164 THE CEREBRUM. 



due to the lateral fillet, and the brachium conjunctivum of the cer- 

 ebellum. Its surface is made up of medullated fibers continu- 

 ous with the lateral fillet and brachium inferius, which forms 

 the stratum zonale; gray substance, called the nucleus of the infe- 

 rior colliculus, constitutes its deep portion. This nucleus receives 

 a part of the lateral fillet and gives rise to a portion of the brach- 

 ium inferius; it forms a partial relay in the acustic path. 



Brachium Superius (Figs. 43 and 44). It connects the lat- 

 eral geniculate body with the superior quadrigeminal colliculus; 

 but, excepting its extreme posterior end, it is buried in the sub- 

 stance of the thalamus. Its course is anterior and internal to 

 the medial geniculate body. The brachium superius is composed 

 chiefly of efferent fibers from the optic radiation, a tract partially 

 relayed in the lateral geniculate body; it probably contains, also, 

 a few optic fibers from the lateral root of the optic tract and a 

 few from the stratum griseum of the superior quadrigeminal 

 colliculus. Optic reflex impulses are conducted by the brachium 

 superius. 



Brachium Inferius (Figs. 43 and 44). The inferior arm, 

 connecting the inferior quadrigeminal colliculus and the medial 

 geniculate body, is visible through its whole length. It forms 

 the superior boundary of the triangle of the lateral fillet; the lateral 

 fillet and the sulcus laterah's of the mid-brain form the other two 

 sides. Two sets of fibers make up the brachium inferius, viz., 

 the lateral fillet fibers and the axones from the' nucleus colliculi 

 inferioris. They all terminate in the medial geniculate body. 

 The brachium inferius forms the third segment of the acustic path. 



CEREBRUM. 



SECTION III. THE STRUCTURE OF THE CEREBRUM. 

 ITS GRAY AND WHITE MATTER. 



We have noticed in the mid-brain that the basis pedunculi 

 and much of the tegmentum are white matter, while the substantia 

 nigra and quadrigeminal colliculi are composed chiefly of gray 

 substance. Gray matter forms nearly all of the inter-brain. The 

 deep part of the cerebral hemisphere is white substance, imbedded 



