CHAP, i.] THE SPINAL CORD. 909 



into nervous mechanisms by the establishment of lines of greater 

 or less resistance, so that the disturbances in it generated by 

 certain afferent impulses are directed into certain efferent channels. 

 It may be added that though conspicuously purposeful movements 

 seem to need the concurrent action of several segments of the cord, 

 and as a rule, the greater the length of the cord involved the 

 more complex and the more distinctly purposeful the movement, 

 still the movements evoked by even a segment of the cord may 

 be purposeful in character; hence we must conclude that every 

 segment of the nervous network is mapped out into mechanisms. 

 But the arrangement of these mechanisms, especially of the more 

 complex ones, is 'not a fixed and rigid one. We cannot always 

 predict exactly the nature of the movement which will result from 

 the stimulation of any particular spot, because the result will vary 

 according to the condition of the spinal cord, especially in relation 

 to the strength and character of the stimulus. Moreover, under 

 a change of circumstances a movement quite different from the 

 normal one may make its appearance. Thus when a drop of acid 

 is placed on the right flank of a brainless frog, the right foot is 

 almost invariably used to rub off the acid ; in this there appears 

 nothing more than a mere 'mechanical' reflex action. If however 

 the right leg be cut off, or the right foot be otherwise hindered 

 from rubbing off the acid, the left foot is, under the exceptional 

 circumstances, used for the purpose. This at first sight looks 

 like an intelligent choice. A choice it evidently is ; and were 

 there many instances of choice, and were there any evidence 

 of a variable automatism, like that which we call 'volition', 

 being manifested by the spinal cord of the frog, we should be 

 justified in supposing that the choice was determined by an 

 intelligence. But, as we shall have occasion later on to point out, 

 a frog, deprived of its brain so that the spinal cord only is left, 

 makes no spontaneous movements at all. Such an entire absence 

 of spontaneity is wholly inconsistent with the possession of 

 intelligence. Then again the above experiment, if not the only 

 instance, is at all events by far the most striking instance of choice 

 on the part of a brainless frog. We are therefore led to conclude 

 that the phenomena must be explained in some other way than 

 by being referred to the working of an intelligence. Moreover 

 this conclusion is supported by the behaviour of other animals. 

 Thus similar vicarious reflex movements may be witnessed in 

 mammals, though not perhaps to such a striking extent as in frogs. 

 In dogs, in which partial removal of the cerebral hemispheres has 

 apparently heightened the reflex excitability of the spinal cord, 

 the remarkable scratching movements of the hind leg which are 

 called forth by stimulating a particular spot on the loins or side of 

 the body, are executed by the leg of the opposite side, if the leg of 

 the same side be gently held. In this case the vicarious movements 

 are ineffectual, the leg not being, as in the case of the frog, crossed 



