THE CENTRAL NERVOUS SYSTEM. 



external division, the outer nucleus, the fore-segment of the thala- 

 mus containing the anterior or upper nucleus. The gray sub- 

 stance composing these segments is traversed in many places by 

 bundles of medullated nerve-fibres ; in the outer nucleus the nar- 

 row fibre-bundles and the zones of gray substance alternate, fusi- 

 form ganglion-cells (20-30 ,u), arranged parallel to the course of 

 the fibres, occupying the gray bands. In addition to the compara- 

 tively large cells found within the anterior nucleus, th^ bundle of 

 Vicq d'Azyr, reflected from the mammillary body below, enters the 

 anterior ventral border of the segment and contributes numerous 

 fibres to its mass. The multipolar cells of the anterior nucleus, 

 as well as those of the posteriorly situated pulvinar, are of large 

 size. 



The central gray matter of the third ventricle is the direct 

 continuation of that lining the Sylvian aqueduct and other parts 

 of the neural tube. The middle or gray commissure of the 

 ventricle contains transverse fibres, in addition to numerous 

 pigmented ganglion-cells ; posteriorly it is intimately blended 

 with the gray substance of the thalamus, while anteriorly it is sepa- 

 rated from the latter by a medullary layer, the inferior stalk of the 

 thalamus. 



The corpus subthalamicum, situated within the region of simi- 

 lar name, is composed of a very close net-work of fine medul- 

 lated fibres, among which are distributed moderate-sized multi- 

 polar nerve-cells ; the capillary net-work of this nucleus is 

 remarkable for the closeness of its meshes. The continuation of the 

 area of pigmented cells forming the substantia nigra within the 

 cerebral crus separates the subthalamic region from the fibre-tracts of 

 the crusta. 



The corpora quadrigemina, the homologues of the optic lobes 

 and corpora bigemina of the lower animals, comprise a posterior 

 and an anterior pair of eminences. 



The posterior quadrigeminal bodies consist in great part of 

 gray substance which forms a lenticular nucleus on either side, and 

 contains numerous small multipolar cells (16-18 //), as well as a few 

 elements of larger size ; the nuclei of the two sides are united by a 

 gray commissure. A thin superficial lamina of medullated nerve- 

 fibres, the stratum zonale, covers in the gray matter. 



The anterior quadrigeminal bodies differ from the posterior 

 in the complexity of their structure brought about by the presence 

 of the root-tracts of the optic nerve. In transverse section these 

 bodies present four layers, which, from the upper or dorsal surface 

 towards the Sylvian canal, are 



i. The stratum zonale, enveloping the superficial portions of 



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