NERVE 97 



tion of light, but it may be stimulated by sudden pressure 

 or by electrical changes. 



In whatever way any special variety of receptor is stimu- 

 lated, the sensation which results is of the same kind. Thus 

 the retina of the eye may be stimulated by ethereal vibra- 

 tions of light, or by mechanical pressure, or by an electric 

 current, but however stimulated, a sensation which we call 

 visual is produced. This fact was formulated by Johannes 

 Mllller as the doctrine of Specific Nerve Energy. The con- 

 verse holds good that the same kind of stimulus applied to 

 different kinds of receptors produces different kinds of 

 sensation. 



The sets of Receptors developed in the body wall and 

 viscera may first be considered. They may be called the 

 Body Receptors. 



I. BODY RECEPTOR MECHANISMS. 



A. General Arrangement and Physiolog'y. 

 I. Visceral (Intero-ceptive). 



A. Structure. — Throughout the internal organs are various 

 peripheral terminations of ingoing nerves, some of the 

 nature of simple dendritic expansions, some of dendritic 

 expansions enclosed in definite fibrous capsules (Pacinian 

 corpuscles). 



B. Physiology. — These are called into action by different 

 kinds of stimulation, nocuous and innocuous, to produce 

 reflex adjustments of the bodily mechanism either without or 

 with the involvement of consciousness — that is, either with- 

 out or with the production of sensation. 



(a) Central Reflex Adjustment. — When food is taken 

 into the stomach, it stimulates the ends of the afferent 

 nerves, and these carry the impulse up to the central nervous 

 system to produce a reflex dilatation of the gastric blood- 

 vessels. 



(6) Peripheral Reflex Adjustment. — Many of these visceral 

 reflexes occur apart from the central nervous system in 

 peripheral plexuses (p. 92). This is specially well seen in 



