Section VI. 

 The Midbrain. 



The miibr-iin of Mustslas has retained to a marked degree 

 the featipes characteristic of this cerebral vesicle in its 

 primitive condition. Its ventricle has been so little encroach- 

 ed apon by nervous matter that the name aqueiuat of Syl'jius 

 ■^oali not be applied as a descriptive term were it not made 

 necessary by jsage in higher vertebrates. The aqueduct is 

 produced laterally into a pair of spacious recesses, each of 

 which occupies the interior of an optic lobe, while the general 

 cavity communicates freely through narrower extensions with 

 the third ventricle in front and the fourth ventricle behind 

 (Fig. l,mb. ). The optic lobes are two thin-walled bodies of 

 dome-like form, separated from each other by a conspicuous 

 median furrow, and the pair taken together are somewhat broad- 

 er than the base of the midbrain on which they rest. The ante- 

 rior divisions of the cerebellum lie upon and partially con- 

 ceal the optic lobes from the dorsal aspect. The base of the 

 miJbrain is a direct continuation anteriorly of the great fibre- 

 systems of the oblongata and metencephalon, showing but slight 

 diminution in size. -^ . 



