CHAP, i.] THE SPINAL CORD. 701 



any point will bring out tetanic convulsions of the whole body. 

 In such a case the strychnia has produced no anatomical change, 

 it has only modified _the molecular condition of the substance of 

 the cord ; and tliat has led to the apparent disappearance of all 

 the central mechanisms directing the paths of impulses ; these 

 now seem to travel in all directions all over the cord. In other 

 words the mechanisms of which we are speaking are mapped out^ 

 not by anatomical relations, but by the physiological condition 

 of the cord. Hence 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. Indeed, under a change of cir- 

 cumstances 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 inva- 

 riably 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 cir- 

 cumstances, 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 vari- 

 able automatism, like that which we call ' volition,' being mani- 

 fested 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, de- 

 prived of its brain so that the spinal cord only is left, makes no 

 spontaneous movements at all. Such an entire absence of spon- 

 taneity 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 con- 

 clusion is supported by the behaviour of other animals. Thus 

 similar vicarious reflex movements may be witnessed in mam- 

 mals, 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 by means of the liiml 

 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 t his 

 case the vicarious movements are ineffectual, the leg not being, 

 as in the case of the fro^, crossed over so as to bear on the spot 

 stimulated, but simply made to scratch the corresponding but 

 unstimulated spot on its own side, and cannot be considered as 



