5 io THE BRAIN 



are carried upwards into the internal capsule. The inter- 

 mediate third or more of each basis pedunculi is composed 

 of the important cerebro-spinal fasciculus (Q.T. pyramidal tract) 

 as it descends from the motor area of the cerebral cortex, 

 but this is quite indistinguishable from the portions of the 

 crusta which lie on either side of it. 



Inferior quadrigeminal body 



Mesencephalic root of fifth nerve 



Grey matter of 



aciueduct *' 



'Nucleus of fourth nerve 



Aquaeductus_ u _ A ^ inferior brachium 



cerebri A--v < **? -^ 



f*A - - Medial longitudinal bundle 



Raphe 



Brachium conjunctivum 



Basis pedunculi 

 Substantia nigra 



FIG. 211. Transverse section through the Mesencephalon at the level of the 

 inferior quadrigeminal body : the right side only is reproduced. The 

 drawing is taken from a Weigert-Pal specimen, and therefore the grey 

 matter is pale and the strands of white matter are dark. The dark colour 

 of the substantia nigra is not evident owing to the thinness of the section. 



Tegmentum. Unlike the basis pedunculi, the tegmentum 

 is undivided, a faint line in the median plane, termed the 

 median raphe, alone indicating its bilateral character. To- 

 wards the dorsum of the mesencephalon it is fused with the 

 bases of the corpora quadrigemina, and its lateral surfaces 

 only are free. 



The tegmentum is composed of an admixture of grey and white matter, 

 constituting what is termed a formatio reticularis. The white matter is 

 composed of fibres running both transversely and longitudinally. Certain 



