ISTHMUS OF ENCEPHALON. 471 



a deep set. Among these fibers are groups of nerve-cells which 

 collectively are known as the nucleus pontis. Some of the transverse 

 fibers, especially the superficial ones, are commissural in character 

 i. e.j they connect corresponding parts of the gray matter of the lateral 

 halves of the cerebellum; others coming from the gray matter of the 

 cerebellum cross the median line and terminate around the cells of 

 the nucleus pontis; others again are connected with the gray cells of 

 the same side. Through the intermediation of the nucleus pontis 

 and certain of the longitudinal 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 posterior longitudinal bundle; (4)^ the 

 substantia ferruginosa; (5) groups of nerve-cells from which arise 

 various cranial nerves e. g., the fifth, sixth, seventh, and eighth. 



The fillet or lemniscus in this region 

 is divided into a mesial and a lateral 

 portion. The fibers of the mesial por- 

 tion are partly the axons of the nerve- 

 cells of the gracile and cuneate nuclei 

 of the opposite side of the medulla, 

 and partly of the axons of the sensor 

 nerve- cells of the afferent cranial nerves 

 with the exception of the auditory. 

 The fibers of the lateral portion are 



mainly the axons of the cells in the FlG 2I3 ._ TRANSECTION OF THE 

 floor of the fourth ventricle around PONS THROUGH ITS MIDDLE 



which the auditory nerve-fibers end. PORTION, SHOWING 



They are therefore a continuation of 

 the auditory tract. COMPOSED. D. i. f. Dorsal 



The jormatio reticularis is a con- longitudinal fasciculus. L.c. 



7 ,. - - , , andc. Locus ceruleus. L.f. 



tinuation of that of the medulla. Lateral fillet. 



The posterior longitudinal bundle 



is triangular in shape and situated behind the formatio reticularis 

 and close to the median line. The fibers composing it are largely 

 derived from the ground fibers of the antero-lateral column of the 

 spinal cord. 



The superior olive is a cylindric mass of gray matter situated in 

 the pons in the anterior part of the formatio reticularis. It consists 

 of nerve-cells the axons of which pass dorso-laterally, decussate in 

 the median line, and form the lateral fillet of the opposite side. Some 

 few axons go to the lateral fillet of the same side. 



The substantia jerruginosa is composed mainly of pigmented 

 cells. 



The groups of nerve-cells lying just beneath the floor of the 

 fourth ventricle give origin to axons of the motor portion of the fifth, 



