_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 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] Fig. 100—*The skull (Huxley).* The illustration shows most of the bones of the skull. 3. _The Thorax._—This group contains twenty-four bones of similar form, called _ribs_, and a straight flat bone, called the _sternum_, or breastbone (Fig. 101). The ribs connect with the spinal column behind, and all but the two lowest ones connect with the sternum in front, and, by so doing, inclose the thoracic cavity. As already stated (page 85), the bones of the thorax form a mechanical device, or machine, for breathing. The ribs are so arranged that the volume of the thorax is increased by elevating them and diminished by depressing them, enabling the air to be forced into and out of the lungs. [Fig. 101] Fig. 101—*Bone groups of trunk.* 4. _The Shoulder and Pelvic Girdles._—These groups form two bony supports—one at the upper and the other at the lower portion of the trunk—which serve for the attachment of the arms and legs (Fig. 101). The _shoulder girdle_ is formed by four bones—two clavicles, or collar bones, and two scapulæ, or shoulder blades. The clavicle on either side connects with the upper end of the sternum and serves as a _brace_ for the shoulder, while the scapula forms a socket for the humerus (the large bone of the arm) and supplies many places for the attachment of muscles.