374 



PHYSIOLOGY 



of the biain. The crura are divided by an irregular mass of grey matter, the 

 substantia nigra, into two parts. The ventral portion is known as the pes or 

 crusta. 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 



Superior quadri- 

 geminal body 



Extl. gen. body 

 Infr. brachium 



IntJ. gen. body 



Mesial fillet 



Crusta 



Optic tract 



Nucleus of 3rd 

 nerve 



Supr. cerebellar 

 peduncle 



3rd nerve 



Substantia 

 nigra 



Corpus 

 mammillare 



FIG. 188. Transverse section through human mid- brain at the level of the superior 

 corpus quadrigeminum. (CUNNINGHAM.) 



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

 formatio 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 formatio 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 

 f 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 



