PROPERTIES OF THE NERVE CELL. 137 



shown that in birds, for instance, the spinal ganghon cells of a 

 swallow killed at nightfall after a day of activity exhibit a marked 

 loss of substance as compared with similar cells from an animal 

 killed in the early morning (Fig. 64). Dolley* also states that in 

 the dog the cerebellar cells exhibit a definite series of changes in 

 the chromatic substance, both that within the nucleus and that 

 within the cytoplasm (Nissl's granules) following upon prolonged 

 muscular activity or after such conditions as shock or anemia. 

 If these conditions are extreme, the chromatin material may be 

 entirely removed from the cells, and this he interprets as an indica- 

 tion of a functionally exhausted cell. 



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

 nature ot 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 metaboUsm which 

 consists essentially in the splitting and oxidation of the complex 

 substances in 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 stimuH 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 tespond 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. 



The Refractory Period of the Nerve Cell. — It will be recalled 

 that the nerve fiber exhibits what is called a refractory period for a 

 ''Dolley, "American Journal of Physiology," 25, 151, 1909. 



