THE NERVOUS SYSTEM. 613 



may be made out in its structure, beginning from the anterior or ven- 

 tral surface. 



(a.) Transverse or commissural fibres connecting the one side of the 

 cerebellum with the other, forming the middle peduncle. These fibres 

 emerge from the lateral parts of the white substance of the hemispheres, 

 having come from the superficial gray matter of the whole surface, from 

 the median vermis, and from the lateral hemispheres. Some of these 

 fibres are truly commissural and probably connect the same points on 

 the surfaces of the two halves; some end in the gray matter of the same 

 side of the pons on the ventral surface, and others cross to the opposite 

 side of the pons and then become longitudinal, passing on to the teg- 

 mentum, a system of fibres and gray matter to be immediately described. 



(#.) Fibres longitudinal in direction which are arranged in larger or 

 smaller bundles separated by gray matter; some of these fibres are what 

 are called the pyramidal fibres, which pass down to the anterior pyra- 

 mids of the bulb. 



(c.) The dorsal portion of the pons is made up to a considerable ex- 

 tent of the reticular formation of the tegmental region together with 

 one or two distinct bundles of longitudinal fibres: i., the chief, situated 

 toward the junction of the ventral two thirds with the dorsal third, is 

 ihe fillet, which consists of two portions, outer and median; and ii., the 

 second, a bundle of similar fibres, posterior longitudinal bundles, is situ- 

 ated between the two divisions of the fillet below the lateral and to the 

 outer side of the median. 



(d.) In the fore part of the pons, a mass of gray matter containing 

 pigment, the locus cwruleus, possibly forming the origin of the fifth nerve, 

 and in the back part a second mass of gray matter, the superior olive. 



The Crura Cerebri. 



The crura cerebri (in, fig. 354) diverge from the anterior edge of 

 the pons Varolii and pass upward on either side toward the cerebral 

 hemispheres. At their anterior termination each of them appears to 

 have upon its dorsal surface, to the inner and outer sides respectively, 

 two large masses of gray matter which have been already spoken of, viz. , 

 the optic thalamus and the corpus striatum. These bodies are not only 

 as it were placed upon the surface of each crus, but are also deeply em- 

 bedded in its substance. 



The crus is found to be made up of two principal parts : 



(a.) The one, the tegmentum, situated for the most part on the dorsal 

 aspect, is composed chiefly of gray matter and some longitudinal fibres. 



And (#.) the other, the crusta, situated toward the other surface, is 

 composed almost entirely of longitudinal fibres. It is known also as the 



