THE MIDBRAIN 355 



bulum, the latter being the slender stalk of the hypophysis 

 which connects it to the brain. When the hypophysis is 

 removed, the infundibulum which attaches it to the brain 

 is, naturally, broken, exposing the cavity in its base as the 

 slit mentioned above. The region of the tuber cinereum 

 contains centres concerned chiefly with the correlation of 

 olfactory with visceral sensory impulses. 

 {p) The mamillary body (corpus mamillare) forms a con- 

 spicuous rounded elevation, lying at the posterior end of 

 the tuber cinereum. The, structure is externally single in 

 the rabbit, but there is an indication of lateral lobes. It 

 also receives impulses from the olfactory correlation regions 

 of the cerebral hemisphere and combines them with others, 

 chiefly visceral. 

 The tuber cinereum and the mamillary body belong to the 

 hypothalamus, a region which includes centres for the control of 

 the visceral organs through the autonomic system and for the 

 regulation of highly integrated vegetative functions such as the 

 metabolism of water, carbohydrate, and fat and the maintenance 

 of body temperature. 



THE mesencephalon: 



(a) The boundary between prosencephalon and mesencephalon 

 is marked dorsally by the anterior edges of a pair of promin- 

 ent rounded elevations, which are associated with a second 

 pair just behind them to form the roof of the midbrain. 

 These four elevations are the corpora quadrigemina. The 

 anterior pair, distinguished as the colliculi superiores, is 

 much larger than the posterior pair, the colliculi inferiores. 

 The superior colliculi correspond to the optic lobes of sub- 

 mammalian vertebrates and receive many of the fibres of 

 the optic tracts as well as fibres conveying impulses of 

 other sensory types to be correlated with the visual ones. 

 The inferior colliculi are important reflex centres belonging 

 to the auditory system. _. 

 ih) The ventral part of the midbrain is shorter anteroposteriorly 

 than the dorsal one and is occupied by a pair of thick 

 ridges converging from in front, the cerebral peduncles. 



