CH. XLIII.] 



THE BRAIN. 



621 



pons, viz., of the yth, 6th, and 5th nerves, there are several 

 masses of grey matter, viz., in the front part, the superior olive 

 and in the back part the locus coeruleus, as well as small 

 amounts of the same material mixed with fibres in the more 

 ventral surface. 



(iii.) In the mid-brain, the grey matter preponderates in the 

 corpora quadrigemina, and corpora geniculata. It is also found 

 surrounding the aqueduct of Sylvius (where it gives origin to 

 the third and fourth nerves) and in other parts of the crura, 

 notably such masses as the red nucleus and substantia nigra. 



(iv.) In the cerebral hemispheres, the cerebral cortex is made up 

 of grey matter which encloses white matter ; the corpora striata 

 and optic thalami at the base of the brain are made up chiefly of 

 grey matter. 



(v.) In the cerebellum, the grey matter forms the encasing 



Olf 



VI 



Fig. 461. Longitudinal and vertical diagrammatic section of a vertebrate brain. Letters 

 as before. I'V, pons Varolii. TAmina. terminalis is represented by the strong black 

 line joining Pu and Py. (Huxley.) 



material of the white matter. In the interior too there are 

 masses of grey matter forming the coi'pora dentata. 



Speaking generally, . there are two main collections of grey 

 matter that on the surface, and that in the interior bordering 

 on the cerebro-spinal cavity, and subdivided into various masses 

 (corpora striata, optic thalami, kc.), whose names have been men- 

 tioned, but whose closer acquaintance we shall make presently. 



The cerebral or cranial nerves, some of which have also been 

 mentioned, are those which originate from the brain ; there are 

 twelve pairs of these altogether, and all of them, with the excep- 

 tion of the first and second, originate from nerve-cells in the grey 

 matter of the floor of the fourth ventricle or its immediate 

 neighbourhood. 



In the fo3tus the central nervous system is formed by an in- 

 folding of a portion of the surface epiblast. This becomes a tube 

 of nervous matter, which loses all connection with the surface of 



