640 CEREBELLUM. [BOOK in. 



in the eye of the same side, it is obvious that complete decussation 

 of the sensory impulses must take place before the centre is 

 reached. The question however whether the decussation of fibres 

 (and consequently of impulses) in the optic chiasma is complete or 

 incomplete, whether the optic tract of one side is the continuation 

 of the fibres in the optic nerve of the opposite side exclusively or 

 whether it is composed of representatives of the optic nerves of both 

 sides, is one which has been much debated, both from an anatomical 

 and a physiological standpoint. In the case of mammals the evidence 

 goes to shew that in some kinds of animals (rabbits) the decussation 

 is complete, but in others (dogs) more or less incomplete. In man 

 a peculiar affection of vision, which may be spoken of under the 

 general name of hemiopia, is a frequent symptom of diseases of the 

 brain. In this affection portions of the field of vision are wanting ; 

 thus a patient sometimes can see nothing in the right half or the 

 upper half of the fields of vision of both eyes ; looking at a man or a 

 house he can only see half the object, the left half or the lower half 

 as the case may be. Hemiopia of both eyes has been observed in 

 cases in which disease was apparently limited to one side of the 

 brain; and these cases added to other evidence lead to the con- 

 clusion that in man the decussation is incomplete. 



Many observers have noticed that injury or removal of the 

 corpora quadrigemina on one side frequently caused forced move- 

 ments, and that removal of the whole mass led to great want of co- 

 ordination. These results are quite in harmony with the fact men- 

 tioned above (p. 610) concerning the coordinating functions of the 

 optic lobes in frogs. But at present we have no exact knowledge 

 concerning the nature of the coordination, and what relations are 

 borne in this respect by the corpora quadrigemina to the cere- 

 bellum, crura cerebri, and pons Yarolii. 



Various observers have witnessed as the result of stimulation of 

 the corpora quadrigemina movements of the several parts of the 

 alimentary canal, and of the urinary bladder, changes in blood- 

 pressure, and alterations in the working of the respiratory mecha- 

 nism, indicating that these bodies have a special connection with 

 the centres (in the medulla oblongata and spinal cord respectively) 

 concerned in carrying on these movements. 



Cerebellum. We have already referred to the cerebellum as 

 being probably concerned in the coordination of movements. It 

 was long ago observed that when a small portion of the cerebellum 

 was removed from a pigeon, the animal's gait became unsteady ; 

 when larger portions were taken away its movements became much 

 more disorderly, and when the whole of the organ was removed an 

 almost total loss of coordination supervened. When the portion 

 removed was small, the disorderly movements which at first ap- 

 peared eventually vanished, but when a large portion was re- 

 moved the loss of coordination became permanent. Subsequently 



