Bones of the Axial Skeleton

The skeleton may be divided into two main groups of bones (see Fig. 3-1):
* The axial skeleton consists of 80 bones and includes the bony framework of the head and the trunk.
* The appendicular skeleton consists of 126 bones and forms the framework for the extremities (limbs) and for the shoulders and hips.

Framework of the Skull
The bony framework of the head, called the skull, is subdivided into two parts: the cranium and the facial portion. Refer to Figures 3-5 through 3-8, which show different views of the skull, as you study the following descriptions. Color-coding of the bones will aid in identification as the skull is seen from different positions.
Cranium This rounded chamber that encloses the brain is composed of eight distinct cranial bones.
* The frontal bone forms the forehead, the anterior of the skull’s roof, and the roof of the eye orbit (socket). The frontal sinuses (air spaces) communicate with the nasal cavities (see Figs. 3-7 and 3-8). These sinuses and others near the nose are described as paranasal sinuses.
* The two parietal bones form most of the top and the side walls of the cranium.
* The two temporal bones form part of the sides and some of the base of the skull. Each one contains mastoid sinuses as well as the ear canal, the eardrum, and the entire middle and internal portions of the ear. The mastoid process of the temporal bone projects downward immediately behind the external part of the ear. It contains the mastoid air cells and serves as a place for muscle attachment.
* The ethmoid bone is a light, fragile bone located between the eyes (see Fig. 3-7). It forms a part of the medial wall of the eye orbit, a small portion of the cranial floor, and most of the nasal cavity roof. It contains several air spaces, comprising some of the paranasal sinuses. A thin, platelike, downward extension of this bone (the perpendicular plate) forms much of the nasal septum, a midline partition in the nose (see Fig. 3-5 A)
* The sphenoid bone, when seen from a superior view, resembles a bat with its wings extended. It lies at the base of the skull anterior to the temporal bones and forms part of the eye socket. The sphenoid contains a saddlelike depression, the sella turcica, that holds and protects the pituitary gland (see Fig. 3-7).

Figure 3-1 The skeleton. The axial skeleton is shown in yellow; the appendicular, in blue.

Figure 3-5 The skull. (A) Anterior view. (B) Left lateral view. (C) Superior view.

* The occipital bone forms the posterior and a part of the base of the skull. The foramen magnum, located at the base of the occipital bone, is a large opening through which the spinal cord communicates with the brain (see Figs. 3-6 and 3-7). Uniting the bones of the skull is a type of flat, immovable joint known as a suture (see Fig. 3-5). Some of the most prominent cranial sutures are the:
* Coronal suture, which joins the frontal bone with the two parietal bones along the coronal plane;
* Squamous suture, which joins the temporal bone to the parietal bone on the lateral surface of the cranium (named because it is in a flat portion of the skull);
* Lambdoid suture, which joins the occipital bone with the parietal bones in the posterior cranium (named because it resembles the Greek letter lambda);
* Sagittal suture, which joins the two parietal bones along the superior midline of the cranium, along the sagittal plane.

Figure 3-7 Floor of cranium, superior view.

Figure 3-8 The skull, sagittal section.

Figure 3-6 The skull, inferior view. The mandible (lower jaw) has been removed.

Facial Bones The facial portion of the skull is composed of 14 bones (see Fig. 3-5):
* The mandible, or lower jaw bone, is the only movable bone of the skull.
* The two maxillae fuse in the midline to form the upper jaw bone, including the front part of the hard palate (roof of the mouth). Each maxilla contains a large air space, called the maxillary sinus, that communicates with the nasal cavity.
* The two zygomatic bones, one on each side, form the prominences of the cheeks.
* Two slender nasal bones lie side by side, forming the bridge of the nose.
* The two lacrimal bones, each about the size of a fingernail, lie near the inside corner of the eye in the front part of the medial wall of the orbital cavity.
* The vomer, shaped like the blade of a plow, forms the lower part of the nasal septum (see Fig. 3-5 A).
* The paired palatine bones form the back part of the hard palate (see Figs. 3-6 and 3-8).
* The two inferior nasal conchae extend horizontally along the lateral wall (sides) of the nasal cavities. The paired superior and middle conchae are part of the ethmoid bone (see Figs. 3-5 A and 3-8).

In addition to the bones of the cranium and the facial bones, there are three tiny bones, or ossicles, in each middle ear and a single horseshoe, or U-shaped, bone just below the skull proper, called the hyoid bone, to which the tongue and other muscles are attached (see Fig. 3-5 B). Openings in the base of the skull provide spaces for the entrance and exit of many blood vessels, nerves, and other structures. Projections and slightly elevated portions of the bones provide for the attachment of muscles. Some portions protect delicate structures, such as the eye orbit and the part of the temporal bone that encloses the inner portions of the ear. The sinuses provide lightness and serve as resonating chambers for the voice (which is why your voice sounds better to you as you are speaking than it sounds when you hear it played back as a recording).

Infant Skull The skull of the infant has areas in which the bone formation is incomplete, leaving so-called soft
spots, properly called fontanels (Fig. 3-9). These flexible regions allow the skull to compress and change shape during the birth process. They also allow for rapid growth of the brain during infancy. Although there are a number of fontanels, the largest and most recognizable is near the front of the skull at the junction of the two parietal bones and the frontal bone. This anterior fontanel usually does not close until the child is about 18 months old.

