Axial Skeleton
The skeleton is divided into the axial skeleton and the appendicular skeleton. 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. The bones of the skeleton are not smooth; they have protuberances called processes and indentations called depressions, and various types of openings.
Skull
The skull is (ormed by the cranium and the facial bones. These bones contain sinuses, air spaces lined by mucous membranes} that reduce the weight of the skull and give the yoke a resonant sound. The paranasal sinuses empty into the nose and are named for their locations; other sinuses include the maxillary, frontaL sphenoidaL and ethmoidal sinuses. The two mastoid sinuses drain into the middle ear. Mastoiditis (mas"toi-di'tis), a condition that can lead to deafness, is an inflammation of these sinuses.
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 skull to compress 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. The cranial bones are one frontal bone, two parietal bones, one occipital bone, two temporal bones, one sphenoid bone, and one ethmoid bone.
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 dorsal 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 dorsal 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 are rounded processes on either side of the foramen magnum that articulate with the first vertebra of the spinal column.
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 One sphenoid bone helps form the sides and base of the cranium, and the floors and sides of the orbits. Within the cranial cavity, the sphenoid bone has a saddle-shaped midportion called the sella turcica, which houses the pituitary gland in a depression.
Ethmoid Bone One ethmoid bone forms part of the roof of the nasal cavity. 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, 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.
The skull is formed by the cranium and the facial bones. The cranium includes the frontal bone, two parietal bones, two temporal bones, one sphenoid bone and one ethmoid bone.
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