CHAP, v.] THE SPINAL CORD. 589 



blooded animals and of many young mammals, after division of the 

 spinal cord below the medulla, a gentle stimulation will provoke a 

 diagonal movement, slight pressure on one fore-foot for example 

 giving rise to movements in the opposite hind-leg; a strong 

 stimulus however will produce an ordinary one-sided movement. 



From these and similar phenomena we may infer that the proto- 

 plasmic network spoken of above is, so to speak, mapped out 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. But 

 the arrangement of these mechanisms 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, be- 

 cause the result will vary according to the condition of the spinal 

 cord, especially in relation to the strength of the stimulus. More- 

 over, 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 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 of a conscious 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. 

 It is however, on the other hand, quite possible to suppose that the 

 lines of resistance in the spinal protoplasm are so arranged as to 

 admit of an alternative, though still mechanical, action ; and seeing 

 how few and simple are the apparent instances of choice witnessed 

 in a brainless frog, and how absolutely devoid of spontaneity 

 or irregular automatism is the spinal cord of the frog, this seems 

 the more probable view. 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 move- 

 ments of the hind leg which are called forth by stimulating a particu- 

 lar 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 and can 

 hardly be considered as betokening intelligence. Again the 

 'mechanical' nature of reflex actions is well illustrated by the 

 behaviour of a decapitated snake. When the body of the animal 

 in this condition is brought into contact at several places with any 



