THE PONS AND THE CEEEBELLUM. 



313 



As stated above, the aqueduct widens out into the fossa rhomboidalis; 

 and the surrounding gray substance also increases in area. A new nucleus, 

 tiucleus n. trocMearis, is found below the corp. quad. post. The fibers of 

 the Troehlearis do not, however, like the fibers of the oculo-motorius, pass 

 downward through the tegmentum, but pass backward immediately after 

 their origin in nearly horizontal direction, then rise and decussate finally, 

 in the velum medullare anticum, with those of the opposite side. They thus 

 leave the brain on the dorsal side just posterior to the corp. quad. post. In 

 Pigs. 201a and 2016 portions of the course of the troehlearis are visible. 

 In Fig. 196 the whole course of the nerve is depicted. 



The posterior longitudinal fasciculi and the fibers of the posterior com- 

 missure, retaining the same relation as in the midbrain, pass down into the- 



Velum med. atit. 



Decussation of the troehlearis 



Rad. meseaceph. N. trig. 



Nucl. laquearis 



Faac. long. post. 



Cerebellar peduncle 



Fasc. from the tegmental decussation 









*5w 



Lat. fillet 

 Fasc. form pons to tegmentu: 



Median fillet--- 











Fig. 2016 shows clearly the changes in the tegmentum which occur just 

 posterior to the corpora quadrigemina. Note also the decussation of the troeh- 

 learis. Compare in the two sections the corresponding fasciculi. 



tegmentum of the medulla. The same is true of the stratum Ismnisci. Ta 

 the latter new fibers are added from the lateral margin of the corp. quad, 

 post. They lie outside of the horizontal layer formed by the upper and 

 lower fillets and are usually designated as the lateral fillet (lemniscus later- 

 ale) to distinguish them from the former or median fillet (lemniscus 

 mediale). The lateral fillet from the posterior quadrigeminal body goes 

 almost exclusively to the termini of the auditory nerve. In them lie groups 

 of ganglion-cells, — nucleus laquearis, — the neuraxons of which mostly join 

 the ascending and descending fillets. 



In Kg. 202 one sees above and externally the triangular lateral fillet 

 passing downward to the horizontal median fillet. 



