CHAPTEE XIX. 



The Tegmentum and the Peduncle of the Midbrain. 



The frontal sections through the cerebrum, with description of which 

 the previous chapter ended, led us somewhat away from the tracing of the 

 tracts which pass into and beyond the midbrain. 



In the description these tracts had been followed to the Region of the 

 posterior end of the middle or third ventricle. One may see in Fig. 135 

 that just behind this the midbrain — the corpora quadrigemina — begins. At 

 that place the thalami diverge from each other, the tegmental prominence 

 pushes in between them, thus considerably decreasing the depth of the 

 ventricle. 



In 'connection with the accompanying description, study, in Fig. 190, 

 the formation of the roof of the midbrain. In the most anterior part of 

 this roof (see also Fig. 185) lie the fibers of the posterior commissure, close 

 behind which lie the corpora quadrigemina. The narrow ventricle which 

 passes under the roof has received, in the region of the midbrain, the name 

 aquceducius Sylvii. The entrance to the aqueduct lies just under the pos- 

 terior commissure. The aqueduct is everywhere surrounded by the gray 

 matter of the central canal. 



Now the posterior commissure lies in the most anterior portion just 

 behind the epiphysis. A part of its fibers arises from ganglia which lie, 

 one on each side, near the median line, deeply imbedded in the interbrain. 

 This is easier to demonstrate in lower vertebrates than in mammals, but 

 Kolliker has also shown that in mammals the origin is the same. Thence 

 they pass upward to the surface and, anterior to the quadrigeminal bodies, 

 turn to the opposite side. They pass for only a very short distance hori- 

 zontally, when they plunge into the depth of the tegmentum of the mid- 

 brain, through which they pass farther back. 



As very clearly seen in lower vertebrates, the majority of the fibers in 

 question pass laterally and ventrally from the posterior longitudinal fascicle 

 into the medulla. Through this reinforcement this fascicle becomes a 

 thick bundle — later to be described. Spitzka and Darkschewitsch have 

 seen a similar arrangement in mammals. In all vertebrates the posterior 

 commissure is one of the first bundles to become meduUated. 



We come now to the region of the midbrain. Let us find what has 

 become of all the fibers which were traced in the last chapter. We find 

 only a few in the region which we have now reached. The greater part of 



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