52 THE HUMAN BODY 



from the side (Fig. 18) 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 coccygeal 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. 



All the vertebrae are built upon the same plan, although with 

 modifications in various parts of the column. Each consists: 1, of a 

 stout bony body or centrum (Fig. 19, C), in shape a cylinder flat- 

 tened at both ends; 2, a bony arch, the neural arch (Fig. 19, A), at- 

 tached to the dorsal side of the centrum and inclosing the neural 

 ring (Fig. 19, Fv). The neural rings of all the vertebrae make up 

 together a long bony tube, the neural canal, which contains the 

 spinal cord. Between the bodies of adjoining vertebrae, except in 

 the sacrum and coccyx, are thick pads of elastic cartilage. These 

 permit bending movements which, while quite limited at each 

 joint may be very considerable in the column as a whole. They 

 also serve to take up a great deal of shock, preventing injury to 

 the body when one sits down hard or comes down on his heels in 

 walking or jumping. During the hours when one is on his feet 

 these intervertebral pads are packed down by the weight of the 

 body, and especially by the hammering effect of the movements 

 of walking, running, etc., so that a man may be from a half to three- 

 quarters of an inch shorter at night than he is in the morning. 

 Strong ligaments fasten adjoining vertebrae together; there are 

 also muscles passing from vertebra to vertebra, which by their 

 contractions assist in bending the body. These muscles are ar- 

 ranged in antagonistic groups; that is, they are so placed that 

 whenever the vertebral column is bent through the contraction of 

 one group the muscles of the antagonistic group are put on the 

 stretch. The neural arch of each vertebra bears a dorsal spinous 

 process (Fig. 19, Ps), and a pair of lateral transverse processes 

 (Fig. 19, Pt). These serve various purposes; the intervertebral 

 muscles are attached to them; they also bear articular surfaces 

 (Pas and Pai Figs. 19 and 20) which sliding upon corresponding 

 surfaces of adjoining vertebrae serve to limit the movements at 

 each joint, and also help to prevent dislocation of the vertebral 

 column. The spinous processes may be felt in the middle of the 



