Usually, this process begins to occur between the ages of 30 and 40, when osteoclastic activity disproportionately increases in comparison to osteoblastic processes. Once this reduction in ossification, women tend to lose approximately eight percent of their skeletal mass every ten years, while men only lose approximately three percent of their skeletal mass in the same amount of time – mainly through hormonal decreases. Not all components of the skeleton are affected equally – the epiphysis, vertebrae, and mandible lose a higher percentage of their portions – resulting in easily broken limbs, a decrease in height, and the loss of teeth, respectively.
■Divisions of the Skeleton
The human skeleton contains a total of 206 individual bones and several accompanying cartilages and ligaments. The skeletal system is divided into two main divisions:
•the axial skeleton
•the appendicular skeleton
The axial skeleton, which encompasses the main mechanical core of the human body, contains exactly 80 bones, while the appendicular skeleton, which includes the bones of the limbs and their respective attachments to the pectoral and pelvic girdles, contains 126 bones. We begin our discussion with a closer look at the axial skeleton.
■Axial Skeleton
The axial skeleton offers a support system that functions to ensure the protection of the vital organs located in the ventral and dorsal body cavities.
Figure 4-6 Axial skeleton structure.
It also serves as a platform for the attachment of skeletal muscles responsible for:
•movement of the head, neck, and trunk
•aiding respiratory processes
•stabilization and security of inserting components of the appendicular skeleton
While movement is limited in the axial skeleton, the bones and joints are solidly fortified with ligaments, and therefore are exceptionally strong and durable. The principal 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