FUNCTIONS OF THE CEREBELLUM 521 



that side, and impulses started in the skin of that side 

 cannot awaken sensations in the brain. 



But there is also a decussation of impulses in the case of 

 the nerves arising from the medulla above the decussation 

 of the pyramids. Thus, in the case quoted above of a blood 

 vessel bursting in the right cerebral hemisphere, the left 

 side of the man's face is paralysed as well as the left side 

 of his body, that is to say, impulses cannot pass to and 

 from his brain and the left facial and fifth nerves. The 

 impulses along these nerves also decussate, and reach the 

 right side of the brain. 



It sometimes happens, however, that disease or injury 

 may affect the medulla oblongata itself, on one side only 

 {e.g. the right), above the decussation of the pyramids, 

 in such a way that the fifth and facial ner\'es are 

 affected in their course before they decussate, that is to 

 say, on the same side as the injury. The man then, 

 while still paralysed on tlie left side of his body, is 

 paralysed on the right side of his face. 



14. The Functions of the Cerebellum. — When 

 speaking of reflex actions we pointed out (p. 487) that the 

 complicated movements of walking when once started by 

 the will are essentiallj' reflex in their continued produc- 

 tion. Moreover we also drew attention to the fact that 

 the co-ordhiatioii of the efferent impulses which, although 

 distributed to many difi"erent muscles, give rise by their 

 united action to the orderly movements of walking, is 

 dependent upon afferent impulses from various parts of 

 the body. Thus walking becomes unsteady or even 

 impossible in the absence of the normal sensory impulses 

 from the skin, or of visual impulses from the eyes ; and 

 to these we might have added afferent impulses from the 

 sensory nerves of the muscles themselves. When we 

 take cases of movements which are less obviously reflex, 

 that IS more strictly voluntary, than are those of walking, 

 we find that here again their orderly or co-ordinated pro- 

 duction depends largely on tactile and visual impulses. 



