436 HISTORY OF THE HUMAN BODY 



rows on each side.* The two roots of the same side unite 

 soon after their exit from the cord into a single metameric 

 nerve, containing both sorts of fibers, but then divides again 

 almost immediately into dorsal and ventral branches, each con- 

 taining both sorts of fibers. These, like all subsequent di- 

 visions, are merely topographical, and not physiological, as in 

 the case of the roots. A spinal ganglion appears in association 

 with each pair of roots, usually associated with the sensory 

 root, but in lower forms often connected with both and situ- 

 ated at the point of union. 



This typical arrangement of nerve roots and their association 

 in the formation of single pairs of metameric nerves is a con- 

 stant one in all vertebrates and is already suggested by the 

 somewhat more primitive condition in Amphioxus. In the 

 lower phylogenetic stages, however, the plan is a little less 

 precise, and there is sufficient indication to show that here, too, 

 as elsewhere, the final arrangement has been obtained by a 

 natural development from a less definite one. 



Thus in Amphioxus the motor roots consist of a series of 

 fibers distinct from one another; the sensory roots are more 

 definite and are placed in the intervals between the first, in 

 such a manner that the motor roots correspond to the myo- 

 meres, the sensory roots to the myocommata. Moreover, since 

 in this singular animal the body somites on the two sides do 

 not match but alternate with one another, the nerve roots do 

 the same, and a sensory root of one side will lie in the same 

 transverse plane as a motor root of the other. The sensory 

 and motor roots do not unite, and the former becomes as- 

 sociated with a subcutaneous ganglion. 



This alternate arrangement of the roots, excepting the 

 non-correspondence between the two sides, is continued in most 

 fishes. In the selachians, for example, the motor root passes 

 through a foramen in the side of the vertebra, the sensory root 

 through a similar foramen in the intercalary piece ; the latter is 

 thus inter-, the former intra-vertebral. 



As we ascend the series the tendency is more and more 

 * The sensory roots often contain motor fibers. 



