THE PONS AND THE CEHEBELLUM. 315 



decreasing mass up to the midbrain and beyond to the most ventral region 

 of the thalamus. They arise from cells which lie at the posterior end of 

 the tract and from cells located along the course. In a dog lacldng a thala- 

 mus they were intact. We have to do here, probably, with a system which 

 joins together different levels of the tegmentum. 



It is not difficult, if one has once thoroughly understood the significance 

 of the separate fields of a good section through the corpora quadrigemina, 

 to find the same in sections through the upper part of the pons and to 

 interpret them rightly. The changes concern, in general, only the location 

 of the cerebellar peduncle and the conformation of the gray substance under 

 the widening aqueduct where new nerve-nuclei arise; then the accession of 

 the lateral fillet to the stratum lemnisci and the increase of the systems of 

 the substantia reticularis. 



But when one makes cross-sections farther back the picture is essen- 

 tially modified. This occurs through the formation of the cerebellum from 

 th-e roof of the ventricle posterior to the velum meduUare anticum, and 

 through the intimate relation into which the cerebellum enters with fibers 

 from the tegmentum and from the pes. 



Cerebellar peduncles and pontal arms disappear in the cerebellum. 

 From the medulla and spinal cord come fibers which are interwoven with 

 those of the tegmentum and turn also to the cerebellum. 



It is therefore advisable to leave for a time the tracing of the tegmen- 

 tal tract and pass to the study of those parts of the central nervous system 

 into which the tracts disappear. The picture of the tegmental system will 

 doubtless be much more easily comprehensible after the reader has become 

 somewhat familiar with the arrangement of fibers in the cerebellum, and 

 after he has learned to know the structure of the spinal cord and the medulla 

 oblongata. 



THE CEEEBELLUM. 



The cerebellum consists of a middle portion, the vermis, and the two 

 hemispheres. It is in connection with the thalamus anteriorly through an- 

 terior cerebellar peduncles from the nucleus ruber, and with the cerebrum 

 ventrally through the brachia pontis, the middle cerebellar peduncles. 

 Through the first it receives principally fibers from the thalamus and from 

 the region of the tegmental radiation; through the second bundles from the 

 cortex of the frontal, parietal, and temporal lobes. A third connection binds 

 the cerebellum to the medulla oblongata and the spinal cord through the 

 posterior cerebellar peduncles — the corpora restiformia. 



In the following figure, which shows the cerebellum from above, 

 one may note: — ■ 



