132 The Axial Skeleton LESSON 4.2 The Skull The 22 bones of the skull are divided into two groups: the cranial and the facial bones (Figure 4.10). The thin, curved bones of the cranium surround and protect the delicate brain. The round shape of the cranium is structurally strong in resisting impact forces and tends to absorb less force than if it were composed of flat surfaces. The facial bones protect the front of the head, give people’s faces their individual shapes, protect and orient the eyes, and allow chewing of food. Most of the bones of the skull and face are joined together by irregularly shaped, interlocking, and immovable joints called sutures. A suture is a joint in which bones of the skull are bound together by strong, tiny fibers. The sutures permit a very small amount of movement, which contributes to the compliance (ability to change size and shape in response to force) and elasticity of the skull. The one exception is the mandible, or jawbone, which is attached to the skull by a movable joint (for obvious reasons!). The skull of a newborn infant is quite different from the adult skull. Compared to adults, babies have big heads relative to the size of their bodies. Whereas the skull of an adult accounts for about one-eighth of total body height, the skull of a newborn represents about one-fourth of body height. As previously discussed, the skeletal system of children includes regions of hyaline cartilage. The sutures of the skull in infants are composed of this soft, connective tissue, which will ossify (turn to bone) in early childhood. In regions of the infant skull where several bones join together, there are openings connected only by pockets of fibrous membranes. Because the baby’s pulse can be felt through these “soft spots,” they have been given the name fontanel (fahn-ta-NEHL), which is French for “little fountain.” Before You Read Try to answer the following questions before you read this lesson. Why is the skull composed of multiple bones? Why is the spine curved instead of straight? Lesson Objectives • Identify the bones of the cranium and face and describe their locations. • Describe the structure of a typical vertebra and explain how the atlas, axis, and other cervical, thoracic, and lumbar vertebrae are specialized for their specific functions. • Discuss the structure and functional importance of the thoracic cage. Key Terms atlas axial skeleton axis cervical region coccyx cranium facial bones fontanel intervertebral discs lumbar region mandible maxillary bones process sacrum skull sternum sutures thoracic cage thoracic region vertebrae vomer Anatomists divide the body into the axis, consisting of the head and trunk, and the appendages—the arms and legs (Figure 4.9). Consequently, the major bones of the axis are known as the axial skeleton, which includes three major parts—the skull, vertebral (spinal) column, and thoracic cage. The axial skeleton is designed to provide stability to the core of the body. This lesson discusses the axial skeleton. Lesson 4.3 describes the appendicular skeleton. Copyright Goodheart-Willcox Co., Inc.