REFLEX ACTIONS. 



151 



paths. This usual view may not, however, be correct. On 

 the physiological side we have the fact (see p. 82) that stimu- 

 lation of certain of the posterior root ganglia under such cir- 

 cumstances does give peripheral effects 

 on the blood-vessels, causing a vascular ^ 

 dilatation in a certain region. On the 

 histological side Cajal* and others have 

 shown that some of these cells are provided 

 with a pericellular nerve network, which 

 is an afferent path so far as the cell is con- 

 cerned, while the axon of the cell con- 

 stitutes an efferent path. Whether these 

 cells form a special group of efferent cells 

 lying within the sensory ganglion, or 

 whether they are sensory cells discharging 

 into the cord and stimulated reflexly 

 through the nerve network as well as 

 through the peripheral process of the axon, 

 cannot be said. The subject is one full 

 of interest to physiology. In the ganglia 

 of the sympathetic nerve and its appen- 

 dages and in the similar ganglia contained 

 in many of the organs the nerve cells have 

 dendritic processes, and, so far as their 

 histology is concerned, it would seem possi- 

 ble that in any ganglion of this type there 

 might be sensory and motor neurons so 

 connected as to make the ganglion an 

 independent reflex center. Numerous 

 experiments have been made to determine experimentally 

 whether reflexes can be obtained 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 Fig. 287). 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 hy- 

 pogastric nerve; the reaction has every appearance of being a 

 true reflex. Nevertheless, Langley and Anderson, f who have 

 studied the matter with especial care, are convinced that in this 



* Cajal, "Ergebnisse der Anat. u. Entwickelungsgeschichte," vol. xvi., 



Fig. 69. 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 b and c gives 

 a pseudoreflex effect. 



1906; Dogiel, "Bau der Spinalganglien, etc.," 1908. 

 f Langley and Anderson, "Journal of Physi 



Physiology," 16, 410, 1894. 



