THE SKELETON. . 71 



foramina, Fsa, which communicate with the neural canal. 

 Its dorsal surface, convex and roughened, has four similar 

 pairs of posterior sacral foramina. 



The coccyx (Fig. 26) calls for no special description. The 

 four bones which grow together, or ankylose, 

 to form it, represent only the bodies of vertebrse, 

 and even those incompletely. It is in reality 

 a short tail, although not visible as such from 

 the exterior. 



The Spinal Column as a Whole. The ver- 

 tebral column is in a man of average height 

 about twenty- eight inches long. Viewed from 

 the side (Fig. 17) it presents four curvatures; 

 one with the convexity forwards in the cervical 

 region is followed, in the thoracic, by a curve with its concavity 

 towards the chest. In the lumbar region the curve has again 

 its convexity turned ventrally, while in the sacral and coccy- 

 geal regions the reverse is the case. These curvatures give the 

 whole column a good deal of springiness such as would be 

 absent were it a straight rod, and this is farther secured by the 

 presence of compressible elastic pads, the intervertebral disks, 

 made up of cartilage and connective tissue, which lie between 

 Ihe bodies of those vertebras which are not ankylosed together, 

 and fill iip completely the empty spaces left between the 

 bodies of the vertebrae in Fig. 17. By means of these pads, 

 moreover, a certain amount of movement is allowed between 

 each pair of vertebras; and so the spinal column can be bent 

 to considerable extent in any direction; while the movement 

 between any two vertebrae is so limited that no sharp bend 

 can take place at any one point, such as might tear or other- 

 wise injure the spinal cord contained in the neural canal. 

 The amount of movement permitted is greatest in the cervical 

 region. 



In the case of the movable vertebrae, the arch is somewhat 

 narrowed where it joins the body on each side ; this nar- 

 rowed stalk is the pedicle (li, Fig. 19), while the broader 

 remaining portion of the arch is its lamina. Between 

 the pedicles of two contiguous vertebrae there are in this 

 way left apertures, the intervertebral holes which form a 

 series on each side of the vertebral column, and one of which, 

 Fi, is shown between the two dorsal vertebras in Fig. 19. 

 Through these foramina nerves run out from the spinal cord 



