The effect of heating the hypothalamus on respiration in the ox (Bos taurus)

When the ox is exposed to heat, respiratory frequency increases before there is any increase in the temperature of the blood at the bicarotid junction (Bligh, 1957b) or of the brain (Findlay & Ingram, 1961). Furthermore, in cool conditions when the temperatures of the extremities are low, localized infra-red irradiation of part of the trunk may be accompanied by large increases in the temperature of the unheated extremities without any change in the deep body temperature (Whittow, 1962; Ingram & Whittow, 1962). These findings lead to the conclusion that, in the ox, at least part of the stimulus for the initiation of heat loss mechanisms originates at the periphery. Nevertheless it is known that the brain contains centres concerned with the maintenance of homoeothermy, and which are sensitive to changes in temperature (Ranson, 1940; Andersson, Grant & Larsson, 1956; Andersson & Persson, 1957). Previous observations on the ox revealed that localized heating of the region between the optic chiasma and the anterior commissure was accompanied by an increase in respiratory frequency and in the skin temperatures of the ears (Findlay & Ingram, 1961) and also in the rate of moisture loss from the skin (Ingram, McLean & Whittow, 1961). It seems probable, therefore, that the heat-loss mechanisms of the ox are controlled by the integration of stimuli originating from both central and peripheral receptors, but nothing is known about the relative importance of these stimuli. The present study was undertaken to provide information about changes in respiratory frequency, tidal volume and minute volume, and the temperatures of the extremities, when the temperature of the hypothalamus was increased by different amounts at various temperatures of body and environment.