368 PHYSIOLOGY 



until they have acquired a medullary sheath, we must conclude that 

 the reflex responses affecting the segment in which the fibres enter 

 are developed earlier than those which involve also the activity 

 of the cerebellum and medulla. 



The primitive segmental character of the central nervous system 

 is retained in its pure form only in the segmentation of the dorsal 



. 



FIG. 162. Transverse section of spinal cord, showing collaterals terminating 

 in a rich arborisation round the cells of Clarke's column (A, B), as well 

 as others passing to the anterior cornua, and through the commissures. 

 (CAJAL.) 



spinal root ganglia. Each of these ganglia or afferent roots consists 

 of the fibres from the sense-organs in a segmental area of the body 

 surface as well as from the muscular and visceral apparatus in the 

 same segment. Section of one dorsal posterior nerve-root will cause 

 a diminution of sensibility over a band- like area corresponding to 

 the distribution of the fibres of the root, though to produce complete 

 insensibility the two adjacent nerve-roots must be divided, in con- 

 sequence of the overlap of fibres at the periphery. In the limbs 

 the segmental distribution of the sensory fibres is made out with 

 more difficulty. Each limb must be regarded as made up from 

 a series of fused segments, from five to seven in number. The 



