FUNCTIONS OF THE BRAIN. 819 



only from the extra-ciliary fibres around the corpora dentata, decussating 

 in the pons, but also from the associate nuclei, while the fibres from the 

 extra-ciliary columns alone have been traced into connection with the 

 pons. It, therefore, is evident that the cerebellum is to be regarded as 

 a complicated collection of gray ganglionic masses in unbroken connec- 

 tion with the medulla oblongata and pons on the one side and with the 

 cerebrum on the other. 



The pons varolii is formed by a collection of the continuations of 

 the reticular formation of the medulla, the pyramids and anterior columns 

 of the medulla, together with the fibres, coming directly by the middle 

 peduncle from the cerebellum. 



Above the pons the two collections of fibres known as the cerebral 

 peduncles {crura cerebri) approach each other, the point at which the 

 union is accomplished above the medulla being the locality in which the 

 upper end of the fourth ventricle terminates in the aqueduct of Sylvius. 



In the interior of each cerebral peduncle is found a mass of gray 

 matter, the substantia niger, which separates the fibres of each crus into 

 two layers, the upper laj'er forming the so-called tegmentum cruris 

 and the lower the basis cruris. The upper division of the tegmentum 

 contains also a gray nucleus {nucleus tegmentis). 



Although these two divisions of the crura are in close anatomical 

 connection, it has been determined with a considerable degree of posi- 

 tiveness that the pyramidal fibres are in direct communication through 

 the basis of the cerebral peduncles with the central convolutions of the 

 cerebral hemisphere on the same side, although whether a direct commu- 

 nication, as in the case of the cerebellum, of these fibres with the gangli- 

 onic cells in the upper surface of the hemispheres exists or not has not 

 yet been determined. Nevertheless, it appears that they form no direct 

 communication with the other gray masses of the hemispheres, the 

 lenticular nucleus, optic thalamus or striated body. And since they 

 undergo degeneration only after injury of the central convolutions, it 

 is probable that they are in direct communication with the cortex of 

 the brain. 



Plechsig claims to have followed them in an unbroken course from 

 the pyramidal columns through the white mass between the lenticular 

 nucleus and optic thalamus, the so-called internal capsule, direct to the 

 gray cortex of the cerebral convolutions. The course of these fibres, 

 together with the other important cerebro-spinal tracts are shown in 

 Fig. 357. 



The cerebrum is divided by a longitudinal fissure into two hemi- 

 spheres in man, mammals, and birds, and its external surface is divided 

 hy a number of lesser fissures into lobes and convolutions. The cere- 

 brum contains the ganglia of the brain, the optic thalamus or corpus 



