228 



THE SKELETON 



transverse ligament into a dorsal and ventral portion. 

 The spinal cord lies in the dorsal portion, while an upright 

 process (the odontoid) of the second vertebra or axis 

 project^ through the lower larger portion. On the top of 

 the atlas are two broad facets lined with cartilage on 

 which the skull rests. By this arrangement, as the skull 

 is turned to right or left by the muscles the atlas rotates 

 about the peglike odontoid process of the axis, which 

 thus forms a sort of pivot. In nodding, the surfaces a 

 and h (see Fig. 87) of the atlas, permit the skull to rock 

 back and forth on them. This odontoid process is a devel- 

 opment of the centrum of the second vertebra. 



The spinal column as a whole. — The total length of the 

 column is about twenty-eight inches in the average man. 

 Its curvature may be readily observed from a glance at 

 the figure (see Fig. 84). This curvature and the cartilage 

 pads between the centra give springiness to the column, 

 while the articulations permit a certain amount of play to 

 the bone. The curvature also prevents a sudden jar to 

 the base of the brain. The greatest amount of movement 

 is between the cervical vertebrae, but the possibility of 

 bending the whole back indicates the flexibility of the whole 

 column. 



The placing of the heavier vertebras at the bottom of 

 the column is an adaptation to compensate for the greater 

 weight upon that part. The neural arch protects the 

 spinal cord, and the apertures between the arches and 

 openings in the vertebrae themselves protect the nerve 

 roots which branch from the cord. 



The ribs and stemiun. • — The ribs are flat, double-curved 

 bones, twenty-four in number, and arranged in pairs on 

 each side of the twelve dorsal vertebra. They are jointed 



