FIG. 7.
[Illustration: B, the first cervical vertebra, the atlas; A, the atlas, and the second cervical vertebra, the axis; e, the odontoid process; c, the foramen.]
THE SKULL ARTICULATES with (is jointed to) the spine in a peculiar manner. On the top of the upper vertebra (atlas [Footnote: Thus called because, as, in ancient fable, the god Atlas supported the world on his shoulders, so in the body this bone bears the head.]) are two little hollows (a, b, Fig. 7), nicely packed and lined with the synovial membrane, into which fit the corresponding projections on the lower part of the skull, and thus the head can rock to and fro. The second vertebra (axis) has a peg, e, which projects through a hole, c, in the first.
FIG. 8.
[Illustration: The Thorax or Chest. a, the sternum; b to c, the true ribs; d to h, the false ribs; g, h, the floating ribs; i, k, the dorsal vertebræ.]
The surfaces of both vertebræ are so smooth that they easily glide on each other, and thus, when we move the head side wise, the atlas turns around the peg, e, of the axis.
THE RIBS, also twenty-four in number, are arranged in pairs on each side of the chest. At the back, they are all attached to the spine. In front, the upper seven pairs are tied by cartilages to the breastbone (sternum); three are fastened to each other and to the cartilage above, and two, the floating ribs, are loose.
The natural form of the chest is that of a cone diminishing upward. But, owing to the tightness of the clothing commonly worn, the reverse is often the case. The long, slender ribs give lightness, [Footnote: If the chest wall were in one bone thick enough to resist a blow, it would be unwieldy and heavy As it is, the separate bones bound by cartilages yield gradually, and diffuse the force among them all, and so are rarely broken.] the arched form confers strength, and the cartilages impart elasticity,—properties essential to the protection of the delicate organs within, and to freedom of motion in respiration. (See note, p. 80.)
FIG. 9.
[Illustration: The Pelvis. a, the sacrum; b, b, the right and the left innominatum.]