. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 364 Comparative Animal Physiology i^ man, development of temperature regulation may take several ; Temperature regulation is perfected in chickens 4 to 5 days after hatching.^-*" In the house wren temperature regulation appears gradually during the two weeks after hatching (Fig. 95). Regulation is accompanied by increasing sensitivity of the heat center and increasing basal metabolism. In the wren, for example, metabolism as measured by COo output increases with age up to 12-15 d
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 364 Comparative Animal Physiology i^ man, development of temperature regulation may take several ; Temperature regulation is perfected in chickens 4 to 5 days after hatching.^-*" In the house wren temperature regulation appears gradually during the two weeks after hatching (Fig. 95). Regulation is accompanied by increasing sensitivity of the heat center and increasing basal metabolism. In the wren, for example, metabolism as measured by COo output increases with age up to 12-15 days; as regulation appears the CO2 production is highest at low air temperature and is minimal at °. The air temperature at which metabolism is minimal is well below body temperature. Similarly, water loss was increased by times from ° to ° in wrens 0, 3, and 6 days old, but hardly at all in adults; however, at air temperature of 40° the water loss was twice the loss at °, regardless of age.'-*** Young swifts are able to survive 60° F 4o°C - as'c ad-'c 34°C 32 °C oNlGHT • DAY. ICC 2o<'C 30°C 105° F lOO°F 95°F -90°F 40°C EXTERNAL TEMPERATURE Fig. 94. Body temperature as a function of air temperature in a nocturnally active Central American opossum, Metachirtis. From Morrison.^"' fasting longer than adults, because their metabolism and temperature fall and they enter a hunger coma.^*''^ Thermal Stimulation. The mechanisms of heat regulation are activated in two ways: by thermal receptors in the skin, and by direct stimulation of the thermoregulator in the brain by changes in blood temperature. The thermal receptors in the skin may elicit vasomotor reflexes even though the individual is not conscious of heat or cold sensation. Observations on man show that the receptors for cold are probably the bulbs of Krause, located at the outer border of the skin layer containing blood vessels, that is, at a depth of about mm., whereas the warm
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