662 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



physiologists upon reactions of this sort as showing a capability 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 be- 

 cause the leg is prevented from moving or because they fail to remove 

 the stimulus, the prime fact remains that the stimulus continues to act 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 adjustment 

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

 precise ; and although the movements of the leg, when the stimulus is applied 

 far up on the rump, differ from those which follow the application of the 

 stimulus to the lower part of the thigh, yet in either case they are very wide, 

 and in both cases the foot is brushed across a large part of both the rump and 

 leg. Considering, therefore, the rather general character of these movements, 

 and the fact that the movement of the opposite leg only follows after a con- 

 tinued stimulus to the leg of the same side has produced an ineffective response, 

 it is best to explain the result by the diffusion of the impulses within the cord, 

 leaving quite to one side the psychical element. Such reflex actions are in a 

 high degree predictable, but in reality this has little significance, since there is 

 but one general movement that a frog in such a condition can make, whether 

 the stimulus be applied to the toes or the rump namely, the flexion of the 

 leg so that under these circumstances the prediction of the kind of movement 

 is a simple matter. The extent of the contraction is related to the intensity 

 of the stimulus, and is in turn dependent on the excitability of the central 

 system, which can be increased or diminished in various ways. The modifi- 

 cation of the reaction as dependent on the location of the stimulus can be in a 

 measure predicted, but the modification is wanting in precision just in so far 

 as the movements themselves are wanting in this quality. 



Periodic Reflexes. Not all reflexes are to be obtained from the same 

 animal with equal intensity at different times. In general, frogs in the spring- 

 time and in early summer, after reviving from their winter sleep, are highly 

 irregular in their reflex responses so irregular that students are advised not to 

 attempt the study of these reactions at this season. On the other hand, it is 

 during the spring that the mating occurs, and during this period the male 

 clasps the female and exhibits the peculiar reflex which has already been 

 described. Comparable with this variation in the frog must be the changes 

 which occur in the spinal cords of migratory birds which both in the spring 

 and in the fall are capable of such extended flights, or in the system of hiber- 

 nating mammals and all animals exhibiting extensive periodic variations in 

 their habits of life. 



General Applicability of these Results. There are many reptiles and 

 fishes in which the arrangement of the spinal cord is more simple than that in the 

 frog ; such are the animals in which the actions of locomotion are very uniform, 

 and in which these locomotory actions represent the principal responses of the 



