18 Anatomy of the Nervous System 



them are grouped in small masses, or ganglia. As noted in 

 the previous chapter, a ganglion occurs on the dorsal root of 

 each spinal nerve and near the bases of some of the cranial 

 nerves — i.e., near the point of entry into the central system 

 of every afferent nerve. These perikarya have certain 

 peculiarities of form, and it has been shown that in the rat 

 their volume varies during growth in direct proportion to 

 the area or skin-surface of the animal. This relation is con- 

 sidered to be an adaptation for maintaining the sensory 

 discrimination despite the extension during growth of the 

 area supplied by a single neuron. ^ 



The main functional unit in the nervous system is a chain 

 of such neurons consisting in the simplest possible condition 

 of two elements — a receptive, sensory, or afferent cell, which 

 receives a stimulus at the periphery and conducts the result- 

 ing impulse to the central organ, and a motor or efferent 

 cell, which is stimulated by the impulse in the afferent neuron 

 and transmits the resulting impulse to a muscle. The muscle 

 thus brought into action is termed the effector, while the 

 chain of neurons is called a reflex arc. It is the mechanism 

 which brings about that immediate, involuntary response 

 to a stimulus which is known as a reflex action. 



Seldom, if ever, does the reflex arc consist of only two 

 neurons. There is practically always at least one internuncial 

 neuron between the afferent and efferent elements, and in 

 the great majority of cases there are many more. The whole 

 nervous system is made up of an infinitely complex system 

 of such arcs variously connected and superposed. 



The internuncial neurons frequently are connected with 

 several other cells from which they receive stimuli, often of 

 different kinds, so that the impulse in such an element may 

 be the resultant of several stimuli instead of the effect pro- 

 duced by a single one. The internuncial cell thus becomes a 

 correlation neuron, and a group of such cells is a correlation 



^Donaldson and Nagasaka, 1918. 



