136 THE HUMAN BODY 



physical stress from particle to particle along the nerve; others 

 would consider it as a chemical process, too delicate and transi- 

 tory to be detected. None of them is sufficiently well established 

 to be considered here. 



Conduction Involving More Than One Neuron. Reflexes. In 

 the actual passage of nerve impulses through the Body more 

 neurons than one are always involved. Let us examine a simple 

 case of conduction by which the Body adapts itself to its surround- 

 ings. Accidentally my finger comes in contact with a hot surface. 

 Quite involuntarily I jerk my hand away. The chain of events is 

 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 over one or more asso- 

 ciation neurons to the motor neuron leading 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 



FIG. 65. Diagram of the simple reflex arc. a, receptor; b, sensory neuron; 

 c, association neuron; d, motor neuron; e, muscle; /, spinal cord. 



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. The neurons involved in the trans- 

 mission 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 impulse 

 from the receptor to the center, and the motor neuron which 

 conveys the impulse from the center to the motor organ. Many 

 facts indicate that reflex arcs as they actually exist in the Body 

 always include at least three neurons, an association neuron 



