276 INNERVATION. [CHAP. X. 



of sucli a section three planes of nervous matter may be distinctly 

 observed. The inferior one is composed of fibrous matter, con- 

 tinuous below with that of the mesocephale and the anterior 

 pyramids, and which passes upwards to the corpus striatum. 

 Immediately above it is a remarkable mass of a peculiarly dark, 

 almost black, matter, which constitutes the well-known locus niger 

 of the crus cerebri. It contains large caudate vesicles, abounding 

 in pigment, with nerve fibres passing among them, or originating 

 from them. This black layer does not extend beyond the crus. It 

 forms a partition between the interior or fibrous layer, and a supe- 

 rior one, which composes the principal portion of the crus. This 

 consists of a grayish matter, continuous with the central portion of 

 the medulla oblongata, or the olivary columns, and passes into the 

 optic thalalami. 



The optic thalamus and corpus striatum are large ganglia formed 

 upon the anterior and upper extremity of each crus, and with which 

 the nervous matter of its upper and lower planes appears to be in- 

 timately connected. The optic thalamus is manifestly continuous 

 with the superior plane, or olivary columns : its colour and texture 

 are quite of the same nature with those of that plane ; and when a 

 longitudinal section of it is carried down through the mesocephale 

 and medulla oblongata, no distinction is apparent between the 

 ganglion and the olivary column, so complete is the continuity of 

 texture. The colour of these bodies has been not inaccurately 

 compared to that of coffee largely diluted with milk (cafe au lait). 

 This arises from the intermixture of vesicular matter with a very 

 close interlacement of fibres. 



The corpus striatum has a much darker colour than the optic 

 thalamus. When a section of it is made in an oblique direction, 

 upwards and outwards, it exhibits the striated appearance whence 

 its name is derived. This arises from the passage of the fibres 

 of the inferior layer of the crus into the vesicular matter of the 

 ganglion. The fibres do not at first blend with the vesicular 

 matter, as in the thalamus, but are collected into bundles, which 

 are large at their entrance from the crus, but subdivide into much 

 smaller ones, diverging from each other, and radiating through the 

 ganglion in various directions, upwards, forwards, outwards, and 

 backwards. 



When thin sections of the corpus striatum are examined by 

 transmitted light, the smallest bundles of fibres observable in them 

 appear to consist of tubules reduced to their minutest dimensions, 

 and closely united to each other. So compactly applied are they, 



