THE MESENCEPHALON 509 



and more distinct. It looks into the interpeduncular fossa, 

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

 It consequently receives the name of the sulcus oculomotorius. 

 The lateral sulcus is termed the sulcus lateralis. 



Cut Surface of the Mesencephalon. Much nearer the 

 dorsal than the ventral surface of the mesencephalon the 

 transversely divided aquaductus cerebri may be seen. This 

 narrow passage 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 this is always very conspicuous, 

 and in its midst are situated the nuclei of the oculo- 

 motor and trochlear nerves, and the upper motor nucleus 

 of the trigeminal nerve, although of course these cannot, 

 except in very favourable circumstances, be detected by 

 the naked eye. Below, the grey matter of the aqueduct 

 is continuous with the grey matter spread out on the 

 anterior wall of the fourth ventricle ; whilst above, it is con- 

 tinuous 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 which intervenes 

 between them. This is termed the substantia nigra. 



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 the pedunculi cerebri, and it consists of a mass of 

 grey matter in the midst of which are large numbers of pig- 

 mented nerve cells. 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 

 emerging 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. 



Basis Pedunculi (O.T. crusta). The basis pedunculi is 

 somewhat crescentic when seen in section, and stands quite 

 apart from its fellow of the opposite side. It is composed of 

 a compact mass of longitudinally directed nerve fibres which 



