PROPERTIES OF THE NERVE CELL. 133 



far as they go, would indicate that in the cerebrum and the cord 

 the nerve cells react with a certain rhythm. 



The Refractory Period of the Nerve Cell. The peculiar 

 rhythm of the active nerve cell just referred to in the para- 

 graph above is explained most satisfactorily by an assumption 

 first used in connection with the rhythmical beat of the heart. As 

 will be explained more fully in the section on the physiology of 

 the heart, it has been found that after the contraction of the 

 heart begins it is unirritable *to artificial stimuli, and that its 

 irritability is recovered during the period of rest, the diastole. 

 The heart has, therefore, alternate periods of irritability and 

 unirritability. The latter phase, the condition in which the heart 

 muscle will not respond to stimulation, is known as the refractory 

 period, or refractory phase. A similar conception has been applied 

 to the nerve cell. The experiments cited in the preceding para- 

 graph would indicate that, after the discharge of an impulse, the 

 cell falls into a refractory phase for a period of time lasting about 

 0.1 sec. The idea* is a convenient one, although we have no explana- 

 tion of what is the immediate cause of this temporary loss of irrita- 

 bility. Reasoning from analogy with the muscle, we might suppose 

 that it is due to some product of the chemical reaction that is 

 assumed to underlie nervous activity. Using this terminology, 

 it is probable that the cells in different parts of the nervous system 

 may have different refractory periods. In the case of the normal 

 nerve fiber (see p. 112) it will be recalled that the refractory period 

 is very brief, say, 0.006 sec., but varies with the condition of 

 the fiber, since in the narcotized fiber it may be as much as 0.1 sec. 



