THE KEFLEX ACTIONS OF THE SPINAL CORD. 569 



or that afferent nerve, are transformed on their advent at the network ; and 

 accordingly the path of any series of impulses in the network will be deter- 

 mined largely by the energy of the afferent impulses. And the action of 

 strychnine may be, in part, explained by supposing that it reduces and 

 equalizes the normal resistance of this network, so that even weak impulses 

 travel over all its tracts with great ease. 



500. Further, the movement, forming part of a reflex action, varies in 

 character according to the particular part of the body to which the stimulus 

 is applied. The reflex actions developed by stimulation of the internal vis- 

 cera are different from those excited by stimulation of the skin. We have 

 reason to think that the contraction of, or other changes in a skeletal muscle 

 may produce, by reflex action, contractions of other muscles ; and such 

 reflex actions also differ from those started by stimulation of the skin. In 

 reflex actions started by applying a stimulus to the skin the movements vary 

 largely, according to the particular area of the skin which is affected. Thus, 

 pinching the folds of skin surrounding the anus of the frog produces differ- 

 ent effects from those witnessed when the flank or toe is pinched ; and, 

 speaking generally, the stimulation of a particular spot calls forth partic- 

 ular movements. In the case of the simple reflex movements, it appears to 

 be a general rule that a movement started by the stimulation of a sensory 

 surface or region on one side of the body is developed on the same side of 

 the body, and if it spreads to the other side, still remains most intense on the 

 same side ; the movement on the other side, moreover, is symmetrical with 

 that on the same side. It has been maintained that " crossed " or diagonal 

 reflex movements, as where stimulation of one fore-foot leads to movements 

 of the opposite hind limb, do not occur unless some portion of the bulb be 

 left attached to the spinal cord. Seeing that locomotion of four-footed 

 animals is largely effected by diagonal movements of the limbs, one would 

 rather have expected to find the spinal cord itself provided with mechan- 

 isms to assist in carrying them out ; and, indeed, it is affirmed that in the 

 case of cold-blooded animals and of many young mammals, after a division 

 of the spinal cord below the bulb, a gentle stimulation will provoke a diagonal 

 movement, slight pressure on one fore foot, for example, giving rise to move- 

 ments in the opposite hind leg ; a strong stimulus, however, will produce an 

 ordinary one-sided movement. Again, when in a dog the cord has been 

 divided in the lower thoracic region so that the hind limbs depend on the 

 lumbar cord alone, a rhythmically repeated drawing up and letting down 

 of the hind limbs is witnessed when these are allowed to hang down ; and 

 these movements, which appear to be of a reflex nature excited by the 

 pendent position of the limbs, are often seen to alternate regularly in the 

 two limbs, the right leg being extended while the left leg is being drawn up 

 and vice versa. It may further be observed that if the foot of one pendent 

 limb be pinched while the other limb is passively flexed the flexion of the 

 limb which is pinched is accompanied by an extension of the other limb. 

 In these respects, however, different animals, as already urged, differ from 

 each other. 



501. From these and similar phenomena we may infer that the nervous 

 network spoken of above is, so to speak, mapped out into nervous mechan- 

 isms 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 pur- 

 poseful 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 move- 

 ments evoked by even a segment of the cord may be purposeful in character ; 



