THE MESENCEPHALON 511 



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

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

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

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



The medial longitudinal bundle (Figs. 21 1 and 212) is a small compact 

 fasciculus which is placed upon the lateral aspect of the lower portion of 

 the central grey matter of the aqueduct. 



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



Medial geniculate body 



Lateral geni- \ 



culate bodvv 



Superior quadrigeminal body 



Tegmentum 

 Inferior brachium ( 



Grey matter of aqueduct 



n-AqtUBduct&S cerebri 



! Nucleus of third 

 . | nerve 



4- Medial longi- 

 4, tudinal bundle 



nucleus 



Fibres of brachium 

 j conjunctivum 



Optic tract 



Basis pedunculi 



Medial lemniscus 



Substantia nigra 



Corpus mamillare 



FIG. 212. 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. 



large strands which are continued upwards from the cerebellum into the 

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

 overlaps the lower quadrigeminal bodies, these brachia will be seen on the 

 surface as they converge in an upward direction. Stretching across the 

 interval between them, and bringing them into continuity with each other, 

 is a thin lamina called the anterior medullary velum. When the brachia 

 conjunctiva reach the bases of the inferior quadrigeminal bodies, they 

 sink into the substance of the mesencephalon, and, in a transverse section 

 through the lower part of this portion of the brain, they may be seen as two 

 white strands, semilunar in outline and placed one on either side of the 

 grey matter of the aqueduct. As they ascend, they gradually assume a 

 deeper (i.e. a more ventral) position in the tegmental part of the mesen- 



