ISTHMUS OF THE ENCEPHALON 567 



The pons varolii comprises in a general way that portion of the central 

 nerve system situated between the medulla oblongata and the crura cerebri. 

 The ventral surface is convex from side to side; the lateral surface, owing to 

 the convergence of the fibers of which it is composed, is contracted to form the 

 middle peduncle of the cerebellum; the posterior surface is flat and forms 

 the upper half of the floor of the fourth ventricle. The pons consists of 

 white fibers and gray matter supported by connective tissue and neuroglia. 

 Transverse sections of the pons show that it is divided into an anterior 

 or ventral, and a posterior or dorsal portion, the latter being usually termed 

 the tegmentum. 



The ventral portion consists for the most part of white fibers, arranged 

 longitudinally and transversely (Fig. 234). The longitudinal fibers are 

 largely continuations of the pyramidal tracts, or the fibers composing in part 

 the anterior pyramid of the medulla. In the lower part of the pons these 

 fibers are compactly arranged, but at higher 

 levels they are separated into a number of 

 bundles by the interlacing of the transverse 

 fibers. The transverse fibers are divided into 

 a superficial and a deep set. Among these 

 fibers are groups of nerve-cells which collec- 

 tively are known as the nucleus pontis. Some 

 of the transverse fibers, especially the super- 

 ficial ones, are commissural in character i.e., 

 they connect corresponding parts of the gray 

 matter of the lateral halves of the cerebellum; 

 others coming from the gray matter of the PORTION, SHOWING THE RELA- 

 cerebellum cross the median line and terminate TION OF THE NERVE TRACTS 

 around the cells of the nucleus pontis; others g^^S^S^ 

 again are connected with the gray cells of the i.e. and c. Locus cemleus. L.f. 

 same side. Through the intermediation of Lateral fillet. 

 the nucleus pontis and certain of the longi- 

 tudinal fibers of the pons, the cerebellum is brought into relation with 

 the cerebrum. 



The dorsal or tegmental portion consists of: (i) The fillet; (2) the formatio 

 reticularis; (3) the medial longitudinal bundle; (4) groups of efferent and 

 afferent nerve-cells. 



The fillet or lemniscus in this region is divided into a mesial and a lateral 

 portion. The fibers of the mesial portion are partly the axons of the nerve- 

 cells of the gracile and cuneate nuclei of the opposite side of the medulla, 

 the fibers of the spinal lemniscus and partly of the axons of the sensor 

 nerve-cells of the afferent cranial nerves with the exception of the audi- 

 tory. The fibers of the lateral portion are mainly the axons of the cells in 

 the floor of the fourth ventricle around which the auditory nerve-fibers 

 end. They are therefore a continuation of the acoustic tract. 



The formatio reticularis is a continuation of that of the medulla. 



The medial longitudinal fasciculus is a tract of nerve-fibers, triangular in 

 shape, placed on either side of the median line just beneath the floor of the 

 fourth ventricle and the aqueduct of Sylvius. It consists of both afferent 

 (ascending) and efferent (descending) fibers. The afferent fibers are the 

 axons of sensor end-nuclei located in the upper segments of the spinal cord 



