238 THE ESSENTIALS OF HISTOLOGY. 



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. 266). Higher up, the bundles of the third nerve spring from the 

 continuation of the same nucleus (fig. 268, n. ni.), and these pass 

 forwards and downwards 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 

 Jillet (/), 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 (figs. 267, 268, 

 cr., fig. 269, p.p.). 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 crus cerebri. 



The prominences of the corpora quadrigemina are formed mainly of 

 grey matter containing numerous 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 traceable below into the ventral part of the 

 anterior area of the medulla oblongata, and then through the raphe to 

 the nuclei of the gracile and cuneate funiculi of the opposite side. 

 Since these nuclei contain the terminal arborisations of many of the 

 ascending fibres of the posterior spinal roots (see diagram, p. 222), and 

 the fibres of the fillet emanate from cells in the nuclei, the fillet forms a 

 second link in the chain of afferent fibres leading towards the brain. 

 Some of the fibres of the fillet are continued up beyond the mid-brain 

 into the subthalamic region of the cerebrum. The superior corpora 

 quadrigemina receive many of the fibres of the optic tract which form 

 a superficial white stratum covering the grey matter. These fibres are 

 derived from nerve-cells in the retina (diagram, fig. 270), and having 



