THE SPINAL CORD AND ITS NERVES 



133 



neurons, and usually there is a complex chain of such neurons. 

 As may be observed in Fig. 61, the dorsal root fiber imme- 



dorsal 



root 



ventral 

 root 



Fig. 60. Diagram of some of the types of connection between the sen- 

 sory fibers of the dorsal root and the motor fibers of the ventral root in the 

 spinal cord of the rabbit (chiefly after the researches of Philippson). The 

 visceral connections are not included. 



1. Collateral branches of the dorsal root fibers effect synaptic relations 

 directly with dendrites of ventral column cells of the same or the opposite 

 side. 



2. Dendrites of ventral column cells may cross to the opposite side and 

 here receive terminals of dorsal root fibers. 



3. A correlation neuron may be intercalated between the two peripheral 

 neurons in either of the first two cases. These neurons may have short 

 axons for reflexes within a single segment (3a) or their axons may pass out 

 into the white matter (fasciculus proprius) and extend for longer or shorter 

 distances in either the ascending or the descending direction (or after branch- 

 ing in both directions) for connections with more remote motor centers of 

 the same or the opposite side (36, 3c). 



4. The root-fibers arising from the cells of the ventral column themselves 

 may give off collateral branches which return to the gray matter and there 

 arborize about other cells of the ventral column belonging to different func- 

 tional groups or about correlation cells, thus facilitating the coordinated 

 contraction of several distinct muscles in the performance of some complex 

 reaction. 



The neurons of the dorsal column apparently do not play an important 

 role as intercalary elements in the simpler spinal reflexes. The axons of 

 these cells are for the most part directed upward, after decussating in the 

 ventral commissure, and are chiefly concerned with the transmission of 

 nervous impulses from the spinal cord to the higher correlation centers of 

 the brain. 



