THE REFLEX ACTIONS OF THE SPINAL CORD. 565 



say that the life of each part is a function of the life of the whole, is no less 

 true than to say that the life of the whole is a function of the life of each 

 part. This is especially borne in upon us when we come to study the actions 

 of the central nervous system. We may, on anatomical grounds, separate 

 the spinal cord from the brain ; but when we come to consider the respective 

 functions of the two, we are brought face to face with the fact that in actual 

 life a large part of the work of the brain is carried out by means of the 

 spinal cord, and conversely the spinal cord does its work habitually under 

 the influence of, if not at the direct bidding of, the brain. We may gain 

 certain conclusions by studying the behavior of the spinal cord isolated from 

 the brain, or of parts of the spinal cord isolated from each other ; but we 

 must be even more cautious than when we were dealing with other parts of 

 the body, and must greatly hesitate to take it for granted that the work 

 which we can make the spinal cord or a part of the spinal cord do, when 

 isolated from the brain, is the work which is actually done in the intact body 

 when the brain and spinal cord form an unbroken whole. Moreover, this 

 caution becomes increasingly necessary when in our studies we pass from the 

 simpler nervous system of one animal to the more complex nervous system 

 of another ; for it is by the complexity of their central nervous systems, 

 more than by anything else, that the " highest " animals are differentiated 

 from those " below" them. 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 compared 

 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 behavior of the isolated spinal cord of 

 one of these animals we infer the behavior of the intact spinal cord of man. 



A further difficulty meets us when an experimental investigation 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 under- 

 stood, 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. Thus a 

 section through the spinal cord, even when made with the sharpest instru- 

 ment and with the utmost skill, so as to avoid all bruising as much as possi- 

 ble, 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 distin- 



