130 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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 hi 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 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 



Fig. 60. Spinal ganglion cells from English sparrows, to show the daily variation in 

 the appearance of the cells due to normal activity: A, Appearance of cells at the end of 

 an active day; B, appearance of cells in the morning after a night's rest. The cytoplasm 

 is filled with clear, lenticular masses, which are much more evident in the rested cells than 

 in those fatigued. (Hodge.) 



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

 such a phenomenon has, not been demonstrated. The strength of 

 the nerve impulse can be determined only by means of the effect 

 on the end-organ, e. gr.,the muscle, in which case the properties 

 of the end-organ must be taken into account, or by the aid of the 

 electrical response. Now, when a nerve is stimulated so rapidly 

 that the second stimulus falls into the nerve before the electrical 

 change due to the first stimulus has passed off, the second stimulus, 

 instead of adding its effect to that of the first, simply has no effect 

 at all; it finds the nerve unirritable.* According to this result, 

 we should expe^. that a summation of the effects of rapidly 

 *Gotch and Burch, "Journal of Physiology," 24, 410, 1899. 



