138 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



It must be remembered, however, that our knowledge of the 

 nature of the chemical changes that occur in the cell during activity 

 is very meager. Presumably carbon dioxid and lactic acid are 

 formed as in muscle, and we know that oxygen is consumed. 

 Enough is known perhaps to justify the general view that the energy 

 exhibited by the nervous system is derived, in the long run, from 

 a metabolism of material 'in the nerve cells, a metabolism which 

 consists essentially in the splitting and oxidation of the complex 

 substances hi the protoplasm of the cell. 



Summation of the Effects of Stimuli. In a muscle a series 

 of stimuli will cause a greater amount of shortening than can be 

 obtained from a single stimulus of the same strength. In this case 

 the effects of the stimuli are summated, one contraction taking 

 place on top of another, or to put it in another way, the muscle 

 while in a condition of contraction from one stimulus is made to 

 contract still more by the following stimulus. In the nerve fiber 

 such a phenomenon has not been demonstrated. In the nerve 

 cell it is usually taught that the power of summation is a charac- 

 teristic property. It is pointed out that, while a single stimulus 

 applied to a sensory nerve may be ineffective in producing a reflex 

 response from the central nervous system, a series of such stimuli 

 will call forth a reaction. In this case it is assumed that the effects 

 of the succeeding stimuli are summated within the nerve cells 

 through which the reflex takes place, and, generally speaking, it 

 is assumed in physiology that the nerve centers are adapted by 

 their power of summation to respond to a series of stimuli or to 

 continuous stimulation. The best examples of this kind of action 

 are obtained perhaps from sensory nerves, in which case we judge 

 of the intensity of the cell activity by the concomitant sensation, 

 or by a reflex response. 



Response of the Nerve Cell to Varying Rates of Stimula- 

 tion. The various parts of the neuromuscular apparatus 

 namely, the nerve cell, the nerve fiber, and the muscle fiber have 

 different degrees of responsiveness to repeated stimuli, and this 

 responsiveness varies, moreover, for the different kinds of mus- 

 cles and of nerve fibers, and, probably for the different kinds 

 of nerve cells. The motor cells of the brain and cord discharge 

 their impulses under normal stimulation at a certain rhythm 

 which was formerly supposed to average about 10 per second, 

 but is now estimated as varying between certain wide limits, 

 perhaps from 40 to 100 per second (p. 46). For any particular 

 group of these motor cells the evidence indicates that it has a prac- 

 tically constant rate whatever may be the intensity of the stimulus 

 and, indeed, when artificial stimulation is used and the rate is 

 varied, the evidence that we have so far appears to show that 



