THE TEGMENTUM OF THE CRUS CEREBRI AND ITS GANGLIA. 431 



to the pulvinar, is bounded by the whole breadth of the medulla 

 of the cerebrum, since at this part, in consequence of the len- 

 ticular nucleus not extending so far, the inner capsule is no 

 longer denned. The substance of the optic thalamus here 

 exhibits a superior and an inferior layer, which are separated 

 from one another by an intervening dentation seen on section 

 of the white substance (fig. 248, at mft). The upper and lower 

 layers of the optic thalamus completely coalesce, as regards their 

 grey matter, and are traversed throughout their entire height 

 (fig. 24<8,.TJi), as far as to the innermost lamina medullaris (T'} } 

 to be hereafter described, by a uniform series of radiating fasci- 

 culi, which alternate with layers of equally thick grey substance 

 in which they terminate. 



The cells of the substance of the optic thalamus, both in its 

 upper and lower layers, have an average length of 30 /u, and 

 breadth of 10 ^u, and are fusiform, with their long axes always 

 arranged parallel to the radiating fasciculi. These uniform 

 fasciculi which enter the confluent upper and lower layers of 

 the optic thalamus arise however from two different centres. 

 Those of the upper layer apparently arise exclusively from the 

 medulla of the cerebral lobes, smd thus from the cortex of the cere- 

 brum, whilst those of the lower layer proceed from the tractus 

 opticus, and consequently from the retina, forming the yet to be 

 described middle root of the tractus opticus. This enters into 

 the substance of the optic thalamus between the external corpus 

 geniculatum (fig. 248, c) and the crusta of the cerebral peduncle 

 (P), proceeding from the medulla of the tractus (about twelve 

 millimeters in front of the posterior border of the pulvinar). 

 This general similarity of its connection with the optic thala- 

 mus to that of the cortex of the cerebrum appears to lend 

 to the retina characters permitting it to be associated as a 

 centre with the cerebral cortex itself ; whilst, on the other hand, 

 the hereafter to be described completely distinct mode of origin 

 in the optic thalamus of the peripherically distributed tracts of 

 the cms cerebri, indicates a not less noteworthy difference be- 

 tween the tractus opticus and peripheric nerves, and between 

 the retina and the terminal organs. The affinity here noted 

 between the mode of convergence of the fasciculi from the cor- 

 tex of the cerebrum and the retina into the optic thalamus is 



