THE STRUCTURE OF THE BRAIN STEM 



421 



THE MID-BRAIN 



A little further forward the fourth ventricle comes to an end, and 

 the section passes through the mid-brain (Fig. 188), the cavity of the 

 second cerebral vesicle being represented by the narrow Sylvian 

 aqueduct, bounded dorsally by the corpora quadrigemina and ventrally 



Superior quadri- 

 geminal body 



Extl. gen. body 

 Infr. brachium 

 Intl. gen. body 



Mesial fillet 



Crusta 



Optic tract 



Sylvian grey 

 matter 



Aqueduct of 

 Sylvius 



Nucleus of 3rd 

 nerve 



Supr. cerebellar 

 peduncle 



3rd nerve 



Substantia 

 nigra 



Corpus 

 mammillare 



FIG. 189. Transverse section through human mid-brain at the level of the superior 

 corpus quadrigeminum. (CUNNINGHAM.) 



by the crura, the stalks of the brain. 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 

 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 



