858 MAN 



the mid-\cntiicular region rapidly contracts in its lateral dimension and, 

 thickening, it I)egins to assume the characteristic appearance ol the gray 

 matter surrounding the aqueduct oi" Sylvius. As the mesencephalon is 

 approached the cavity of the ventricle rapidly diminishes, the walls of the 

 ventricular system increase considerably in thickness until the region of the 

 colliculi is approached, w here the lumen of the \entricle rapidly diminishes 

 in size and the walls of the ventricle become enormously thickened. 

 This thickening takes place ventral and dorsal to the aqueduct through- 

 out the lower half of the mesencephalon, the dorsal portion forming the 

 support for the mesial extremities of the superior and inferior colliculi. This 

 dorsal layer of gray matter is pierced at various points by the fiber bundles 

 which are passing across between the two inferior and the two superior 

 colliculi as the collicular commissures. In the u])per halt ot the mesencepha- 

 lon the ventral portion of the central gray matter rapidly extends ventrally, 

 forming a long, tongue-shaped process which passes forward lateral to the 

 raphe and almost in contact with its fellow of the opposite side, being sepa- 

 rated onlv by the indifferent material of the raphe. In this ventral extension 

 of the central gray matter there appears a large nucleus, the nucleus ot the 

 oculomotor nerve, which is continued cephalad to the junction of the 

 mesencephalon with the diencephalon. 



In the uppermost portion of the reconstruction the dorsal portion ol the 

 central gray matter is broken up by the appearance ot the posterior commis- 

 sure w hich passes across from one side to another and di\ ides this part ot the 

 central gray matter into a ventral lamina lying directly around the cavity of 

 the aqueduct of Sylvius and a dorsal la\ er w hich lit's between the posterior 

 commissure and the commissure ol the su])erior colliculi. Laterally the central 

 gray matter is supported almost throughout its entire extent l)y tlu' reticular 

 •■.formation of the \arious portions of the stem. In the upper portion ot the 

 ■ mesencephalon the for\vard prolongation of the central gray matter, which 



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