THE SUBTHALAMIC REGION, ETC. 



105 



brain. In Fig. 72 these fibres originate in the substantia reticu- 

 laris and the posterior commissure, and this is probably the 

 truth. In all vertebrates the posterior commissure is one of the 

 first bundles to become medullary. Its development, from the 

 simple cyclostoma up to human beings, is always conducted 

 in the same wav. 



w) 



Under the aqueduct the structures of the tegmentum and 

 the fibres of the pes pedunculi extend farther back. Above 



FIG. 59. 



Diagram of a section through the vicinity of the posterior commissure. Shows the 

 ' ganglia and the course of some of the fibres of the subthalamic region. 



Hauben Bahn, Tegmental tract. Stabler, zum Tlialamns, Coronal fibres to the thalamus. 



them lie the corpora quadrigemina developed from the roof of 

 the mid-brain. The following cut shows the corpora quadri- 

 gemina viewed from above. They lie on the crura, and seem to 

 be crowded in between the thalami. Ventrad of them appears, on 

 each side, a thick strand of fibres from the depths below, which 

 extend into the cerebellum. They are the anterior or superior 

 peduncles of the cerebellum. In Fig. 53 you can see how they 

 arise in the red nucleus, which is situated in the tegmentum 

 beneath the thalamus and the corpora quadrigemina. Behind 



