1112 THE CORPORA QUADRIGEMINA. [BOOK in. 



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 cerebellum, 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 towards the tegmentum and 

 of the sharp contrast presented between the tegmentum and 

 the conspicuously motor pyramidal tract in the pes, the view 

 has gained ground that the tegmentum is essentially a sensory 

 structure. But there does not appear to be adequate evidence 

 either clinical or experimental for such a conclusion. The 

 thalamus, which we have regarded as the front so to speak of the 

 tegmentum, cannot, as we have already urged ( 686), be con- 

 sidered exclusively or especially sensory. And many of the ties 

 of the tegmentum, such as the fibres from the corpora striata 

 ending in the substantia nigra, for this may be considered as 

 properly belonging to the tegmentum, are of the kind which we 

 may suppose to be efferent or motor. Indeed we may probably 

 regard the whole tegmentum as being broadly the analogue in 

 the forward segments of the cerebro-spinal axis of both the 

 anterior and posterior grey matter of the spinal segments behind. 



Though we are thus in the dark concerning what goes on in 

 the cerebellum, it may be worth while to call attention once more 

 to the remarkable characters of the superficial grey matter ( 648). 

 The many points of resemblance between it and the cerebral 

 cortex cannot but suggest that the processes taking place in it 

 have some analogies with cortical events. And it is at least a 

 fact of some significance that congenital deficiency, or atrophy 

 of the cerebral hemisphere of one side, is frequently accompanied 

 by a corresponding deficiency of the crossed cerebellar hemisphere. 



688. Both the anterior and posterior corpora quadri- 

 gemina are complex in structure ; not only do they differ from 

 each other, but also in each the grey matter differs in different parts, 

 both as to its nature and appearance and as to its connections 

 with tracts of fibres. If we have little right to speak of the 

 "functions of the cerebellum," we have even less right to speak 

 of the " functions of the corpora quadrigemina " or of either pair 

 of them. The anterior pair, as we have seen, has to do in some 

 way with vision ; but we have reason to think that a part only of 

 the whole body is thus concerned ; and there is some foundation 

 for the view that of this part, one portion belongs, so to speak, to 

 the optic tract and another portion to the cortical fibres of the 

 optic radiation. Possibly still another part is concerned in 

 bringing, as we have ( 673) suggested, visual impulses to bear 

 on the coordination of movements. 



Stimulation of the surface of the posterior pair, besides 



