PROSENCEPHALON OF MAMMALS. 99 



and ' teretial ' tracts, expand in passing beneath the bigeminal 

 bodies, and receive accessions from grey matter continuous with 

 that of the macromyelon, but so dark as to have received the 

 name ( locus niger ' when exposed in section. They are divided 

 by the third ventricle, and swell out respectively at their upper 

 part, through the superaddition of formative neuriiie, into the 

 bodies called ' thalami optici,' fig. 68, c, figs. 71 and 75, t. The 

 free surface is white, but the grey matter constitutes their chief 

 bulk, and is partially divided by the longitudinal fibres into an 

 outer and an inner portion : from the latter the soft commissure 

 is continued. The optic tracts, fig. 68, d, commencing at the optic 

 lobes and geniculate bodies, bend round the outer and back part 

 of the ( thalami,' from which they derive accessory filaments to form 

 the optic nerve. In connection with the mesencephalon must be 

 noted the tract of white fibres continued from the fornix, on each 

 side the third ventricle anterior to the soft commissure, to a 

 nodule, conspicuous in Gyrencephala behind the infundibulum, 

 and forming a pair (' corpora albicantia' in anthropotomy) in Apes, 

 fig. 112, and Man. 



208. Prosencephalon. As the 'crura cerebri' enter the pros- 

 encephalou, they are augmented by further accessions of formative 

 neurine in masses which in the human brain have received the 

 names ' nucleus tremreformis,' ( nucleus lenticularis,' and ( nucleus 

 caudatus.' The latter projects into the prosencephalic ventricle, 

 as the ' corpus striatum,' figs. 70, s, 75, r. But this name extends 

 or applies also to the deeper-seated grey masses, w r hich are so in- 

 terblended with the diverging white fibres as, in section, to give 

 alternate white and grey strias. The accession of white fibres 

 from these formative nidi, diverging to form the basis of the 

 cerebral hemispheres, causes the form expressed by the term 

 6 fibrous cone,' fig. 66, c. The grey matter again appears as a 

 thin superficial covering or ( cortex ' of the expansion of the 

 white fibres : and this grey matter contains cells similar to those 

 in the corpus striatum. 



In most Ly- and Liss-encephala, and in a few of the smallest 

 kinds of Gyrcncepliala, the prosencephalic vesicles retain the out- 

 ward uniformity of surface which they have in birds and reptiles : 

 unlike those of the mes- and ep-encephalon, they are so little 

 united together that they are called and seem to form distinct 

 ( hemispheres.' These are connected together in all Mammals as 

 in Birds by the cord-like fasciculus of transverse fibres, figs. 

 69 and 73, c, called e anterior commissure.' But the main dis- 

 tinction lies in the superaddition to the ( diverging ' or ' crural ' 



H 2 



