432 HISTORY OF THE HUMAN BODY 



esting way with changes in the musculature of that part. In 

 such forms as fishes and salamanders, the metameric mus- 

 culature is not discontinued at the cloaca, which marks the 

 posterior limit of the body cavity, but is continued in a grad- 

 ually reducing series to the extreme tip of the tail. Each of 

 these caudal metameres is supplied with a pair of spinal nerves, 

 to furnish which the cord must of necessity be continued quite 

 or nearly to the end. In mammals, although in some cases the 

 tail is long and extensive, its metameric muscles have been 

 given up except those of its most anterior somites, and the tail 

 is moved by a complex system of tendons proceeding from 

 these latter. The only nerves necessary for the tail, then, are 

 those of its anterior metameres, which are easily supplied 

 from the cauda equina, thus obviating all necessity on the part 

 of the cord for extending very far posteriorly. Indeed, with 

 the exception of the primitive Ornithorhynchus and a few 

 rodents, the spinal cord of mammals fails to reach even the 

 sacrum. 



In much the same way as the development of the caudal 

 muscles conditions the point and manner of termination of the 

 cord, so is its caliber modified in other places through the rela- 

 tive amount of muscular development in the various body 

 regions, especially in the case of the limbs. In such a form as 

 Amphioxus, where the successive metameres are practically 

 alike, the spinal nerves, and consequently the cord, are of about 

 an equal caliber throughout the body, gradually tapering to the 

 end of the tail, and for the same reason in forms like eels and 

 snakes, which have secondarily lost their metameric differenti- 

 ation, the cord is correspondingly simple; where, however, a 

 certain metamere, or a series of successive ones, becomes 

 greatly developed, the nerves which supply this part are neces- 

 sarily increased in caliber, and this causes a corresponding in- 

 crease in the cord at or near the point from which they 

 originate. 



The most conspicuous example of this principle is, of course, 

 that of the limbs, which are often excessively developed in 

 the higher forms and cause a corresponding increase of size in 



