THE CEREBRO-S FINAL NERVOUS SYSTEM. 



501 



of seeing; and diseases in which they are disorganized are usually accom- 

 panied by 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 (2) 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 may be due to giddiness and partial loss of sight. 



(3) The more evident and direct influence is that produced on the iris. 

 It contracts when the corpora quadrigemina are irritated: it is always 



FIG. 341. Base of the brain. 1, superior longitudinal fissure; 2, 2', 2", anterior cerebral lobe; 3, 

 fissure of Sylvius, between anterior and 4, 4', 4", middle cerebral lobe; 5,5', posterior lobe; 6, 

 medulla oblpngata; the figure is in the right anterior pyramid; 7, 8, 9, 10, the cerebellum; -f the in- 

 ferior vermiform process. The figures from I. to IX. are placed against the corresponding cerebral 

 nerves; III. is placed on the right crus cerebri. VI. and VII. on the pons Varolii; X. the first cervi- 

 cal or suboccipital nerve. (Allen Thomson.) #. 



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 the impressions derived from the retina through the 

 optic nerves and tracts. 



(4) The centre for the co-ordination of the movements of the eyes is 

 also contained in them. This centre is closely associated with that for 

 contraction of the pupil, and so it follows that contraction or dilatation 

 follows upon certain definite ocular movements. 