Figure 3-9 Infant skull, showing fontanels.

Framework of the Trunk
The bones of the trunk include the spine, or vertebral, column, and the bones of the chest, or thorax.
Vertebral Column This bony sheath for the spinal cord is made of a series of irregularly shaped bones. These number 33 or 34 in the child, but because of fusions that occur later in the lower part of the spine, there usually are just 26 separate bones in the adult spinal column.

Figures 3-10 and 3-11 show the vertebral column from lateral and anterior views. The vertebrae have a drum-shaped body (centrum) located anteriorly (toward the front) that serves as the weight-bearing part; disks of cartilage between the vertebral bodies act as shock absorbers and provide flexibility (see Fig. 3-11). In the center of each vertebra is a large hole, or foramen. When all the vertebrae are linked in series by strong connective tissue bands (ligaments), these spaces form the spinal canal, a bony cylinder that protects the spinal cord. Projecting dorsally (toward the back) from the bony arch that encircles the spinal cord is the spinous process, which usually can be felt just under the skin of the back. Projecting laterally is a transverse process on each side. These processes are attachment points for muscles. When viewed from a lateral aspect, the vertebral column can be seen to have a series of intervertebral foramina, formed between the vertebrae as they join together, through which spinal nerves emerge as they leave the spinal cord (see Fig. 3-10). The bones of the vertebral column are named and numbered from above downward, on the basis of location. There are five regions:

* The cervical vertebrae, seven in number (C1 to C7), are located in the neck (see Fig. 3-11). The first vertebra, called the atlas, supports the head (Fig. 3-12). (This vertebra is named for the mythologic character who was able to support the world in his hands.) When one nods the head, the skull rocks on the atlas at the occipital bone. The second cervical vertebra, the axis (see Fig. 3-12), serves as a pivot when the head is turned from side to side. It has an upright toothlike part, the dens, that projects into the atlas and serves as a pivot point. The absence of a body in these vertebrae allows for the extra movement. Only the cervical vertebrae have a hole in the tranverse process on each side (see Fig. 3-11). These transverse foramina accommodate blood vessels and nerves that supply the neck and head.
* The thoracic vertebrae, 12 in number (T1 to T12), are located in the chest. They are larger and stronger than the cervical vertebrae and each has a longer spinous process that points downward (see Fig. 3-11). The posterior ends of the 12 pairs of ribs are attached to these
vertebrae.
* The lumbar vertebrae, five in number (L1 to L5), are located in the small of the back. They are larger and heavier than the vertebrae superior to them and can support more weight (see Fig. 3-11). All of their processes are shorter and thicker.
* The sacral vertebrae are five separate bones in the child. They eventually fuse to form a single bone, called the sacrum, in the adult. Wedged between the two hip bones, the sacrum completes the posterior part of the bony pelvis.
* The coccygeal vertebrae consist of four or five tiny bones in the child. These later fuse to form a single bone, the coccyx, or tail bone, in the adult.

Figure 3-10 Vertebral column, left lateral view.

Figure 3-11 The vertebral column and vertebrae.

Figure 3-12 The first two cervical vertebrae. (A) The atlas (1st cervical vertebra), superior view. (B) The axis (2nd cervical vertebra), superior view. (C) The axis, lateral view.

Curves of the Spine When viewed from the side, the vertebral column can be seen to have four curves, corresponding to the four groups of vertebrae (see Fig. 3-10). In the fetus, the entire column is concave forward (curves away from a viewer facing the fetus). This is the primary curve. When an infant begins to assume an erect posture, secondary curves develop. These curves are convex (curve toward the viewer). The cervical curve appears when the head is held up at about 3 months of age; the lumbar curve appears when the child begins to walk. The thoracic and sacral curves remain the two primary curves. These curves of the vertebral column provide some of the resilience and spring so essential in balance and movement.

Thorax The bones of the thorax form a cone-shaped cage (Fig. 3-14). Twelve pairs of ribs form the bars of this cage, completed by the sternum, or breastbone, anteriorly. These bones enclose and protect the heart, lungs, and other organs contained in the thorax. The superior portion of the sternum is the broadly T-shaped manubrium that joins laterally on the right and left with the clavicle (collarbone). The point on the manubrium where the clavicle joins can be seen on Figure 3-14 as the clavicular notch. Laterally, the manubrium joins with the anterior ends of the first pair of ribs. The body of the sternum is long and bladelike. It joins along each side with ribs two through seven. Where the manubrium joins the body of the sternum, there is a slight elevation, the sternal angle, which easily can be felt as a surface landmark. The lower end of the sternum consists of a small tip that is made of cartilage in youth but becomes bone in the adult. This is the xiphoid process. It is used as a landmark for CPR (cardiopulmonary resuscitation) to locate the region for chest compression. All 12 of the ribs on each side are attached to the vertebral column posteriorly. However, variations in the anterior attachment of these slender, curved bones have led to the following classification:
* True ribs, the first seven pairs, are those that attach directly to the sternum by means of individual extensions called costal cartilages.
* False ribs are the remaining five pairs. Of these, the 8th, 9th, and 10th pairs attach to the cartilage of the rib above. The last two pairs have no anterior attachment at all and are known as floating ribs. The spaces between the ribs, called intercostal spaces, contain muscles, blood vessels, and nerves.

Figure 3-14 Bones of the thorax, anterior view.