THE NERVOUS SYSTEM 1439 



Varolii into the subthalamic tegmental region. Laterally it reaches 

 the lateral sulcus on the outer asf)ect of the cms, where it is thin, 

 and the oculo-motor sulcus on the inner aspect, where it is thick 

 and is traversed by the fasciculi of the third cranial or oculo-motor 

 nerve. Its tegmental surface is concave, and the surface directed 

 towards the crusta is convex. From the latter surface prolonga- 

 tions extend into the crusta. 



Aqueduct of Sylvius. — The aqueduct of Sylvius is the narrow 

 passage which leads through the mesencephalon from the third to 

 the fourth ventricle [iter a tertio ad quartutn ventriculum). It lies 

 nearer the dorsal than the ventral aspect of the mesencephalon ; its 

 direction is from above downwards; and its length is rather more 

 than I inch. Its upper opening is situated on the posterior boundary 

 of the third ventricle, immediately underneath the posterior com- 

 missure, and its lower opening occupies the superior median angle 

 of the floor of the fourth ventricle. In transverse section the 

 aqueduct is T-shaped in its upper part, near the third ventricle, 

 and triangular in its lower part, near the fourth ventricle. The 

 passage is lined with ciliated columnar epitheUum, external to which 

 there is a thick layer of grey matter, which is spoken of as the 

 Sylvian grey matter. This is continuous superiorly with the grey 

 matter of the floor and lateral walls of the third ventricle, and in- 

 feriorly with that which covers the floor of the fourth ventricle. 

 It contains numerous nerve-cells, disposed in a scattered manner, 

 but, in addition to these, there are certain definite cell-groups. 

 These groups constitute the nuclei of origin of the follo%ving cranial 

 nerves: the third or oculo-motor, the fourth or trochlear, and 

 the descending or mesencephalic root of the fifth nerve. The 

 oculo-motor nucleus is situated in the ventral portion of the Sylvian 

 grey matter, imdemeath the upper quadrigeminal body, and it 

 extends upwards into the layer of grey matter on the adjacent 

 portion of the lateral wall of the third ventricle. The trochlear 

 nucleus is also situated in the ventral portion of the SyMan grey 

 matter, but at a lower level than the oculo-motor nucleus, being 

 placed imdemeath the upper part of the lower quadrigeminal body. 

 The nucleus of the descending or mesencephalic root of the fiftii 

 nerve is extensive, and is situated in the lateral portion of the 

 Sylvian grey matter. 



Development — ^The aqueduct of Sylvius is the perststent remains of the 



cavity of the mesencephalon. 



Locus Perforatus Posticus. — This space has been previously 

 described, in a general way, in connection with the base of the 

 encephalon. It will here be considered more fully. It Ues at the 

 bottom of a deep depression, called the interpeduncular fossa {fossa 

 Tarini), which forms the back part of the mterpeduncular space. 

 The fossa is boimded posteriorly by the median portion of the upper 

 border of the pons VaroUi, and laterally bv the crura cerebri .4m- 

 teriorly it is limited by the corpora mammillaria. 



