698 REFLEX ACTIONS. [BOOK in. 



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

 but later on accommodates 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 imme- 

 diate ' 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 con- 

 clusions 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 ( 90) 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. 



458. Many of the features of reflex movements are 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. In these animals moreover 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. 

 In the frog reflex actions may be carried out by a very small 

 portion of the cord, by a single segment corresponding to a 

 pair of nerves, isolated by two transverse sections from the 

 parts above and below. Stimulation of the sensory fibres of 

 the nerve belonging to the segment will give rise to contrac- 

 tions in the muscles supplied by the motor fibres of the nerve. 

 The movements thus evoked are naturally simple in character. 

 When a larger portion of the cord is experimented upon, and 

 especially when the whole cord is employed, there are brought 

 to light what are perhaps the most notable features of reflex 

 movements, their complexity and purposeful character ; a few 

 afferent impulses developed in a few afferent fibres may lead 

 to many muscles being thrown into contractions, the time, 

 order and vigour of the several contractions being so related 



