718 THE BRAIN. 



this cerebro-cerebellar connection carries impulses from the cebral cortex 

 to the cerebellum ; and it has been further inferred that these impulses 

 are of the nature of motor impulses. As we have more than once urged, 

 the character of degeneration, that is, whether " ascending " or " descend- 

 ing," is not a satisfactory proof of the direction taken 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 each 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 ( 506) 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, shows 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 ex- 

 perience of our daily life, many of the phenomena of the condition known 

 as hypnotism and of allied conditions, as well as various experimental re- 

 sults such as that quoted in 574, where a sensory impulse seems to inhibit 

 the activity of a motor area, show that the cortex may itself in turn be in- 

 hibited 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 on- 

 ward, some of the cerebellar fibres of the middle peduncle on the one hand 

 upward toward the cerebrum, and, on the other hand, downward toward 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 gray 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 tegmen- 

 tum another tie, presumbly of a different nature from the foregoing, between 

 the cerebellum and the cortex ; indeed, it used to be called processes a 

 cerebello ad cerebrum. It is an obviously crossed tract (Fig. 136, $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 pro- 

 ceeding from the superficial gray matter of one side of the cerebellum, with 

 the red nucleus and other parts of the tegmentum of the crossed side, and 

 thus with the cortex of the crossed side. It has been supposed that the 

 direction of impulses passing along it is from the cerebrum to the cere- 

 bellum, but we have no clear proof of this ; indeed, as to what it does, we 

 have no satisfactory evidence either experimental or clinical. 



We may here incidentally remark that, in consequence of afferent tracts 

 being traced to or toward the tegmentum, and of the sharp contrast pre- 

 sented between the tegmentum and the conspicuously motor pyramidal tract 



