THE SPINAL CORD 



93 



flexes. Others cross in the anterior white commissure and are commissural 

 fibers concerned in crossed reflexes. Afferent impulses may be transmitted 

 along the cord in either direction by the branches of the dorsal root fibers; or by 

 means of synapses in the gray matter they may be transferred to the long asso- 

 ciation and commissural fibers and conveyed to the primary motor neurons of 

 the same or opposite side in more or less distant segments. The course of a 

 nerve impulse in a unilateral intersegmental reflex is indicated on the left side 



Dorsal root 



Ventral root 



Ascending branch of dorsal root fiber 



Association fibers -'-'' 

 Descending branch of dorsal foot fiber 



Dorsal root 



Commissural fibers 



I 



Ventral root 



Fig. 68. Diagram of the spinal cord, showing the elements concerned in intersegmental reflexes. 



of Fig. 68, while on the right side of the same figure are shown the elements 

 concerned in crossed reflexes. 



The observations of Coghill (1913 and 1914) and of Herrick and Coghill (1915) tend to 

 show that the simple form of reflex arc illustrated in Fig. 66 is not the primitive type. In 

 larval Amblystoma the first arcs to become functionally mature are composed of chains 

 of many neurons, so arranged that every cutaneous stimulus elicits the same complex response 

 of the entire somatic musculature, i. e., the swimming movement. It is of particular interest 

 to note that in this primitive reflex mechanism the sensory fibers arise from giant cells located 

 within the spinal cord and that the ventral root fibers are collaterals from the central motor 

 tract. In adult Amblystoma these sensory and motor elements are replaced by the usual 

 type of primary sensory and motor neurons. 



