200 THE ESSENTIALS OF HISTOLOGY 



The Sylvian aqueduct (s), with its lining of ciliated epithelium, re- 

 presents the central canal of the cord and the fourth ventricle of the 

 medulla. In the grey matter which surrounds it (central grey matter) 

 there is seen in all sections of the region a group of large nerve-cells 

 lying anteriorly on each side of the middle line, close to the reticular 

 formation. From this group the root-bundles of the fourth nerve arise 

 at the lower part of the mesencephalon and pass obliquely backwards 

 and downwards around the central grey matter, decussating with those 

 of the opposite side to emerge just above the pons Varolii (fig. 234). 

 Higher up the bundles of the third nerve spring from the continuation of 

 the same nucleus (fig. 235, n. in.}, and these pass forwards and down- 

 wards with a curved course through the reticular formation, to emerge 

 at the mesial side of the crusta. 



The reticular formation of the pons is continued up into the mes- 

 encephalon, and is here known as the tegmentum. It is composed as 

 before of longitudinal and transverse bundles of fibres with much grey 

 matter intermingled. The transverse fibres include the decussating 

 fibres of the superior crura of the cerebellum (s.c.p.}, and the fibres of 

 the fillet (/), which are passing in an oblique manner from the raphe 

 to the side of the mesencephalon, to reach eventually the grey matter 

 of the prominences of the corpora quadrigemina. The pyramid bundles 

 of the pons are continued upwards on each side into the crusta (cr.). 

 This forms a mass of longitudinally coursing bundles of fibres lying on 

 the ventral aspect of each half of the mesencephalon, and diverging 

 above into the internal capsule of the cerebral hemisphere. The crusta 

 is separated from the tegmentum by a layer of grey matter containing 

 a number of very deeply pigmented nerve-cells which give it the name 

 of substantia nigra (s.n.} The crusta and tegmentum, together with 

 the intervening substantia nigra, constitute the cms cerebri. 



The prominences of the corpora quadrigemina are formed mainly 

 of grey matter containing numerous small nerve-cells. From each 

 a bundle of white fibres (brachium) passes upwards and forwards 

 towards the geniculate bodies, eventually joining the optic tract of 

 the same side. On the other hand, each of the prominences receives 

 from below fibres of the fillet, which are themselves traceable into the 

 posterior part of the lateral column of the medulla oblongata. 



