570 THE SPINAL CORD. 



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 can- 

 not 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 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 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 circum- 

 stances, 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 phe- 

 nomena must be explained in some other way than by being referred to the 

 working of an intelligence. Moreover, this conclusion is supported by the 

 behavior 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 remark- 

 able scratching movements of the hind leg which are called forth by stimu- 

 lating 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 effectual, the leg not being, as 

 in the case of the frog, crossed over so as to bear on the spot stimu- 

 lated, and cannot be considered as betokening intelligence. Again, the 

 " mechanical " nature of reflex actions is well illustrated by the behavior 

 of a decapitated snake. When the body of the animal in this condition 

 is brought into contact at several places at once with an arm or a stick, 

 complex reflex movements are excited, the obvious purpose as well as 

 effect of which is to twine the body round the object. A decapitated 

 snake will, however, with equal and fatal readiness twine itself round a 

 red-hot bar of iron, which is made to touch its skin in several places at 

 the same time. 



502. In considering the nature of the events in the spinal cord which 

 determine the behavior of the frog in the instance just mentioned we must 

 bear in mind that the movements in question are " coordinated ; " that is to 

 say, not only are many distinct muscles brought into play, but certain rela- 

 tions are maintained between the amount, duration, and exact time of occur- 

 rence of the contraction of each muscle and those of the contractions of its 

 fellow muscles sharing in the movement. In the absence of such coordina- 

 tion the movement would become irregular and ineffectual. We shall have 

 occasion later on in dealing with voluntary movements to point out that 

 the coordination and hence the due accomplishment of a voluntary move- 

 ment is dependent on certain afferent impulses passing up from the con- 



