904 REFLEX ACTIONS. [BOOK in. 



withdrawal of various influences which previously were being 

 exerted upon it by the rest of the system, but later on accom- 

 modates itself to its new conditions, and learns, so to speak, to 

 act without the help of those influences. And indeed it is possible 

 that some of the effects of even immediate ' shock ' may be due, 

 not as suggested above, to the action of an inhibitory or exhausting 

 stimulus, but to the sudden cessation of habitual influences. 



Still, in spite of all these difficulties, it is possible not only to 

 ascertain the working of an isolated portion of the central nervous 

 system, but even to infer from the results some conclusions as to 

 the share taken by that portion in the working of the entire and 

 intact system. There can be no doubt, for instance, that the 

 spinal cord can, quite apart from the brain, carry out various reflex 

 actions, and that moreover it does carry out actions of this kind 

 when in the intact organism it is working in concert with the 

 brain. Indeed the carrying out of various reflex actions seems to 

 be one of the most important functions of the spinal cord, so 

 much so that, though the brain or, at least, parts of the brain can 

 also and do develope reflex actions, the spinal cord offers the best 

 field for the study of these actions. We have already ( 101) 

 touched on the general features of reflex actions, and elsewhere 

 have incidentally dwelt on particular instances ; we may therefore 

 confine ourselves now to certain points of special interest. 



583. Reflex movements are perhaps best studied in the 

 frog and other cold-blooded animals, since in these the actions 

 of the cord are less dependent on, and hence less obscured by 

 the working of, the other parts of the central nervous system. 

 They obtain however in the warm-blooded mammal also, but in 

 these special precautions are necessary to secure their full 

 development. In the frog the shock, which as we have said 

 follows upon division of the spinal cord and for a while suspends 

 reflex activity, soon passes away ; within a very short time after 

 the bulb for instance has been divided the most complicated reflex 

 movements can be carried on by the frog's spinal cord when the 

 appropriate stimuli are applied. With the mammal the case is 

 very different. For days even after division of the spinal cord the 

 parts of the body supplied by nerves springing from the cord below 

 the section may exhibit very feeble reactions only. In the dog, 

 for instance, after division of the spinal cord in the lower dorsal 

 region, the hind limbs hang flaccid and motionless, and pinching 

 the hind foot evokes as a response either slight irregular movements 

 or none at all. Indeed were our observations limited to this period 

 we might infer that the reflex actions of the spinal cord in the 

 mammal were but feeble and insignificant. If however the animal 

 be kept alive for a longer period, for weeks or better still for 

 months, though no union or regeneration of the spinal cord takes 

 place, reflex movements of a powerful, varied and complex character 

 manifest themselves in the hind limbs and hinder parts of the 



