428 HISTORY OF THE HUMAN BODY 



consists essentially in all cases of a tube with a minute lumen 

 (canalis centralis) and extremely thick walls. It consists of 

 both ganglion cells (gray matter) and connecting fibers (white 

 matter} and, as the latter usually form the greater part of its 

 bulk, it is to be considered in the main a great central nerve 

 bundle proceeding from the brain and distributing its fibers to 

 all parts. This distribution takes place through the formation 

 of pairs of spinal nerves, which are arranged metamerically, a 

 pair for each body somite. 



The proportion of the spinal cord in weight as compared 

 with that of the brain may be said in a general way to decrease 

 as we ascend the scale of vertebrates, but this is due rather to 

 the increase in the size of the brain than to a decrease in that 

 of the cord. There is, however, another principle, that of pro- 

 gressive cephalization, which tends to shorten the cord and con- 

 centrate the nervous system at the anterior end, and it is 

 through this that the changes may be best explained. This 

 principle appears equally well among many groups of inverte- 

 brates and is shown in ( i ) ) a tendency to shorten the body 

 axis, and (2) to concentrate and hence shorten the longitudinal 

 nerve axis. The results of this process may be especially well 

 followed among such a group of animals as that of insects, in 

 which the central nervous system originally consists of a pair 

 of small ganglia for each somite, this condition running 

 through the entire body. 



Thus in the myriapod (Fig. 119, A), an ancestral form, the 

 primitive condition is still realized ; in such a low form of insect 

 as the dragon fly or grasshopper the concentration of ganglia 

 has commenced, and in the fly the highest cephalization is 

 reached. That these stages are passed through during the de- 

 velopment is shown by a comparison of the nervous system of 

 the fly in its various stages, that of the larva still showing 

 a quite primitive condition (Fig. 119, cf. B and C). 



This principle is shown in vertebrates by the progres- 

 sive shortening of the spinal cord in a series of gradually ceph- 

 alizing forms, but can be used as a criterion of development 

 only within the limits of a single group. Thus the frog, with 



