CHAPTER X. 

 The Midbrain. 



THE general form of the midbrain, or mesencephalon, is 

 less modified from the early embryonic condition than 

 is that of either of the other primary subdivisions of 

 the brain. It retains the condition of a thick-walled tube 

 connecting the forebrain and the hindbrain with only a little 

 modification of the shape of the walls by a greater thickening 

 ventrally than dorsally, and by the moulding of the dorsal 

 wall, or tectum, into four eminences, the corpora quadrigemina 

 (Pis. HI., Xn.-XVL). The narrow ventricle is variously 

 known as the cerebral aqueduct, the aqueduct of Sylvius, or the 

 iter. The thick wall ventral to the aqueduct is the cerebral 

 peduncles, which are divided into a thick tegmentum, in the 

 deep position, and a pair of large flattened bands of longi- 

 tudinal fibres situated superficially, the basis peduncuU or 

 pes peduncuU. Tegmentum and basis are separated by a 

 layer of gray matter, the substantia nigra (Pis. XHI.-XM.). 



Dorsal to the substantia nigra, the tegmentum consists of 

 reticular formation continuous with that of the medulla 

 oblongata, while dorsal to this again is a rather thick layer 

 of central gray matter. Into the tegmentum may be traced 

 many of the longitudinal tracts which have been observed 

 in the corresponding region of the hindbrain. In the posterior 

 part, the fibres of the lateral lemniscus are to be seen at each 

 side surrounding their dorsal nucleus and running antero- 

 dorsally into the tectum. From the enlarged anterior end of 

 the dorsal nucleus of the lateral lemniscus, arises a thin bundle 

 of transverse fibres, the commissure of the lateral lemniscus or 

 commissure of Probst (PI. XII.). After decussating, these 



