CENTRAL NERVOUS SYSTEM. 661 



tube filled with warm water at 47 to 61 C., which was applied to the skin. 

 Under these conditions it was remarkable that a continuous stimulus was 

 often followed, not by a single contraction of the muscles, but by a series of 

 contractions, suggesting that in the central system the cells are roused to a 

 discharge and then are for a time concerned with the preparation for sending 

 out new impulses, and that during this latter period the muscles were relaxed. 



Apparently a high degree of uniformity in the conditions was obtained in 

 these experiments, but at the same time the reactions were far from uniform, 

 in either the latent time of contraction or the order in which the contrac- 

 tion of the several muscles followed, although certain muscles tended to con- 

 tract first, and certain series of contractions to reappear. The co-ordination 

 of the contractions is therefore variable in time, even under these condi- 

 tions. These variations are probably due either to the fact that the impulses 

 are not distributed in the centre in the same manner on each occasion, or if 

 they are thus distributed, the central and efferent cells vary from moment 

 to moment in their responsiveness. That these cells should so vary is easy 

 to comprehend, for all the cell-elements in such a reflex frog are slowly dying. 

 In this process they are undergoing a destructive chemical change, and with 

 these destructive changes are generated weak impulses sufficient to cause their 

 physiological status continually to vary, thus modifying the effects of any 

 special set of incoming impulses acting upon them. 



It is not to be overlooked also that the dissection of the muscles tested, 

 and the removal of the skin about them, deprived the spinal cord of the 

 incoming impulses due to the stretching of the skin by the swelling of the 

 contracting muscles and disturbed the order and intensity of such sensory im- 

 pulses as come in from the tendons and the muscles themselves. However 

 much these impulses may add to the regularity of the muscular responses, as 

 apparently they do, in the case of an intact leg, these experiments indicate 

 that the regularity thus obtained is dependent rather on the constancy of the 

 incoming stimuli than on any fixed arrangement in the nerve-centres them- 

 selves. It is thus evident that the discharge of one efferent cell is not neces- 

 sary in order that another efferent cell may discharge, but that each dis- 

 charging cell stands at the end of a physiological pathway and may react 

 independently. 



Purposeful Character of Responses. When the muscular responses of 

 a reflex frog to a dermal stimulus are studied, they are seen to have a purpose- 

 ful character, in that they are often directed to the removal of the irritation. 

 This is demonstrated by placing upon the skin on one side of the rump a 

 small square of paper moistened with dilute acid. As a result the foot of the 

 same side is raised and the attempt made to brush the paper away ; if the first 

 attempt fails, it may be several times repeated. When the irritation has been 

 removed, the frog usually becomes quiet. If the leg of the same side be held 

 fast after the application of the stimulus, or if the first movements fail to 

 brush away the acid paper, then the leg of the opposite side may be contracted 

 and appropriate movements be made by it. Emphasis has been laid by various 



