LESIONS OF THE CEREBELLUM. 895 



cortex does not entail clear or obvious symptoms ; but that pronounced 

 disturbances ensue on injuring the deep structures. Experiments by- 

 Thomas 1 show, however, that purely cortical ablations of cerebellum 

 produce symptoms similar in type, though not so severe, as those 

 entailed by injury of the deep nuclei and tracts. Very extensive 

 removals of cortex induce symptoms of full and long-lasting 

 severity. 



Unilateral ablation. — The phenomena observable in a mammal 

 after ablation of a lateral {e.g. right) half of its cerebellum may be 

 briefly sketched thus. 



No symptom is noticeable until the deep chemical narcosis employed 

 during the operation has diminished. The animal then lies on the right 

 . side, with neck and trunk curved concavely to the right, and with some 

 torsion of the unsupported head, so that the occiput tends to the right, 

 but the muzzle rather to the left of the median position, and the left ear 

 rather higher than the right. The limbs, especially the fore-limbs, are 

 stiffly extended, the right more than the left. There is some abduction 

 of the right limbs. There is often conjugate deviation of the eyeballs to 

 the left. The animal cannot stand ; it falls to the right side, that is, so that 

 its right side is undermost. It may, if it try to move, roll round the 

 long axis of its body, with a movement continued in the same direction 

 as its fall. The movement of rotation may be accompanied by lateral 

 nystagmus, the slower slipping over of the eyeballs being from right to 

 left, the return snatches in the reverse direction. The majority of 

 observers believe that muscular weakness as well as spasm exists, 

 especially in the right limbs. The next three or four days witness 

 considerable recovery from some of these symptoms, and it is found 

 that the tendon phenomena are brisk in the spastic limbs, therefore 



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