CHAP, vi.] THE BRAIN. 639 



vision, the pupils are at the same time contracted ; and when the 

 eyeballs are directed upwards, and return to parallelism, the pupils 

 are dilated to a corresponding extent; when both eyeballs are 

 moved together sideways the pupils remain unchanged. We have 

 seen (p. 546) that the various movements of the eyeballs may be 

 brought about by direct stimulation of particular parts of the 

 anterior corpora quadrigemina, and are then also accompanied by 

 the appropriate changes in the pupils. The association therefore 

 of the movements of the pupil and of the ocular muscles is not 

 simply psychical in nature but is dependent on the close connection 

 of their respective centres. From the fact of the movements of 

 the eyeball and pupil being so readily and variously excited by 

 stimulation of the anterior corpora quadrigemina it has been 

 inferred that the centres for these movements lie in those bodies ; 

 it would appear however that what may be called the real or im- 

 mediate centres of these movements lie beneath the corpora quad- 

 rigemina, in the front part of the floor of the aqueduct of Sylvius, 

 and therefore are affected in an indirect manner only when the 

 corpora quadrigemina are stimulated. 



It was long ago observed that unilateral extirpation of the 

 corpora quadrigemina in mammals or of the optic lobes in birds 

 produced blindness in the opposite eye ; and the same result has 

 been gained by many subsequent observers. We have seen more- 

 over that both frogs, birds, and mammals continue to receive and 

 within limits to react upon visual impressions after the total 

 removal of the cerebral hemispheres. From these facts we infer 

 that visual sensory impulses become transformed into visual sensa- 

 tions in the corpora quadrigemina ; or, in other words, that these 

 nervous structures are centres of sight. But they are so in a limited 

 sense only. We have seen that destruction or injury of the cerebral 

 hemispheres profoundly affects vision ; even admitting that in such 

 cases the results may be in part at least due to concomitant failures 

 in the optic thalami, we may still venture to say that in the absence 

 of the cerebral convolutions, a crude vision, devoid of distinct visual 

 perceptions, is probably all that is possible. The processes consti- 

 tuting distinct and perfect vision, in fact, begin in the retina, are 

 partially elaborated in the corpora quadrigemina and further de- 

 veloped in the optic thalami, but do not become perfected until the 

 cerebral convolutions have been called into operation. Anatomical 

 considerations lead us to suppose that the anterior pair of the 

 corpora quadrigemina are alone connected with the optic tract, 

 and so with the external corpus geniculatum and optic thalamus. 

 Hence we may infer that it is the anterior pair alone which are 

 thus concerned in vision and that the posterior pair have some 

 other function. 



In those animals (ex. gr. rabbits) in which unilateral destruction 

 of the corpora quadrigemina entails blindness of the opposite eye, 

 and yet does not affect at all the visual sensory impulses originating 



