*Bone Groups.*—On account of the close relation between the bones of the same group, they cannot profitably be studied as individual bones, but each must be considered as a part of the group to which it belongs. By first making out the relation of a given bone to its group, its value to the whole body can be determined. The most important of the groups of bones are as follows: 1. _The Spinal Column._—This group consists of twenty-four similarly shaped bones, placed one above the other, called the _vertebræ_, and two bones found below the vertebræ, known as the sacrum and the coccyx (Fig. 98). These twenty-six bones supply the central axis of the body, support the head and upper extremities, and inclose and protect the spinal cord. [Fig. 98] Fig. 98—The spinal column. The upper seven vertebræ form the neck and are called the _cervical_ vertebræ. They are smaller and have greater freedom of motion than the others. The first and second cervical vertebræ, known as the _atlas_ and the _axis_, are specially modified to form a support for the head and provide for its movements. The head rests upon the atlas, forming with it a hinge joint (used in nodding to indicate "yes"); and the atlas turns upon an upward projection of the axis forming a pivot joint (used in shaking the head to indicate "no"). 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] 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: