54 Anatomy of the Nervous System 



to Influence the tone of the body muscles. This bundle runs 

 through the reticular formation, where it is not distinct from 

 its surroundings. As it enters the region of the pons from 

 above, the rubro-spinal tract lies just medial to the lateral 

 lemniscus. Upon reaching the level of the trigeminal nuclei, 

 it takes up a more superficial position immediately ventral 

 to the spinal \' nucleus and root, separating the latter from 

 the ventral spino-cerebellar tract, and in this position it 

 descends directly into. the spinal cord (Pis. X., XIII.). 



\^entral to the medial longitudinal fasciculus is situated a 

 bundle of fibres, somewhat diffuse in the rat, rising in the 

 anterior colliculi of the midbrain and descending to the spinal 

 cord, where degeneration experiments in the rat reveal it 

 as an extensive zone of finely scattered fibres external to the 

 fasciculus proprius in the ventral funiculus. This is the 

 tectospinal tract {fasciculus tecto-spinalis sen praedorsalis) 



(Pis. vn.-xiii.). 



The reticular formation, of course, contains many other 

 pathways, which, however, are mostly diffuse in arrangement 

 and cannot be observed in sections of normal material. 



Finally, there may be mentioned here the pons {pons 

 Varolii) (Pis. II., XII., XIII., XXYL). This is a large mass 

 of transverse fibres running round the ventral surface of the 

 anterior part of the medulla oblongata and turning dorsally 

 at each side to pass up into the cerebellum as the brachium 

 pontis or middle cerebellar peduncle (Pis. III., XII.). In the 

 ventral part of their course, the fibres spread apart to allow 

 the cerebral peduncles (pyramidal tracts+cortico-bulbar 

 and -pontine tracts) to descend between them and also to 

 surrotind a considerable amount of gray matter, the pontine 

 nuclei. These nuclei receive impulses from various sources, 

 particularly from the cerebral cortex^ through the cortico- 

 pontine tracts, and give rise to the fibres of the pons itself. 



Trontal and temporal lobes in man. The former, however, is 

 exceedingly rudimentary in the rat. 



