THE TEGMENTUM OF THE CRUS CEREBRI AND ITS GANGLIA. 429 



tuberculum, however, is only the head of a kind of nucleus cau- 

 datus in the optic thalamus, in the shape of the upper nucleus, 

 which, turned backward and outward, is prolonged into a tail 

 which terminates in the pulvinari. This is the most nearly, 

 though certainly not completely, isolated nucleus of the optic 

 thalamus, inasmuch as the zonular layer splits as it were into 

 two layers that surround it, defining its limits clearly both 

 in longitudinal and transverse sections. Its most anterior part, 

 however, coalesces, without any line of demarcation, with the 

 internal substance of the optic thalamus. 



The internal capsule conveys radiations from the posterior 

 portions of the frontal and parietal lobes, not only to the 

 anterior extremity of the optic thalamus, but also to the middle 

 of its length. The transverse section of the optic thalamus 

 and that of the lenticular nucleus form the two corresponding 

 diagonal halves of a square applied to the internal capsule. 

 And just as the upper surface (in section the upper border) of 

 the lenticular nucleus was its recipient territory, so is it the 

 obliquely sloping inferior surface of the optic thalamus along 

 which the contact and connection with the internal capsule 

 occurs. 



It would be hazardous to maintain that the middle portion 

 of the optic thalamus is connected exclusively with the parietal 

 lobes. For, independently of the long course of the fasciculi of 

 the anterior peduncle of the optic thalamus proceeding from 

 the frontal portion of the cerebrum, as well as of the inferior, 

 running backward from the temporal lobes, the temporal lobes 

 participate to a remarkable extent in the interpenetration of the 

 most externally situated region of the grey matter of the optic 

 thalamus. Projection fasciculi of this cerebral lobe here occur, 

 running in an arched manner from behind forwards into the optic 

 thalamus, and decussate with the radially converging fasciculi 

 (as, for instance, with those proceeding from the parietal lobes) 

 to form a web which is the trellis-work of the optic thalamus. 

 This interweaving of fibres occurs, that is to say, not in the me- 

 dulla of the hemispheres, but in the most external layers of the 

 optic thalamus itself, so that a thin stratum of its substance still 

 remains as a claustrum of the optic thalamus, external to the 

 interrupted medullary capsule of its trellis- work, in the fasciculi 



