CORPORA QUADRIGEMINA. 411 



Each cms cerebri contains among its fibres a mass of vesic- 

 ular substance, the locus niger, the nerve-corpuscles of which 

 abound in pigment-granules, and afford some of the best in- 

 stances of the caudate structure. 



With regard to their functions, the crura cerebri may be 

 regarded as, principally, conducting organs. As nerve-centres 

 they are probably connected with the functions of 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 valueless. 



Corpora Quadrigemiua. The corpora quadrigemiua (from 

 which, in function, the corpora geniculata are not distinguished), 

 are the homologues of the optic lobes in birds, amphibia, and 

 fishes, and may be regarded as the principal nervous centres 

 for the sense of sight. The experiments of Flourens, Longet, 

 and Hertwig, show that removal of the corpora quadrigemina 

 wholly destroys the power of seeing ; and diseases in which 

 they are disorganized 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. 



This loss of sight is the only apparent injury of sensibility 

 sustained by the removal of the corpora quadrigemina. The 

 removal of one of them affects the movements 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 quadri- 

 gemina are irritated : it is always dilated when they are re- 

 moved : so that they may be regarded, in some measure at 

 least, as the nervous centres governing its movements, and 

 adapting them to the impressions derived from the retina 

 through the optic nerves and tracts. 



Concerning the functions, taken as a whole, discharged by 

 the olfactory and optic lobes, the gray substance of the pons, 

 the corpora striata and optic thalami (b, d, Fig. 146), with 



