144 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



a 



whether or not reflexes can occur through the peripheral nerve 

 ganglia, particularly those belonging to the sympathetic system. 

 With regard to the posterior root ganglia, it may be said that no 

 reflexes are possible through them. If the posterior root connect- 

 ing such a ganglion to the cord is severed stimulation of the sensory 

 area supplied by these ganglia causes no reflex response. Indeed, 

 according to our conception of the mechanism of a reflex, the pos- 

 terior root ganglia could not serve as reflex centers: they contain 

 apparently no efferent neurons. In the ganglia of the sympathetic 

 nerve and its appendages and in the similar 

 (o) ganglia contained in many of the organs the 



nerve cells have dendritic processes, and, so 

 far as their histology is concerned, it would 

 seem possible that in any ganglion of this 

 type there might be sensory and motor neu- 

 rons so connected as to make the ganglion 

 an independent reflex center. Numerous 

 experiments have been made to determine 

 experimentally whether reflexes can be ob- 

 tained through such ganglia. Perhaps the 

 most successful of these experiments have 

 been made upon the inferior mesenteric 

 ganglion. 



This ganglion may be isolated from all 

 connections with the central nervous system 

 and left attached to the bladder through the 

 two hypogastric nerves (see schema, Fig. 

 105) . If now one of these nerves is cut and 

 the central stump is stimulated a contraction 

 of the bladder follows. Obviously in this 

 case the impulse has traveled to the ganglion 

 and down the other hypogastric nerve; the 

 reaction has every appearance of being a true 

 reflex. Nevertheless, Langley and Ander- 

 son,* who have studied the matter with espe- 

 cial care, are convinced that in this and similar cases we have to do 

 with what they call pseudoreflexes or axon reflexes. The idea under- 

 lying this term may be explained in this way : Every sympathetic 

 ganglion is connected with the central nervous system, brain and 

 cord, by efferent spinal fibers, preganglionic fibers, which terminate 

 by arborization around the dendrites of the sympathetic cells. The 

 efferent fibers arising from the latter may be designated as post- 

 ganglionic fibers. These authors give reasons to believe that any 

 one preganglionic fiber, a, Fig. 64, may connect by collaterals with 

 * Langley and Anderson, " Journal of Physiology," 16, 410, 1894. 



Fig. 64. Schema to 

 show idea of an axon re- 

 flex : The preganglionic 

 fiber, a, sends branches 

 to two postganglionic 

 fibers, b, c. If stimulated 

 at x the impulse passes 

 backward in a direction 

 the reverse of normal and 

 falling into 6 and c gives 

 a pseudoreflex effect. 



