CENTRAL NERVOUS SYSTEM. 215 



contractions is therefore variable in time, even under these conditions. These 

 variations are probably due either to the fact that the impulses arc not dis- 

 tributed 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 compre- 

 hend, 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 

 impulses as come in from the tendons and the muscles themselves. The 

 observations of both Bickel x and Hering 2 show that these impulses are not 

 necessary for accurate reflex movements of the frog's leg, and thus weaken 

 the force of the suggestion just made. However much these impulses may 

 add to the regularity of the muscular responses, Lombard concludes that 

 the discharge of one efferent cell is not necessary in order that another 

 efferent cell may discharge, but that each discharging 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 pur- 

 poseful character, in that they are often directed to the removal of 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 physiologists upon reactions of this sort as showing a capa- 

 bility of choice on the part of the spinal cord, thus granting to the cord 

 psychical powers. Against such a view it must be urged that the movements 

 of the leg on the side opposite to the stimulus do not occur until after the 

 muscles of the leg on the same side have responded. When these responses 

 are inefficient because the leg is prevented from moving or because tiny tail 

 to remove the stimulus, the prime fact remains that the stimulus continues /<> 

 ad and the diffusion of the impulses in the cord goes on, involving in either 

 case the nerve-cells controlling the muscles of the opposite leg. The adjust- 

 ment of the reaction of the leg, on whichever side it occurs, is, however, far 



1 Bickel : Pfluger'a Archiv, Kd. lxvii. 



1 Hering: Archiv fiir experimenleUe Paihologie und Pharmakologie, Bd. xxxviii. 



