158 



THE HUMAN BODY 



as follows: the skin of the hand is stimulated by the heat; the 

 sensory neurons in the nerve supplying that part of the hand 

 convey the nerve impulses thus aroused to the central nervous 

 system ; here the impulses are conveyed to the motor neurons lead- 

 ing to the muscle which retracts the arm; upon the arrival of the 

 impulses within the muscle the latter is stimulated to contract. 

 The whole process is entirely mechanical; none of the structures 

 involved has any knowledge that the hand is in danger of being 

 severely burnt, or that retraction of the arm will save it. It is an 

 example of an adaptive mechanism. Such a chain of events as the 

 one described constitutes a simple reflex and typifies the funda- 

 mental basis of all nervous activity within the organism. Our 



study of the operation of the 

 nervous system will consist 

 throughout of enlargements 

 and modifications of this 

 elementary conception of 

 nervous activity as conduc- 

 tion of nerve impulses from 

 receptor to active tissue. In 

 its broadest sense any such 

 act of conduction may be 

 termed a reflex, and so we 

 shall define the word. The 

 neurons involved in the 

 transmission of the impulse 

 from receptor to muscle 

 make up the reflex arc. The simplest imaginable reflex arc must 

 include at least two neurons, the sensory neuron which brings 

 the impulses from the receptor to the center, and the motor 

 neuron which conveys the impulses from the center to the 

 motor organ. Undoubtedly most reflex arcs in the Body include, 

 in addition, one or more association neurons interposed between 

 the sensory and the motor neuron. 



Anatomical Arrangement. The anatomical relationships of the 

 various neurons which make up the reflex arc are indicated in 

 figure 63. For simplicity the spinal cord is taken as the part of the 

 central nervous sj'stem pictured. The receptor communicates 

 with the cell-body of the sensory neuron by means of the axon-like 



FIG. 63. Diagram of the simple reflex 

 arc. R, receptor; A, afferent (sensory) 

 neuron; E, efferent (motor) neuron; M, 

 muscle. 



