CENTRAL NERVOUS SYSTEM. 711 



able iu monkey or mau, there are several replies. In the first place, no such 

 extensive experiments have been made on monkeys of the right age and under 

 equally favorable conditions. If a mature animal is taken, the secondary 

 degenerations are so massive that they certainly cause great disturbance in the 

 remaining part of the system. This is not equivalent to an assertion that the 

 same results could be obtained in the monkey by more extensive experiments, 

 but a suggestion of one difference behind the results thus far reported. There 

 is no reason for assuming any deep-seated difference in the arrangement of the 

 central system of the highest mammals as compared with that in the lower. 

 Indeed, in some human microcephalic idiots the proportion of sound and 

 functional tissue in the eucephalon is less than one-fourth that found in a 

 normal person, yet, on the other hand, no normal adult could lose anything 

 like that amount of tissue which is out of function in these microcephalic brains 

 and at the same time live. 



The central system, therefore, even in man, is to be looked upon as possessed 

 of some power to adapt itself when portions have been lost, but this is most 

 evident when the defect begins early and develops slowly. 



Keeping the cerebrum still in view, it is possible to go into further detail. 

 In forms below the monkey the loss of portions of the cerebral cortex from the 

 motor area is accompanied by a greater or less paralysis of the muscles repre- 

 sented. This, however, is an initial symptom only, and gradually disappears, 

 though not always with the same completeness. In man, of course, the tend- 

 ency to recover is least. 



The anatomical relations behind this difference are the following : The 

 efferent cells in the ventral horns are dominated principally by two sets of 

 impulses, those arriving directly over the dorsal roots of that segment in which 

 they are located, and those coming over the long paths by way of the cerebral 

 cortex and pyramidal tracts. In the lower mammals this second pathway is 

 insignificant, and when interrupted, therefore, the disturbance iu the control of 

 the ventral-horn cells is but slight. Passing up the series, however, this path- 

 way tends to become more and more massive and important, as the figures pre- 

 viously given show (see p. 695), until in man and the monkey a damage of it 

 such as is effected by injury to the cortex causes a high degree of paresis if not 

 permanent paralysis, because by this injury a greater proportion of the impulses 

 is thus cut off from the efferent cells. 



It has previously been shown that the cortical areas do not vary accord- 

 ing to the mass of the muscles which they control. Experiments also show 

 that it is the fore limbs which are most disturbed in their reactions when the 

 lesion involves the cortical centres for both fore and hind limbs, and this falls 

 under the law that the more highly adaptable movements (i. e. those of the fore 

 limb as contrasted with the hind limb) are most under the control of the cortex. 

 If the examination be restricted to the fore limb alone, it is found that the 

 finger and hand movements or those of the more distal segments are in turn 

 the ones most disturbed. Thus, in the limbs, the more distal groups of mus- 

 cles are those best controlled from the cortex. It follows, then, that for the 



