422 PHYSIOLOGY 



of the cerebellum. The dorsal part, the tegmentum, is a direct pro- 

 longation forwards of the formatio reticularis 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 cere- 

 bellar 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. 189), 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 quadrigemina. 

 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 quadri- 

 gemina 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. 190). 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 deepening 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 with numerous 

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

 infundibulum, which leads to the posterior lobe of the pituitary body. 

 In front of the infundibulum 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 of nervous matter, the lamina cinerea, at the 

 upper border of which, projecting slightly into the ventricle, is a 



