152 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



and similar cases we have to do with what they call pseudo- 

 reflexes or axon reflexes. The idea underlying 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. 69, may connect by collaterals with 

 several sympathetic cells. If such a fiber were stimulated at x, 

 then the impulse passing back along the axon in a direction the 

 reverse of normal would stimulate cells 6 and c, giving effects that 

 are apparently reflex, but which differ from true reflexes in that 

 the stimulating axon belongs to a motor neuron. Under normal 

 circumstances it is not probable that an effect of this kind can be 

 produced, but there are some interesting observations on record 



Sen soi-/ Ne 



SKin 



Fig. 69a. Schemata of axon-reflex producing local vascular dilatation from the applica- 

 tion of an irritant A, Bardy's schema: n, the sensory fiber; c, the conjunctiva; d, the per- 

 ipheral sympathetic ganglion; /, the blood-vessel. The reflex runs from c over a b and d to 

 /; v, a vaso-constrictor fiber. B, Bruce's schema (modified): The arrows indicate the course 

 of the axon-reflex. 



which indicate that axon-reflexes may play an important role in the 

 control of the blood-supply to certain areas. When a membrane 

 or the skin is inflamed by an irritant, it becomes congested and pain- 

 ful, exhibiting, in fact, the four classical symptoms, rubor, turgor, 

 calor, and dolor. Two observers* have found that the local vaso- 

 dilatation caused by such an irritant as mustard oil applied to the 

 conjunctiva is greatly diminished if the sensory nerve-fibers of the 

 region are paralyzed by a local anesthetic or if the same fibers are 

 destroyed by degeneration after section. In other words, the effect 

 of the irritant in causing local dilatation of the blood-vessels is not 

 due to its direct action on the walls of the blood-vessels, but is of 

 the nature of a reflex effect through the nerve supply. Further 

 experiments indicate that the effect is not due to a reflex of the 



* Ninian Bruce, "Quarterly Journal of Exp. Physiol.," 6, 339, 1913; Bardy, 

 "Skandinavisches Archiv. f. Physiol.," 32, 198, 1914. 



