components and their included subsidiaries of the axial skeleton include:
•The skull (8 cranial bones and 14 facial bones), as well as bones associated with the skull (6 auditory ossicles and the hyoid bone).
•The thoracic cage (the sternum and 24 ribs).
•The vertebral column (24 vertebrae, the sacrum, and the coccyx).
The skull is the most composite bony structure in the human body. It is formed by a total combination of 22 cranial and facial bones. An additional seven bones associated with the skull that are technically not part of the skull include the six auditory ossicles11 (the stapes, the incus, and the malleus – located bilaterally in the inner ear), and the hyoid bone – situated in a floating position, posterior to the mandible and bilaterally connected to the inferior border of the temporal bones by a pair of ligaments. (For trivia enthusiasts out there, the hyoid bone is the only bone in the human body that does not articulate, or join, with another bone).
■Cranial Bones
The eight cranial bones fuse together to form the cranium, or braincase, protecting the fragile brain and providing stability for the attachment of head and neck muscles.
Figure 4-7 Cranial bones.
The individual bones making up the cranium12 are the:
•Frontal
•Occipital
•Sphenoid
•Ethmoid
•Pairs of parietal and temporal bones
Collectively, the cranial bones form the cranial cavity, a fluid-filled compartment that encases and shields the brain. Articulations, or joints, are formed at the junction of wherever two bones intersect. Except for where the mandible connects to the skull, all joints in the adult human skull are immoveable structures, or seams, called sutures.
Bones are bound tightly together by dense fibrous connective tissue, allowing for the expansion and protection of delicate brain tissue during growth and development. At a young age, ossification of the sutures results in the fusion of the skull bones into a single component, providing an immense protective barrier.
The individual bones of the cranium are important to distinguish because they are anatomically associated with specific blood vessels and corresponding areas of the brain responsible for specific central nervous system functions.
The frontal bone is a shell-shaped structure that forms the anterior aspect of the skull, better known as the forehead. It articulates (joins) with the laterally paired parietal bones via the pronounced coronal suture. The parietal bones are rounded, semi-circular shaped structures that form the lateral and superior aspects of the skull. They are fused together and assemble superiorly at the midline of the cranium.
The occipital bone is a convex, trapezoid-shaped structure that forms the majority of the posterior aspect and base of the skull. It joins anteriorly with the parietal and temporal bones, and articulates inferiorly with the sphenoid bone at the base of the skull.
The two paired temporal bones are relatively smaller cranial bones located on the lateral aspects of the skull. They lie inferior to the parietal bones and constitute the inferiolateral aspect of the skull and a portion of the cranial floor.
Temporal bones were named due to their Latin origin, temporum, meaning “time”; a sign of aging, or time passing, is the appearance of gray hairs – which usually first become visible in the region of the temporal bones.
The sphenoid bone is a centrally positioned, butterfly-shaped structure that shapes the middle cranial fossa. The sphenoid bone is unique in that it articulates with all the other cranial bones in the skull. Because of its rather irregular shape, the sphenoid bone is a difficult structure to study. Its configuration is essential to many of the central nervous system’s fundamental components, and a thorough understanding of the human body’s neuroanatomy would necessitate a complete investigation of this structure.
Finally, the ethmoid bone, similar to the sphenoid bone, has a complex and irregular shape. It is positioned anterior to the sphenoid bone and posterior to the nasal bones of the face, making it the most deeply placed bone in the skull. The ethmoid bone performs an important role in the formation and integrity of the nasal cavity and nasal septum,13 as well as the human body’s sense of smell. At the superior surface of the ethmoid bone are found a pair of bony structures called the cribiform plates. The cribiform plates are perforated with tiny holes called olfactory foramina, which allow for the olfactory nerves to pass through from the brain to the olfactory receptors located in the nasal cavities, making the sense of smell possible.
Craniofacial bones
Fourteen facial bones serve to guard and sustain the openings of two other major organ systems, the digestive and respiratory systems. The facial bones also form the contour of the face, provide supporting cavities for the special sensory organs of sight, taste, and smell, teeth, and allow for the attachment of facial muscles. The fourteen bones of the facial skeleton include the:
•unpaired mandible and vomer
•paired maxillae
•zygomatic bones
•nasal bones
•lacrimal bones
•palatine bones
•inferior conchae bones
The mandible, or lower jaw bone, is the largest and most durable bone of the face. The body of the mandible forms the chin, and the two lateral and upright stems, called rami (or singularly, ramus), intersect the body at its posterior aspect. The rami articulate with the temporal bones at their superior aspects, forming what is referred to as the temporomandibular joint (commonly abbreviated as TMJ).
A major function of the mandible is to support the oral cavity, as well as serve as an anchor for lower teeth. Because of its location, the mandible is one of the most commonly dislocated bones of the face. Although usually not an emergency, dislocation of the mandible commonly presents with other, more serious traumatic injuries depending on the nature of the injury. Treatment of a simple mandibular dislocation (closed reduction) is not recommended in the field, and should be assessed in a proper clinical setting.
Figure 4-8 Craniofacial and skull bone structure.
The paired maxillary bones demarcate the upper jaw and form the central division of the facial skeleton; all facial bones, except the mandible, articulate with the maxillary bones. The maxillae anchor the upper teeth. The specific structure of the maxillae also serves as a passageway for requisite blood vessels and nerves of the face.
The zygomatic bones are a pair of irregularly shaped facial bones commonly referred to as the “cheek bones”. They individually articulate with the zygomatic processes (bony projections originating from a particular bone) of the adjacent temporal bones posteriorly, the zygomatic processes of the frontal bone superiorly, and the zygomatic processes of the maxillary
