46 MORPHOLOGY OF THE ORG A AS OF VERTEBRATES. 



ism consists in alternate expansions and contractions of the 

 cord and its contained canal.^ This early segmentation disap- 

 pears with growth, but the same segments are later indicated by 

 the roots of the spinal nerves. 



The spinal nerves are paired structures passing off from 

 either side of the cord (Fig. 47), each nerve arising by two 

 roots, one dorsal the other ventral in position. These roots 

 differ markedly in structure and function. The dorsal roots 

 are connected with the dorsal horn of the gray matter, and 

 soon after leaving the cord each becomes enlarged into a 

 ganglion composed of those ganglion cells which give rise to 

 the fibres of which this root is composed. The ventral roots, 



Fig. 47. Section through spinal cord showing the roots of a spinal nerve. 

 d, dorsal root with its ganglion ; g, gray matter of cord ; v, ventral root ; rt', white 

 matter. 



on the other hand, are not ganglionated, but their fibres are 

 connected with the ganglion cells of the gray matter of the 

 ventral horns of the cord, from which they pass out into the 

 root. Just beyond the ganglion of the dorsal root the two 

 roots of a spinal nerve unite, and the fibres of each follow a 

 common course. 



Experiment shows that the dorsal roots are sensory ; i.e., 

 they carry impulses from the terminal sensory structures to the 

 central nervous system. The ventral roots are motor in func- 

 tion ; that is, the nervous impulses which they transmit come 

 from the central nervous system, and are carried to peripheral 

 portions (muscles, glands, etc.) which they cause to act. 

 Since the dorsal roots convey stimuli from without to the 



1 It must be understood that this metamerism is not necessarily primitive in its 

 character. 



