CHAP, ii.] THE BRAIN. 1111 



by impulses; but it is perhaps of more importance to remember 

 that, as we have also urged, we have no right to assume that the 

 impulses passing along such a tract as the one in question must 

 be either sensory or motor, or indeed that such a tract serves as 

 an instrument for producing effects in one direction only. 



That during life the fibres of which we are speaking serve as 

 an important chain by which cerebral cortex and cerebellum affect 

 the one the other, there can be but little doubt ; but we are 

 wholly in the dark as to what really takes place along the fibres. 

 We have seen ( 593) reason to think that the brain may and 

 does exert an inhibitory influence over the spinal cord ; and the 

 mechanical certainty with which an animal deprived of its cerebral 

 hemispheres responds to stimuli, in contrast to the uncertainty 

 attending the result of stimuli applied to an intact animal, as well 

 as all the experience of our own daily life shews that the cerebral 

 cortex can work in an inhibitory manner on other parts of the 

 brain; the remarkable "forced movements" on which we dwelt in 

 a previous section seem in some instances to be the result of the 

 abrupt snap of some inhibitory bond. Conversely all the experi- 

 ence of our daily life, many of the phenomena of the condition 

 known as hypnotism and of allied conditions, as well as various 

 experimental results such as that quoted in 661, where a 

 sensory impulse seems to inhibit the activity of a motor area, 

 shew that the cortex may itself in turn be inhibited by other 

 parts of the central nervous system. But we have at present no 

 satisfactory indications as to the paths of inhibitory impulses or 

 as to how inhibition is brought about ; nor have we any proof that 

 the cerebro-cerebellar tract is an inhibitory one in either direction. 



We may add that some of the fibres of the middle peduncle 

 appear to be neither commissural nor connected with the cortical 

 fibres in the pes, but to end in other ways ; and tracts have been 

 described as continuing onwards some of the cerebellar fibres of 

 the middle peduncle on the one hand upwards toward the 

 cerebrum, and on the other hand downwards towards the spinal 

 cord. It has been further urged that these tracts are efferent 

 in function. 



Lastly, we may call attention to the superior peduncles. These, 

 which as we have seen appear to come largely from the grey 

 matter of the nucleus dentatus and to end in the tegmentum, 

 largely in the red nucleus, may be regarded as constituting 

 through the relay of the front part of the tegmentum another 

 tie, presumably of a different nature from the foregoing, between 

 the cerebellum and the cortex ; indeed it used to be called the 

 processus a cerebello ad cerebrum. It is an obviously crossed 

 tract (Fig. 113, $P); it connects one nucleus dentatus, and so 

 presumably by that relay the fibres of the inferior peduncle 

 ending in that body, and perhaps other fibres proceeding from 

 the superficial grey matter of one side of the cerebellum, with 



