MIDBRAIN AND THALAMUS OF NECTURUS 221 



ing backward into the fourth ventricle and forward into the 

 unpaired telencephahc ventricle in front of the optic chiasma. 



The mesencephalon 



The mesencephalic ventricle is widely dilated both dorso- 

 ventrally and laterally, with the widest part somewhat above 

 the middle (figs. 7 to 14 and 63). As in vertebrates generally, 

 it is composed of a dorsal tectum mesencephali and a ventral 

 pedunculus cerebri. There is no external boundary between 

 these. The internal structure indicates that the widest part 

 of the ventricle does not represent the position of the embryonic 

 sulcus limitans separating the dorsal sensory from the ventral 

 motor lamina, this boundary lying somewhat ventrally of that 

 level. 



The approximate boundary between the mesencephalon and 

 the diencephalon is marked externally by a shallow transverse 

 groove extending from the dorsal to the ventral surface at the 

 level where the hypothalamus becomes free from the cerebral 

 peduncle. Internally the upper limit of the mesencephalon is 

 commonly described as marked by the rastral border of the 

 posterior commissure dorsally and of the tuberculum posterius 

 ventrally. 



The tuberculum posterius (figs. 62, 63, 64, tub.p.) is the sharp 

 bend in the brain floor where the rostral end of the cerebral 

 peduncle turns ventralward to join the dorsal, part of the 

 hypothalamus. 



The caudal boundary of the mesencephalon is marked dorsally 

 by the decussatio veli, containing the cerebellar commissures 

 and the decussation of the IV nerves (fig. 62), On the lateral 

 surface it is indicated by the constriction of the isthmus (fissura 

 isthmi), behind which on the dorso-lateral aspect are the en- 

 largements formed by the small cerebellum and the very large 

 auricular lobes of the medulla oblongata. The ventral part of 

 the midbrain (cerebral peduncle) extends caudalward less than 

 half as far as does the dorsal part or tectum. 



In the ventro-medial plane the caudal boundary of the cere- 

 bral peduncle is indicated by a well defined pit, the fovea isthmi 



