[pg 224]The next twelve vertebræ, in order below the cervical, are known as the thoracic vertebræ. They form the back part of the framework of the thorax and have little freedom of motion. The five vertebræ below the thoracic are known as the lumbar vertebræ. These bones are large and strong and admit of considerable motion. Below the last lumbar vertebra is a wedge-shaped bone which has the appearance of five vertebræ fused together. This bone, known as the sacrum, connects with the large bones which form the pelvic girdle. Attached to the lower end of the sacrum is a group of from two to four small vertebræ, more or less fused, called the coccyx.
Fig. 99—Two views of a lumbar vertebra. A. From above. B. From the side. 1. Body. 2, 3, 4, 5. Projections from the neural arch.
The Joining of the Vertebræ.—A typical vertebra consists of a heavy, disk-shaped portion in front, called the body, which is connected with a ring-like portion behind, called the neural arch. The body and the neural arch together encircle a round opening which is a part of the canal that contains the spinal cord (Fig. 99). From the neural arch are seven bony projections, or processes, three of which serve for the attachment of muscles and ligaments, while the other four, two above and two below, are for the interlocking of the vertebræ with each other. The separate vertebræ are joined together in the spinal column, as follows:
a. Between the bodies of adjacent vertebræ are disks of elastic cartilage. Each disk is about one fourth of an inch thick and is grown [pg 225]tight onto the face of the vertebra above and also onto the face of the vertebra below. By means of these disks a very close connection is secured between the vertebræ on the front side of the column.
b. On the back of the column, the downward projections from the neural arch of each vertebra above fit into depressions found in the neural arch of the vertebra below. This interlocking of the vertebræ, which is most marked in the lumbar region, strengthens greatly the back portion of the column.
c. To further secure one bone upon the other, numerous ligaments pass from vertebra to vertebra on all sides of the column.
2. The Skull.—The skull is formed by the close union of twenty-two irregular bones. These fall naturally into two subgroups—the cranium and the face (Fig. 100). The cranium consists of eight thin, curved bones which inclose the space, called the cranial cavity, that holds the brain. The face group, consisting of fourteen bones, provides cavities and supports for the different organs of the face, and supplies a movable part (the inferior maxillary) which, with the bones above (superior maxillary), forms the machine for masticating the food.
Fig. 100—The skull (Huxley). The illustration shows most of the bones of the skull.