240 A MANUAL OF ANATOMY. 



placed nearly transversely across the crus at about its mid- 

 dle ; this is the locus niger and divides the crus into the 

 crusta (anterior) and tegmentum (posterior). 



The crusta and tegmentum are also separated from each 

 other on their outer surface by two grooves which run 

 parallel with each other. One groove is on the inner 

 surface of the crus and is called the oculomotor groove, 

 because out of this groove the third or oculomotor nerve 

 emerges ; the other groove is placed externally and called 

 the lateral groove of the crus. 



The crusta is composed of fibres which pass through the 

 pons and appear as the anterior pyramids of the medulla, 

 and of connecting fibres to the opposite cerebellar hemi- 

 sphere. These fibres, continued upward, pass under the optic 

 thalamus and take part in forming the internal capsule. 



The tegmentum is composed of fibres passing from the 

 internal capsule to the medulla and cerebellum. 



The red nucleus (Figs. 34, 40, 46, 51) is a round 

 nucleus of brownish-gray matter, with a diameter of about 

 three-eighths of an inch ; it is situated in the tegmentum 

 behind the middle of the locus niger and under the posterior 

 part of the optic thalamus. 



External to the red nucleus, on the same plane with it, 

 but extending backward so as to appear behind it, is a 

 second gray nucleus, the subthalamic ganglion, or the 

 body of Luys. The plane of junction of the opposite 

 tegmenta is called the median raphe. 



The Aqueduct of Sylvius. Figs. 34, 39, 47. 



This is the small canal which connects the third with the 

 fourth ventricle. It passes beneath the posterior commis- 

 sure and the lamina quadrigemina, and lies in the raphe 

 between the tegmental tracts. 



