96 ANATOMY OF VERTEBRATES. 



nucleus breaks up into primary branches, which become the 

 centres of the lobules of which it consists. The ramifications of 

 the nucleus, whether of the median lobe or of the hemispheres, 

 pass from it only in the vertical plane or from before backwards ; 

 in the latter direction, however, to a very slight extent. The 

 fibrous matter of the median lobe is continuous with that of the 

 hemispheric lobules. By reason of this disposition of the fibrous 

 matter, the surface exposed by a horizontal section through the 

 entire cerebellum consists of a plane of white matter bounded on 

 the sides and behind by a narrow cortex of grey matter. 



( The white matter consists exclusively of fibres, chiefly of the 

 tubular kind, and of all degrees of size. These, in the more 

 distant ramifications, penetrate the vesicular matter of their grey 

 cortex, and form some unknown connection with its elements. 

 The grey matter consists of three layers, readily distinguishable 

 by the naked eye from their difference of colour. The external 

 layer is the darkest, and consists chiefly of granular and vesicular 

 matter. The next or intermediate layer is of a light colour, and 

 is composed of a stratum of fine nucleus-like particles. The 

 third layer has the greatest thickness, and is immediately in con- 

 tact with the fibrous matter ; it is intermediate in point of colour 

 to the other two, and consists of numerous vesicles of the caudate 

 kind, especially with branching processes and nerve-tubes of all 

 sizes. The dark colour of the external layer is doubtless owing 

 in a great measure to the great numbers of capillary vessels 

 which enter it ; the greater paleness of the inner stratum is to be 

 attributed to the intermixture of the white fibres, whilst the light 

 colour of the middle stratum is intrinsic.' l 



The lower or hinder crus, fig. 66, r, on entering the cerebellum 

 curves backward, expanding on the outer side of the converging 

 and onwardly continued fibres which constitute the upper or 

 ' anterior crus,' t. In the part of the ' nucleus ' connected with the 

 latter is developed a plicated capsule of grey or vesicular matter, 

 d, also exposed in section at R, fig. 49, and called ' corpus denta- 

 tum ;' it supplies accessory white fibres to those diverging from, 

 or converging to, the crura ; with these are interlaced the com- 

 missural fibres of the pons. 



Thus an influence ascending from the myelon, by the resti- 

 form tracts, fig. 66, s, r, to the cerebellum, may be propa- 

 gated from that body, by the crus, t, to the mesencephalon, 

 and thence to the cerebrum. Conversely, cerebral influence 

 may pass through the mesencephalon by the t processus and 



1 xxvn". p. 692. 



