CHAP, i.] THE SPINAL CORD. 903 



When we compare a rabbit, a dog, a monkey and a man, the 

 differences in the vascular, digestive and respiratory systems of the 

 four, striking as they may appear, sink into insignificance com- 

 pared with the differences 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 circulation 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 investiga- 

 tion 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 conscious- 

 ness, of volition, of reflex movements and other nervous actions. 

 Tli us 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 

 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 temporary 

 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 



