REFLEX ACTIONS. 151 



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 

 which indicate that axon-refiexes 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 

 ordinary kind through the central nervous system or the posterior 

 root ganglion or the superior cervical ganglion, and it is probably, 

 therefore, a peripheral reflex of the nature of an axon-reflex taking 

 place wholly through the sensory nerve-fibers or through such 

 fibers in connection with the small peripheral sympathetic gangha 

 distributed along the course of the blood-vessels. The apparatus 

 that may be involved is represented in the two accompanying 

 schemata (Fig. 69). 



The Tonic Activity of the Spinal Cord. — In addition to the 

 definite reflex activities of the cord, each traceable to a distinct 

 sensory stimulus, there is evidence to show that many of its motor 

 neurons are in that state of more or less continuous activity which 

 we designate as tonic activity or tonus. There is abundant reason 

 for this belief in regard to many of the special centers of the cord 

 and brain, such as the vasomotor center, the center for the sphinc- 

 ter muscle of the iris, the centers for the sphincter muscles of the 

 bladder, the anus, etc. But the evidence includes the motor 

 neurons to the voluntary as well as the involuntary musculature. 

 In a decapitated frog the muscles take a definite position, and 

 Brondgeest showed that if such an animal is suspended, after cut- 

 ting the sciatic plexus in one leg, the leg on the uninjured side 

 takes a more flexed position. The explanation offered for this 

 result is that the muscles on the sound side are being innervated 

 by the motor neurons of the cord. Inasmuch as a result of this 

 kind cannot be obtained from a frog whose skin has been removed, 

 or in one in which the posterior roots have been severed it seems 

 evident that this tonic discharge from the motor neurons is due 



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

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



