834 Comparative Animal Physiology 



is unable to stand. Preparations of this kind have been used extensively in 

 the study of righting reflexes. 



The inferior colliculus can mediate some reflex shocklike responses to 

 sound, and action potentials from the midbrain of the cat in response to 

 sound show that the inferior colliculus is an important auditory reflex cen- 

 ter.-^ Similar electrical recording shows that the superior colliculus is a cen- 

 ter for mediating responses to light,^^ with a considerable degree of point-to- 

 point representation for the retina. Local stimulation of the superior collicu- 

 lus by strychnine elicits discrete eye movements. Mammals lacking the vis- 

 ual (striate) cortex show limited visual responses mediated by the mid- 

 brain; cats and dogs still show some intensity discrimination, monkeys show 

 less discrimination, and in man vision for light and dark seems to be lost. 

 A variety of evidence indicates that in most mammals rod vision depends 

 less than cone vision on the cortex, but that in man neither can be subserved 

 by subcortical levels. 



The midbrain of fishes and amphibians is the region of highest integra- 

 tion and controls the most complex behavior which these animals show. In 

 higher animals, particularly mammals, the midbrain retains the regulation 

 of restricted visual and auditory reflexes, and complex integration has moved 

 forward to areas of greater surface. 



The Forebrain. Diencephalon. The various portions of the thalamus and 

 associated structures show great variations in structure from one vertebrate 

 class to another. The function of this part of the brain is litde known in non- 

 mammalian groups. The diencephalon gives rise to the posterior pituitary 

 gland, also to the pineal and contains (in fish) many neurosecretory cells.""® 

 The thalamus consists of dorsal and ventral thalamus and the hypothalamus, 

 which is increased greatly in birds and mammals. 



The thalamus is not known functionally in fish. Frogs from which the en- 

 tire thalamus has been removed (also the forebrain connections) are inac- 

 tive and do not jump voluntarily.'^" They appear to be blind.-*'^ Release of 

 the female by a male frog after oviposition requires a region in the dorsal 

 hypothalamus and preoptic forebrain.'" There is evidence that water trans- 

 fer through the skin may also be regulated by the thalamus. In birds and 

 mammals the thalamus becomes important as an autonomic center. In birds 

 temperature regulation is localized in this region,^®'"^ and stimulation causes 

 a rise in blood pressure and slowing of respiration.'^® 



In mammals the dorsal thalamus is important in correlating some sensory 

 functions, particularly those of taste and temperature. The hypothalamus is 

 essential to life and is the seat of many vegetative functions, such as water 

 regulation by eliciting liberation of a pituitary hormone which decreases re- 

 absorption of water by kidney tubules, and regulation of temperature, sleep, 

 and blood sugar level, and other functions. There is some cortical regula- 

 tion of the autonomic centers of the thalamus, and removal of parts of the 

 cortex from cats releases a so-called center of sham rage.-^^ The anterior part 

 of the hypothalamus prevents abnormally high body temperatures; the pos- 

 terior part prevents chilling. Damage to the hypothalamus removes emo- 

 tional drive and induces somnolence.'*^^ 



Telence-phalon. The telencephalon is absent from Avi'phioxus but is pres- 

 ent in all classes of vertebrates, where it has evolved in association with the 



