THE PROPERTIES OF EACH PART OF THE REFLEX ARC 



797 



section of the posterior roots of the spinal cord, but fail to do so if 

 the nerve fibers are cut and allowed to degenerate, or if the stimuli are 

 blocked by applying cocaine to the skin. What actually happens is 

 evidently that the impulse set up by the irritant as it travels up the 

 afferent fiber passes on to one of the branches above referred to, along 

 which it then proceeds to the blood vessels, which it causes to dilate. 

 That such vasodilator impulses may be transmitted down the fibers of 

 an afferent nerve has been confirmed by Bayliss, who found that vaso- 

 dilatation occurred in the hind limb when the posterior spinal roots 

 were stimulated (see page 234). 



Post, roof 

 gang. 



Fig. 206. Diagram to show axon reflex of sensory nerve fiber of skin. A stimulus applied to 

 the skin is transmitted by the sensory fiber (AT), part of it going to the spinal cord (SO, and 

 part of it passing by the collateral (C) to the arteriole (A), which it causes to dilate. 



In this peripheral branching of the afferent fibers of the skin, we 

 have therefore a sort of neuropile which, like that of certain forms of 

 Celenterates (see page 782), is capable of serving as a pathway for the 

 transmission of a sensory impulse to an effector organ without the in- 

 tervention of nerve cells. Such a reflex is known as an axon reflex, and 

 it is evident that it may occur through any fiber which gives off branches, 

 one traveling to a sensory surface, the other to some effector organ, as 

 occurs in the hypogastric nerves to the bladder (see page 883). 



THE SYNAPSIS 



At the point of contact between a branch of one neuron and a nerve 

 cell of the next, we have seen that there exists a structure known as 

 the synapsis. Although this is described by histologists as a tuft-like 



