434 THE BRAIN. 



Morbid alterations of the cerebellum, furthermore, particularly 

 of a chronic nature, such as slow inflammations, abscesses, tumors, 

 &c., have often been observed in the human subject, without giving 

 rise to any marked disturbance of the voluntary movements. 



On the other hand, many facts derived from comparative anatomy 

 seem to favor the opinion of Flourens. If we compare different 

 classes of animals with each other, as fish with reptiles, or birds 

 with quadrupeds, in which the development and activity of the 

 entire nervous system vary extremely, the results of the comparison 

 will be often contradictory. But if the comparison be made be- 

 tween different species in which the general structure and plan of 

 organization are similar, we often find the development of the cere- 

 bellum to correspond very closely with the perfection and variety 

 of the voluntary movements. The frog, for example, is an aquatic 

 reptile, provided with anterior and posterior extremities; but its 

 movements, though rapid and vigorous, are exceedingly simple in 

 character, consisting of little else than flexion and extension of the 

 posterior limbs. The cerebellum in this animal is exceedingly 

 small, as compared with the rest of the brain ; being nothing more 

 than a thin, narrow ribbon of nervous matter, stretched across the 

 upper part of the fourth ventricle. In the common turtle we have 

 another aquatic reptile, where the movements of swimming, diving, 

 progression, &c., are accomplished by the consentaneous action of 

 anterior and posterior extremities, and where the motions of the 

 head and neck are also much more varied than in the frog. In 

 this instance the cerebellum is very much more highly developed 

 than in the former. In the alligator, again, a reptile whose motions, 

 both of the head, limbs, and tail, approach very closely to those of 

 the quadrupeds, the cerebellum is still larger in proportion to the 

 remaining ganglia of the encephalon. 



The complete function of the cerebellum, accordingly, as a nerv- 

 ous centre, cannot be regarded as positively ascertained ; but so far 

 as we may rely on the results of direct experiment, this organ has 

 evidently such an intimate and peculiar connection with the volun- 

 tary movements, that a sudden and extensive injury inflicted upon 

 its substance is always followed by an immediate, though tempo- 

 rary, disturbance of the co-ordinating power. 



TUBERCULA QUADRIGEMINA. These bodies, notwithstanding 

 their small size, are very important in regard to their function. 

 They give origin to the optic nerves, and preside, as ganglia, over 



