THE MESENCEPHALON 



457 



a left half. Towards the dorsum of the mesencephalon it is 

 fused with the deep surface of the lamina quadrigemina, and 

 only its lateral surfaces are free. 



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

 constituting what is termed a formatio reticularis. The white matter 

 consists of fibres running both transversely and longitudinally. Certain 

 of the longitudinal fibres are grouped together and form well-marked 

 tracts, which, in a section through the mesencephalon of a fresh brain, can 

 be detected by the naked eye. The tracts are: (i) the medial longi- 

 tudinal bundles ; (2) the brachia conjunctiva ; (3) the lemnisci. 



Superior quadrigeminal body 



Tegmentum 



Inferior brachium i 



Medial geniculate body 



Lateral geni- \ 



culate bodyv ^__^_J. 



Optic tract 



Basis peduncul 



Medial lemniscus 



Grey matter of aqueduct 



Aquaeductus cerebri 



"^__| Nucleus of oculo- 

 .' : motor nerve 

 f;iLj. Medial longi- 

 "ff, # tudinal fasciculus 



r-Red nucleus 



4- Fibres of brachium 

 . .' conjunctivum 



.Fila of 

 oculo-motor nerve 



Substantia nigra 



Corpus mamillare 



FIG. 1 8 1. Section through upper part of Mesencephalon at level of superior 

 quadrigeminal body. The drawing is taken from a Weigert-Pal specimen. 

 The dark colour of the substantia nigra is not evident owing to the 

 thinness of the section. 



Fascicuhts Longitudinalis Medialis. The medial longitudinal fasciculus 

 (Figs. 180 and 181) is a small, compact bundle which is placed upon the 

 corresponding lateral aspect of the ventral portion of the central grey 

 matter of the aqueduct. 



The brachia conjunctiva (O.T. superior cerebellar peduncles} are two 

 large strands which are continued upwards from the cerebellum into the 

 mesencephalon. By pulling away the margin of the cerebellum, where it 

 overlaps the inferior colliculi of the quadrigeminal lamina, the dissector 

 will see the brachia on the surface as they converge in an upward direction. 



