THE STRUCTURE OF THE BRAIN STEM 373 



wusta. It is composed almost entirely of longitudinal white fibres, among 

 which is the continuation forwards of the pyramids of the medulla. The 

 pyramids however form only about two-fifths of the total mass of white 

 fibres, the rest consisting of fibres which run from the different parts of the 

 cerebral cortex, especially from the frontal and temporal lobes, to end in the 

 f ormatio reticularis of the pons, probably in relation with the grey matter in 

 this situation and with the endings of the transverse fibres derived from the 

 cerebellum and forming the middle peduncles of the cerebellum. The dorsal 

 part, the tegmentum, is a direct prolongation forwards of the f ormatio reticu- 

 laris of the medulla and pons, and like this contains much scattered grey 

 matter. On a level with the inferior corpora quadrigemina a number of 

 decussating fibres are to be seen in the tegmentum, which are derived from 

 the superior cerebellar peduncles. Their decussation is complete at the level 

 of the upper border of the inferior corpora quadrigemina. Here each 

 peduncle turns upwards, and a large proportion of its fibres end in the red 

 nucleus (Fig. 188), a mass of grey matter forming a conspicuous feature of 

 sections through the anterior part of the mid-brain. Many of the fibres pass 

 round the red nucleus, forming a sort of capsule over it, to the ventral 

 part of the optic thalamus, in which they probably end. It is possible that a 

 certain proportion pass through the optic thalamus and run straight to the 

 cerebral cortex of the Rolandic area. The lateral fillet has disappeared from 

 the region of the tegmentum and passed into the inferior corpora quadri- 

 gemina. The mesial fillet forms a flat band lying to the outer side of the red 

 nucleus and comes into close relation with a ganglion of the fore-brain, known 

 as the internal geniculate body. The roof of the mid-brain is formed by the 

 corpora quadrigemina. The inferior corpora quadrigemina are composed of 

 central grey matter encapsulated by white matter, derived chiefly from 

 the lateral fillet. The superior corpora quadrigemina are composed of several 

 layers of grey matter traversed by nerve fibres, derived partly from the 

 fillet, partly from the optic tract, and partly from the occipital lobe of the 

 cerebral hemisphere. 



THE FORE-BRAIN 



In the fore-brain the most important feature is the optic thalami, the 

 two head ganglionic masses of the brain stem (Fig. 189). In this region 

 the central neural canal, which in the mid-brain forms the Sylvian iter, 

 widens out to the third ventricle, in the lateral walls of which are developed 

 the two optic thalami. It is a narrow cleft, rapidly increasing in depth from 

 behind forwards. As we trace sections forwards we see that the two crura 

 cerebri diverge from one another. The floor of the third ventricle is thus left 

 thin. It is formed from behind forwards by a thin layer of grey matter witn 

 numerous vessels, the locus perforatus posticus, two small eminences, he 

 corpora mammillaria, and in front of these another lamina of grey matter 

 known as the tuber cinereum. In front of the tuber cinereum is the infun- 

 dibulum, which leads to the posterior lobe of the pituitary body. In front 

 of the inf undibulum the optic chiasma is closely attached to the lowest part 

 of the anterior wall of the ventricle. The front wall is formed by a thin layer ' 



