368 



bundles lie close to the nucleus magnocellularis. Here two or three 

 medullated fibers have been seen running directly on the surface of a 

 large cell or its dentrite. It is entirely possible that some of the 

 fibers either end or arise in relation with these cells, but the great 

 majority of the fibers of this tract are so far removed from this nu- 

 cleus when they turn into the fiber zone and lose their sheaths that 

 there is no reason for thinking that they have any relation to the 

 nucleus magnocellularis. That part of the tract which lies nearest to 

 this nucleus is the part which makes the longest loop into the 

 cerebellum and this probably explains Edinger's uncertainty about it. 



Returning now to the point at which the description was begun, 

 the course of the tract caudally is quite simple. Lateral to it medul- 

 lated fibers coming from the sensory root of the trigeminus run through 

 the acusticum to enter the cerebellum. As the tract is traced cau- 

 dally it bends downward and laterad and leaves the ventricle, at the 

 same time coming into closer relation with the trigeminal fibers just 

 mentioned. The radix mesencephalica is composed of coarser fibers 

 and remains as a compact bundle which approaches the sensory root 

 of the trigeminus and leaves the brain with it. The motor fibers of 

 the trigeminus run in four distinct bundles which take a straight course 

 from the motor nucleus to the root. The radix mesencephalica crosses 

 these motor bundles nearly at right angles and does not give fibers 

 to them. The sensory root is interwoven with arcuate fibers from the 

 acusticum and the basket-like arrangement of the bundles makes it 

 possible to see with certainty that the fibers of the radix mesencephalica 

 bend into the sensory bundles. The tract as a whole does not go 

 near the motor nucleus of the trigeminus and there is no evidence in 

 these preparations that it has any relation with that nucleus. Although 

 the peripheral course of these fibers must be traced in order to know 

 their true nature (or their central endings demonstrated in Golgi 

 preparations), there is every reason to suppose that they are sensory 

 fibers and not motor. The course of the biindle throughout is entirely 

 clear and beyond doubt. 



Sargent (5) describes the nucleus magnocellularis tecti in Raja, 

 Squalus and Mustelus. Each cell has three processes, one of a den- 

 dritic character which receives impulses from the optic tract fibers, an 

 axone which enters into the formation of Reissner's fiber, and a 

 third process which he calls the cerebeilar neurite. The last processes 

 unite to from the tractus tecto-cerebellaris which runs caudally at 

 either side of the median line of the tectum, passes beneath the de- 

 cussation of the trochlearis and bends up into the cerebellum. I am 



