THE PONS AND' THE CEREBELLUM. 311 



the corpora quadrigemina a thick white mass of fibers lies yentral to the 

 cerebral peduncles. Descending from the cerebellum, they embrace and 

 cover the pedal region in a thick layer. These fibers taken together are 

 called the pons. 



Only a part of the fibers cover the crusta externally, — stratum super- 

 ficiale pontis, — most of them invade, from both sides, the fiber-system of the 

 crusta, dividing it into isolated fasciculi: stratum complexum et profundum 

 pontis. 



It will be remembered that of the fibers which pass ventrally into the 

 crusta from the cerebrum only a portion can be followed as far as the pons. 

 These were the fasciculi from the frontal, parietal, and temporal lobes. The 

 pyramidal tract, from the region of the central convolutions, passes through 

 the pons. Almost the whole inner third and the whole outer third of 

 the crusta terminate in the pons. Beyond the latter only the middle third 

 of the pes — namely, the pyramidal tract — passes out (see Fig. 200). 



The fibers of the pons come from above out of the cerebellar hemi- 

 spheres, embrace and penetrate the fibers of the pes and in the more ventral 

 regions, — stratum superficiale, — terminate, for the most part, in the pontal 

 ganglia of the same side; while, in the more dorsal regions, in those of the 

 opposite side (Minghazzini). The pontal ganglia are gray masses filled with 

 a reticulum of fine fibers, in which one may follow both the fibers from the 

 arms of the pons (brachia pontis) and the tracts which arise from the cere- 

 brum. 



Through the investigations of S. R. y Cajal it has become quite certain that 

 the strong cortico-pontal fasciculi ramify around the large cells of the pontal ganglia, 

 and that the arms to the cerebellum are formed from the neuraxons of those cells. 

 But experiments show, also, that a part of the pontal fibers degenerates after ex- 

 tirpation of the cerebellum. We must, therefore, conclude that, as in many other 

 bundles, so in the arms of the pons there are fibers which pass in both directions, 

 namely: fibers from the cells of the cerebellum to the pontal ganglia, and fibers from , 

 the ganglia to the cerebellum. 



In animals with a relatively small cerebrum the pons is also small. Compare 

 Kg. 141 with Fig. 180. Here in the calf there is seen a transverse system of fibers, 

 corpus trapezoides, lying between the pedal and tegmental portions of the pons: a 

 system of fibers which, in man, is covered by pontal fibers. It contains the fasciculi 

 which belong to the acusticus. 



In mammals there is added to the pontal fibers that tract from the 

 cerebellum to the tegmentum of the medulla which was demonstrated during 

 the consideration of the brain of the lowest vertebrates. The fibers of this 

 tract do not pass into the pontal ganglia, but diverge from their course, 

 decussating dorsally in the raphe of the pons, and are lost in the gray 

 matter of the tegmentum (see Pig. 3016). 



The pes pedunculi is split up by the pontal system of fibers and in part 



