THE MESENCEPHALON 455 



and the basis pedunculi) is indicated by a medial and a 

 lateral groove or sulcus. The medial sulcus is the deeper 

 and more distinct. It looks into the interpedtmcular fossa, 

 and from it emerge the fila of the oculo - motor nerve. 

 It consequently receives the name of the sulcus oculomotorius. 

 The lateral groove is termed the sulcus lateralis. 



Cut Surface of the Mesencephalon. When the cut 

 surface of a mesencephalon, which has been divided trans- 

 versely, is examined, the first point which should be noted is 

 the position of the aquaductus cerebri (Figs. 180, 181). It is 

 a narrow passage which lies nearer the dorsal surface than 

 the ventral surface of the mesencephalon, and it leads from 

 the fourth ventricle, below, to the third ventricle, above. It 

 is surrounded by a thick layer of grey matter, called the 

 central grey matter of the aqueduct. In a fresh brain the 

 central grey matter is always very conspicuous, and in its 

 midst are situated the nuclei of the oculo-motor and trochlear 

 nerves, and the upper nucleus of the trigeminal nerve, 

 but, except in very favourable circumstances, the positions 

 of the nuclei cannot be detected by the naked eye. The 

 grey matter of the aqueduct is continuous, below, with the 

 grey matter spread out on the anterior wall of the fourth 

 ventricle ; whilst, above, it is continuous with the grey matter 

 on the floor and sides of the third ventricle. 



The division between the tegmentum and the basis 

 pedunculi, on each side, is rendered very evident by a con- 

 spicuous lamina of dark pigmented matter, termed the sub- 

 stantia m'gra, which intervenes between them. 



Substantia Nigra. As seen in transverse section, the sub- 

 stantia nigra presents a somewhat crescentic outline. It is 

 a thick band interposed between the basal and tegmental 

 parts of each pedunculus cerebri, and it consists of grey 

 matter many of the cells of which are deeply pigmented. 

 It begins, below, at the upper border of the pons, and it 

 extends upwards into the subthalamic region. Its margins 

 come to the surface at the oculo-motor and lateral sulci, 

 and its medial part is traversed by the merging fibres of 

 the oculo - motor nerve. The surface turned towards the 

 tegmentum is concave and uniform ; the opposite surface is 

 convex, and is rendered highly irregular by the presence of 

 numerous slender prolongations of its substance into the 

 basis pedunculi. 



