CHAP, i.] THE SPINAL CORD. 697 



digestive and respiratory systems of the four, striking as they 

 may appear, sink into insignificance compared with the dif- 

 ferences exhibited by their respective central nervous systems. 

 We need caution when from the results of experiments on dogs 

 or rabbits, we draw conclusions as to the digestion or circula- 

 tion of man, but we need far greater caution when from the 

 behaviour of the isolated spinal cord of one of these animals 

 we infer the behaviour of the intact spinal cord of man. 



A further difficulty meets us when an experimental investi- 

 gation entails operative interference with the central nervous 

 system. Removal or section of, or other injury to parts of the 

 brain or spinal cord is very apt to give rise in varying degree 

 to what is known as 'shock.' The cutting or tearing or other 

 lesion of any considerable mass of nervous substance affects the 

 activity, not only of the structures immediately injured, but of 

 other, it may be far distant, structures. The nature of ' shock ' 

 is not as yet thoroughly understood, but may perhaps, in part 

 at all events, be explained by regarding the lesion as a very 

 powerful stimulus, which, partly by way of inhibition but still 

 more by way of exhaustion, depresses or suspends for a while 

 normal functions, and thus gives rise to temporary diminution 

 or loss of consciousness, or volition, of reflex movements and 

 other nervous actions. Thus a section through the spinal cord, 

 even when made with the sharpest instrument and with the 

 utmost skill, so as to avoid all bruising as much as possible, 

 may for a while suspend all reflex activity of the cord, or indeed 

 all the obvious activities of the whole central nervous system. 

 We may add that such a ' shock ' of the central nervous system 

 may also be produced by N sudden lesions not bearing directly 

 on the central nervous system, as for instance by extensive 

 injury to a limb. 



Moreover in many cases in which the effects of experimental 

 interference have been watched for some considerable time, 

 days, months or years after the operation, it has been observed, 

 on the one hand, that phenomena which are conspicuous in the 

 early period may eventually disappear, and, on the other hand, 

 that activities which are at first absent may later on make their 

 appearance ; movements for instance which are at first frequent 

 after a while die away, and conversely, movements which at 

 first seemed impossible are later on easily achieved. We have 

 to distinguish or to attempt to distinguish between the tem- 

 porary and the lasting effects of the operation, including 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 profoundly influenced 

 as to its activities by the withdrawal of various influences which 



