566 THE SPINAL CORD. 



guish between the temporary and the lasting effects of the operation, includ- 

 ing among the former not only those of ordinary " shock," but others of 

 slower development or longer duration. In many instances where a part of 

 the central nervous system is by section or otherwise suddenly separated 

 from the rest, the phenomena suggest that the separated part is at first pro- 

 foundly influenced as to its activities by the withdrawal of various influences 

 which previously were being exerted upon it by the rest of the system, but 

 later on accommodates itself to the 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, moreover, it does carry out actions of this kind 

 when in the intact organism it is working in contact with the brain. Indeed, 

 the carrying out of various reflex actions seems to be one of the most impor- 

 tant functions of the spinal cord, so much so that though the brain or, at 

 least, parts of the brain can also and do develop reflex actions, the spinal 

 cord offers the best field for the study of these actions. We have already 

 ( 97) touched on the general features of reflex actions, and elsewhere have 

 incidentally dwelt on particular instances ; we may therefore confine our- 

 selves now to certain points of special interest. 



496. 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 preparations 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 awhile suspends reflex activity, soon passes away ; within 

 a very short time after the bulb, for instance, has been divided the most com- 

 plicated 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 ex- 

 hibit 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 body; a very feeble stimulus applied to the 

 skin of these regions promptly gives rise to extensive and ^ yet coordinate 

 movements. Indeed, the more the matter is studied, the stronger is the evi- 

 dence that the reflex movements carried out by isolated portions of the 

 spinal cord of the mammal are hardly less definite, complete, and purpose- 

 ful than those witnessed in the frog. It is worthy of attention, as bearing 

 out the remarks made above on the great differentiation of the central ner- 

 vous system in the higher animals, that the reflex phenomena in mammals 



