floor of the nasal cavity, each maxilla has the following
processes: alveolar process (Fig. 6.6a). The alveolar processes contain the tooth sockets for teeth: incisors, canines, premolars, and molars. palatine process (Fig. 6.7a). The left and right palatine processes form the anterior portion of the hard palate (roof of the mouth).
     Palatine Bones The two palatine bones contribute to the floor and lateral wall of the nasal cavity (Fig. 6.5). The horizontal plates of the palatine bones form the posterior portion of the hard palate (Fig. 6.7a). Notice that the hard palate consists of (1) portions of the maxillae (i.e., the palatine processes) and (2) horizontal plates of the palatine bones. A cleft palate results when either (1) or (2) have failed to fuse.
     Zygomatic Bones The two zygomatic bones form the sides of the orbits (Fig. 6.7a). They also contribute to the “cheekbones.” Each zygomatic bone has a temporal process. A zygomatic arch, the most prominent feature of a cheekbone consists of a temporal process connected to a zygomatic process (a portion of the temporal bone).
     Lacrimal Bones The two small, thin lacrimal bones are located on the medial walls of the orbits (Fig. 6.6). A small opening between the orbit and the nasal cavity serves as a pathway for a duct that carries tears from the eyes to the nose.
     Nasal Bones The two nasal bones are small, rectangular bones that form the bridge of the nose (Fig. 6.5). The ventral portion of the nose is cartilage, which explains why the nose is not seen on a skull.
     Vomer Bone The vomer bone joins with the perpendicular plate of the ethmoid bone to form the nasal septum (Figs. 6.5 and 6.6a).
     Inferior Nasal Conchae The two inferior nasal conchae are thin, curved bones that form a part of the inferior lateral wall of the nasal cavity (Fig. 6.6a). Like the superior and middle nasal conchae, they project into the nasal cavity and support the mucous membranes that line the nasal cavity.
     Mandible The mandible, or lower jaw, is the only movable portion of the skull. The horseshoe-shaped front and horizontal sides of the mandible, referred to as the body, form the chin. The body has an alveolar process (Fig. 6.6a), which contains tooth sockets for 16 teeth. Superior to the left and right angle of the mandible are upright projections called rami. Each ramus has the following: mandibular condyle (Fig. 6.6b), which articulates with a temporal bone; coronoid process (Fig. 6.6b), which serves as a place of attachment for the muscles used for chewing.

Hyoid Bone
The U-shaped hyoid bone (Fig. 6.4) is located superior to the larynx (voice box) in the neck. It is the only bone in the body that does not articulate with another bone. Instead, it is suspended from the styloid processes of the temporal bones by the stylohyoid muscles and ligaments. It anchors the tongue and serves as the site for the attachment of several muscles associated with swallowing.

Figure 6.7 Skull anatomy continued. a. Inferior view.

Figure 6.7 Skull anatomy continued. b. Superior view.