THE EVOLUTION OF THE BRAIN IO3 



to the voluntary muscles and by means of which muscular 

 movements are excited. 



The cell bodies of the dorsal fibers are contained in the 

 gangha of the dorsal roots and the fibers themselves pass 

 from these cells on the one hand to the regions of sensory 

 termination either in the skin or among the muscles and on 

 the other hand into the spinal cord, where they branch 

 and extend up and down that organ. These sensory fibers 

 are as numerous as to give rise to about one-fourth of the 

 substance of the cord. 



The cell bodies of the ventral neurones are large elements 

 lodged within the cord; their fibers pass directly out of the 

 cord and gather into bundles, the ventral roots. Each 

 ventral root unites with a dorsal root and thus constitutes a 

 spinal nerve. In such a nerve the two classes of fibers, 

 sensory and motor, retain their individuahty though they 

 are as closely apphed one to the other as are the wires in an 

 electric cable. The ventral fibers of course make their way as 

 motor components of the spinal nerves to the muscles that 

 they control. 



These two classes of neurones, sensory and motor, together 

 afford the basis for the simplest type of reflex connection. 

 When the foot of a human being is pricked with a pin, it is 

 instantly withdrawn, the act being essentially a reflex. The 

 pricking of the skin stimulates the peripheral branches of a 

 sensory neurone and thus generates a nervous impulse that 

 passes over the sensory fiber to the cord. Here it is trans- 

 ferred to appropriate motor neurones that transmit it to the 

 muscles by which the foot is withdrawn. Thus these two 

 types of neurones together are sufficient to carry out a 

 simple reflex act. It is, however, an open question whether 

 in man such simple reflexes ever really occur. Certainly 

 in the majority of reflex arcs more than two neurones are 

 included. These intercalated neurones are strictly speaking 

 neither sensory nor motor. They represent the third class of 

 elements already mentioned, the communicating or inter- 

 nuncial neurones, which are characterized by the fact 

 that they connect one nerve center with another but do 

 not extend beyond the central organ of which they form a 

 part. Such neurones commonly run lengthwise or crosswise 



