THE BRAIN AND ITS MEMBRANES. 719 



medulla (see page 712). They join the restiform body, and emerge from the 

 anterior median fissure. Traced backward into the fissure, they enter the raphe, 

 cross over the median line, still in the raphe, and then bend upward and become 

 longitudinal, after which their course is not traceable. As these fibres emerge 

 from between the pyramids a few fibres from each pyramid are said to join with 

 them. As they pass across the ventro-lateral groove and olive they are joined 

 bv some of the internal arciform fibres (see below). Scattered amongst these 

 fibres, or between them and adjacent parts of the medulla, are small masses of 

 gray matter with nerve-cells. These masses are the nuclei of the external arci- 

 form fibres. The largest on each side is ventral to the pyramid. 



The Internal or Deep Arciform Fibres. A portion of these have already been 

 mentioned. Traced upward, they start from the nuclei of the gracilis and cuneate 

 columns, and then constitute the decussation of the fillet (see page 718). The 

 remainder of the internal arciform fibres are known as the olivary peduncle (see 

 page 715). The fibres of this peduncle decussate across the median line (through 

 the raphe) with those of the opposite peduncle. It must be remembered that 

 this peduncle, as a whole, is really a lamina of superimposed transversely run- 

 ning fibres, and not the mere bundle it appears to be on section. Traced from 

 one side to the other, they start from the cells in the gray matter of the olivary 

 nucleus (see page 715), and pass mesially out through the hilum. They then 

 decussate, as above mentioned, with the opposite peduncle, and enter, through 

 its hilum, the opposite olivary nucleus. Here they diverge as they approach the 

 gray lamina of the nucleus, and proceed in different directions, after passing 

 through the lamina. This "passing through" the lamina is not true for all the 

 fibres, for some end in the nuclei, which in their turn give rise to new fibres 

 which continue the course of the old ones. On "passing through," then, the 

 lamina, the most posterior fibres run backward through the lateral area and join 

 the restiform body, and thus reach the cerebellum ; the uppermost pass upward as 

 longitudinal fibres in the formatio reticularis of the lateral area (see page 718), 

 and have been traced on up to the cerebral hemisphere of the same side ; the 

 more anterior fibres run between the longitudinal fibres on the surface of the 

 olive or between those in the grooves on each side of the olive, and there bend 

 backward and join the external arciform fibres, and are continued to the restiform 

 body and cerebellum. 



The Raphe. The raphe is situated in the middle line of the medulla, above 

 the decussation of the pyramids. It consists of nerve-fibres intermingled with 

 nerve-cells. The fibres have different directions which can only be seen in suitable 

 microscopic sections ; thus : 



1. Some are dorso-ventral ; these are continuous ventrally with the superficial 

 arciform fibres, and dorsally with fibres from the striae acusticae. 



2. Some are longitudinal ; these are derived from the arciform fibres, both 

 sets, which on entering the raphe change their direction and become longitudinal. 



3. Some are oblique ; these are continuous with the deep arciform fibres which 

 pass from the raphe. 



Some of the fibres of the raphe arise from the nuclei ventral to the floor of 

 the fourth ventricle. 



THE PONS VAROLH (Figs. 415, 416). 



The pons Varolii is the ventral portion of the hind-brain, the dorsal portion 

 being the cerebellum. The pons is in direct continuity below with the medulla, 

 all the longitudinal fibres of each being directly continuous from one to the 

 other, with the exceptions of the restiform bodies of the medulla, which go to the 

 cerebellum (inferior peduncles), and the superior peduncles of the cerebellum, 

 which, as will be seen, belong to the structure of the pons, after they leave or 

 before they enter, according as they are traced, the cerebellum. 



The pons is about one inch long, and somewhat more in width. Dorso- 

 ventrally, it is about three-fourths of an inch (17-18 mm.), hence its thickness is 



