DESCRIPTION OF THE ARBOR VIT.E CEREBELLI. 79 



white matter between the cavity of the fourth ventricle and the lingula is the 

 middle part of the superior medullary velum. The two vela are prolonged like roots 

 upwards and downwards from the anterior or ventral side of the corpus trapezoides. 

 From the upper part of the corpus trapezoides two branches diverge, one upwards 

 with a slight inclination forwards, this is the branch to the central lobe ; the 

 other, the thickest and strongest of all the branches of the arbor vitse of the 

 worm, passes upwards and backwards into the culmen. This stem of the culmen 

 gives off secondary branches on cither side. Of those which are directed upwards 

 and forwards, the one nearest the base is rudimentary, and enters a small lamina 

 concealed at the bottom of the post-central fissure : the next two are large, and 

 enter laminae which reach the surface, but before attaining it they may themselves 

 fork. The prolongation of the stem also reaches the surface, being first bifurcated, 

 but the branches from the lower or posterior side of the stem of the culmen, two, 

 three or more in number, pass into short laminae which mostly lie concealed within 

 the pre-clival fissure. 



Prolonged backwards from the postero-inferior angle of the corpus trapezoides 

 is a narrow horizontal stem, which after a course of about two centimeters, ends 

 directly in the medullary centre of the folium cacuminis, often passing round a 

 slight curve just before reaching this. From the upper side of this horizontal stem 

 about five branches pass upwards and backwards, and from the lower side about 

 as many pass downwards and backwards, but all are not of equal importance, for 

 some merely enter rudimentary lamellae which are concealed at the bottom of the 

 interlobar fissures, and only a few enter lamellas which reach the surface. 



Of those which pass from the upper side of the horizontal stem all may be 

 considered to belong to the clivus. Of the more anterior of these some are rudi- 

 mentary, one or two less so, but all enter lamellas which are completely concealed 

 in the preclival fissure. The hindmost is larger and longer and reaches the surface ; 

 it gives off a vertical branch which passes into the upper part of the lobe nearest 

 the culmen, bifurcating near the surface, and is then continued on in a nearly 

 horizontal direction parallel to and overlying the folium cacuminis. 



Of the branches from the lower side of the white centre and its horizontal pro- 

 longation three are of great importance. The foremost one passes from the corpus 

 trapezoides almost vertically downwards for about six or eight mm. into the uvula, 

 giving off only small lateral branches ; it then gives off successively two or three 

 branches which course downwards and forwards, usually bifurcating before reaching 

 the surface. Only short branches are given off backwards from the main part of the 

 uvula stem. 



About two or three mm. behind the branch to the uvula, that to the pyramid 

 passes off. This has a general direction downwards and backwards ; it gives off 

 lateral branches tis it proceeds, most of which are small, but one or two longer 

 branches come off on its anterior aspect and pass to the surface nearly vertically 

 downwards. 



Following the horizontal stem back, two or three rudimentary offshoots are seen 

 entering small lamellae which are concealed within the postpyramidal fissure, until 

 finally, about 15 mm. from the corpus trapezoides and 5 mm. from the base of the 

 folium cacuminis, another considerable branch passes off at an acute angle, with a 

 curve forwards and downwards into the tuber valvulae. Its lateral offshoots, which 

 are at first short, become gradually longer, corresponding with the expanding form 

 of this lobe of the worm. 



The deepest fissures of the median section are the precentral, the postcentral, 

 the preclival, the prepyramidal, the postpyramidal, and the postnodular (see 

 fig. 59 A). The postclival fissure is here quite shallow, the great horizontal only 

 slightly deeper. The general grouping of the branches of the arbor vitae is into 



