522 THE NERVOUS SYSTEM. 



the third cerebral nerve, which arises from the locus niger, 

 and through which are directed the chief of the numerous 

 and complicated movements of the eyeball and iris. 



From the result of vivisection it appears that when one 

 of the crura cerebri is cut across, the animal moves round 

 and round, rotating around a vertical axis from the injured 

 towards the sound side. Such movements, however, attend 

 the sections of other parts than the crura cerebri ; and as 

 indications of the functions of these parts, the results of 

 such experiments have been hitherto almost absolutely 

 valueless. 



Corpora Quadrigemina. The corpora quadrigemina (from 

 which, in function, the corpora geniculata are not distin- 

 guished), are the homologues of the optic lobes in birds, 

 Amphibia and fishes, and may be regarded as the prin- 

 cipal nervous centres for the sense of sight. The experi- 

 ments of Flourens, Longet, and Hertwig, show that 

 removal of the corpora quadrigemina wholly destroys the 

 power of seeing ; and diseases in which they are disor- 

 ganized are usually accompanied with blindness. Atrophy 

 of them is also often a consequence of atrophy of the eyes. 



Destruction of one of the corpora quadrigemina (or of 

 one optic lobe in birds), produces blindness of the opposite 

 eye. 



And this loss of sight is the only apparent injury of 

 sensibility sustained by the removal of the corpora quad- 

 rigemina. The removal of one of them affects the move- 

 ments of the body, so that animals rotate, as after division 

 of the crus cerebri, only more slowly : but this is probably 

 due to giddiness and partial loss of sight. The more 

 evident and direct influence is that produced on the iris. 

 It contracts when the corpora quadrigemina are irritated : 

 it is always dilated when they are removed : so that they 

 may be regarded, in some measure at least, as the nervous 

 centres governing its movements, and adapting them to 



