CRANIAL CAVITY. 



continues between the brachia. The corpus geniculatum in- 

 ternum is a small oval eminence on the side of the upper part 

 of the mid-brain, under the optic thalamus. [531] 



Grura Cerebri. These form the main bulk of the mid-brain 

 and appear on the base as two diverging rope-like strands. 

 A dark gray layer (substantia nigra), convex ventrally, divides 

 each crus into a dorsal tegmental part (tegmentum) passing 

 under the thalamus, and a ventral part (crusta) passing out- 

 side of the thalamus to the internal capsule. Two grooves on 

 each crus indicate this division; the outer one (sulcus lateralis 

 mesencephali) is continuous with the groove between the 

 superior and middle peduncle; the inner one (sulcus oculo- 

 motorius) gives origin to the oculomotor nerve. A few faint 

 fibres of the lateral fillet emerge from the sulcus lateralis and 

 curve over the superior peduncle to the inferior quadrigeminal 

 body. The aqueduct of Sylvius is a canal running through the 

 tegmentum in the midline, from the fourth to the third ven- 

 tricle, and lying nearest the dorsal surface; it is triangular on 

 section below, T-shaped above, and surrounded by a thick 

 layer of Sylvian gray matter continuous with that on the ven- 

 tricular floors. [532] 



Internal Structure. The crusta are bundles of white fibres. 

 The tegmentum consists chiefly of formatio reticularis, and 

 partly of longitudinal white bundles; it presents an elongated 

 and cylindrical reddish mass (red nucleus) at its upper part, 

 on each side of the midline, ventral to the aqueduct. Quadri- 

 geminal bodies ; the inferior are gray matter with white matter 

 externally ; the superior each present, from without inward, a 

 white layer (stratum zonale) and three gray layers (stratum ci- 

 nereum, stratum opticum, and stratum lemnisci). [533-541] 



FORE-BRAIN. 



Optic Thalami. These are two large ovoid gray masses 

 which are separated by the third ventricle but lie close to- 

 gether anteriorly. The external (lateral) surface is connected 

 3 [33] 



