THE TEGMENTUM OF THE CRUS CEREBRI AND ITS GANGLIA. 433 



The connections of the posterior region of the optic thala- 

 mus with the occipital and temporal lobes have already been 

 discussed at p. 409. 



The origin of that portion of the tegmentum which springs 

 from the optic thalamus presents three parts, comprising (1) 

 tegmental fasciculi of the posterior commissure ; (2) the laminae 

 medullares; (3) tegmental fasciculi from the ganglion of the 

 peduncle of the pineal gland. The centre of origin of the 

 latter fasciculi alone (figs. 248, H; 241, /) is really well defined, 

 appearing as an oblong clavate nucleus at the inner side of the 

 stratum zonale, the superjacent fasciculi of which represent the 

 upper link of its projection system. The large fasciculus (IT 1 ) of 

 the crus cerebri, proceeding from the ganglion of the habenula 

 (peduncle of the pineal gland) bends, covered by the grey 

 matter of the third ventricle, in a sigmoid curve, first out- 

 wards towards the posterior longitudinal fasciculus (L\ then 

 inwards towards the red nucleus of the tegmentum, which it 

 frequently traverses with some of its fasciculi, and arches 

 rectangularly behind the substantia nigra, to run downwards 

 as the most anterior and innermost fasciculus of the tegmen- 

 tum. Perhaps a decussation occurs in the posterior commissure, 

 between it and the zonular layer of its ganglion. 



The ganglion habenulse is larger in brutes than in Man, and 

 exhibits, in consequence of the close aggregation of cells, a 

 structure differing from that of the remainder of the optic 

 thalamus, and more resembling that of the pineal gland itself. 



The region of the two other origins are not separated from 

 one another in the optic thalamus ; nevertheless, they consti- 

 tute two sharply defined formations, which we may conceive 

 to be divided from one another by a line, commencing behind 

 the external border of the tuberculum anterius, and extending 

 to the outer border of the ganglion of the habenula. The 

 inner of these regions may be regarded as the layer of orig : n 

 for the crus cerebri through the posterior commissure, and the 

 outer of these regions as the layer for the origin of the crus 

 cerebri through the laminae medullares. 



The fasciculi from the cerebral lobes which enter the optic 

 thalamus, by its anterior and inferior peduncles (as in fig. 

 247, Jk) run longitudinally through the internal layer of the 



VOL. II. F F 



