Midbrain 61 



Extending obliquely between the medial longitudinal 

 bundle and the lateral lemniscus, lies the cross section of the 

 superior cerebellar peduncle, which is thick and rounded 

 dorso-medially (caput brachii conjunctivi) and tapers out 

 almost to a point ventro-laterally (cauda brachii conjunctivi) 

 (PI. XII.). Passing upward, the cauda becomes less definite 

 and the whole tract becomes a little more medial in position 

 by the time it reaches the level of the trochlear nucleus, about 

 the front of which it turns medially and decussates immedi- 

 ately ventral to the medial longitudinal bundle (Pis. XIII., 

 XXVI.). In the upper part of its course before decussation, 

 it is less sharply marked off from the surrounding reticular 

 formation than farther back. In man, the superior peduncle 

 is proportionally larger than in the rodent and the dense 

 decussation has a considerably greater dorso- ventral extent. 



After decussating, the tract runs straight forward a short 

 distance to end largely in the red nucleus {nucleus ruber) 

 (Pis. XI\'., XXVI.). Some of the fit>res pass on into the 

 thalamus, while a few turn backward in the reticular forma- 

 tion {brachium conjunctivum descendens). 



The red nucleus is a large group of cells which is very 

 conspicuous in sections prepared with cytoplasmic stains. 

 In the human brain, it is conspicuous also in Weigert pre- 

 parations, where it is marked off sharply from the surround- 

 ing reticular formation by a capsule made up largely of the 

 fibres of the brachium conjunctivum. In the rat, however, 

 there is no such definite capsule and a great many white 

 fibres run between the cells, so that the nucleus is less sharply 

 delimited in Weigert sections. A magno-cellular division 

 and dorso-lateral and ventro-Lateral divisions composed of 

 smaller cells may be distinguished in this nucleus, the magno- 

 cellular portion being relatively larger in the rat than in 

 man. Besides the cerebellar peduncles, fibres from the 

 cerebral hemispheres also end in the red nucleus. 



From the magnocellular division of the red nucleus, arise 



