THE BRAIN 507 



individual parts of which the brain as a whole is built up. 

 It would be neither possible nor desirable to attempt to 

 deal in any detail in this book with the varied arrange- 

 ments of the several deposits of grey matter in the brain, 

 and with their connections by strands of white matter. 

 But some of them stand out so conspicuously as structures, 

 and are so important in their functions, that we must of 

 necessity take them into consideration. 



The Corpora Quadrigexnina. — These have already been 

 described as four conspicuous masses of tissue lying in 

 two pairs above the aqueduct of Sylvius. They consist 

 of deposits of grey matter in the otherwise thin wall of 

 the roof of the aqueduct. Each deposit is surrounded 

 by white matter, and from each bands of fibres run 

 obliquely downwards and forwards, those from the 

 anterior pair of the corpora making connection with 

 structures connected with the optic nerve (Fig. 160, II.), 

 while those from the posterior pair are believed to make 

 similar connections with the nerves concerned in hearing 

 (Fig. 160, VIII.). 



The Optic Thalami. — The longitudinal fibres of the 

 bulb, passing between the transverse fibres of the pons 

 reappear, as we have seen, in front of the pons as the 

 cruya cerebri. These diverge from the middle line to 

 enter the cerebral hemispheres. As each crus passes into 

 the base of the corresponding hemisphere, it receives on 

 its upper surface a large deposit of grey matter placed 

 somewhat obliquely across its course ; this mass of grey 

 matter is the optic thalamus. Lying thus to one side 

 of the third ventricle, and under the lateral ventricle, it is 

 easily seen how each optic thalamus comes to form a pro- 

 jection in the outer side-wall of the third ventricle, and 

 on the floor of the lateral ventricle. Thus the optic 

 thalamus is shown at 0. T. in Fig. 161, as part of the wall 

 of the third ventricle, and as 0. T. in Fig. 163, which re- 

 l)resents in diagram a horizontal section through the 

 hemispheres passing above the floor of the lateral 



