g^^ Comparative Animal Physiology 



erection (pilomotor response) may occur in distant regions of the body by 

 reflexes which traverse hypothalamic centers. Whether there are by-pass con- 

 nections directly to the vasomotor center in the medulla from ascending tracts 

 carrying impulses from thermoreceptors or whether all of these impulses go 

 first to the temperature center in the hypothalamus is not established. In any 

 case the initial reflex reactions have the effect of altering the body insulation, 

 increasing or decreasing heat loss from the skin. Removal of parts of the 

 cerebral cortex from cats and dogs results in abnormal temperature-regulating 

 reflexes, particularly to cold; hence the cortex may be required for proper 

 responses of the temperature center. ^^^ Decerebrate and spinal animals are 

 unable to show temperature regulation. 



When the environmental changes in temperature are sufficient that the 

 insulating reactions do not adequately maintain body temperature, the blood 

 temperature may change and this stimulates to more drastic reaction the 

 thermoregulating center. This is a region of the caudal hypothalamus con- 

 sisting of two portions, one heat-sensitive and the other cold-sensitive."*'' ^'^^ 

 These regions evoke many responses in the blood vessels, endocrines such as 

 adrenal medulla, various autonomic effectors, and skeletal muscle. The prin- 

 cipal outflow is autonomic, and sympathectomized animals fail to show vaso- 

 motor and pilomotor responses to temperature changes. 



The mechanism of stimulation of thermoreceptors and the nature of the 

 afferent discharge from them are not known. It is of interest that nerve fibers 

 can be stimulated by local temperature change, the small C fibers of mammals 

 being excited by heating and large A fibers by cooling.^'^ The cutaneous 

 thermoreceptors are large structures, and a thermal gradient in them may 

 cause a potential from one end of the receptor to the other which is excitant. 



Physical Mechanisms of Temperature Regulation. The mechanisms of 

 temperature regulation are sometimes considered as physical (transfer of heat 

 increased or decreased), and chemical (production of heat increased or de- 

 creased). The physical factors are well summarized by Dubois.^** The 

 initial responses to changes in environmental temperature are local vaso- 

 motor reactions which are initiated reflexly from cutaneous thermoreceptors. 

 Skin vessels dilate and blood leaves the viscera when the air temperature 

 is high, and the reverse reactions occur in low air temperatures. Change 

 in temperature of one extremity causes vasomotor responses in the other 

 extremity; the ears of some mammals (rabbit, elephant) are important in 

 vasomotor cooling. Thus the transfer of heat to the body surface by blood 

 regulates loss or retention of heat. Further, the thermal conductivity of skin 

 is increased by vasodilatation. The temperatures inducing dilation and con- 

 striction vary with humidity, and an "optimal" temperature exists for an 

 animal at a given humidity. As water is lost in heat the blood volume may 

 increase and water may move from the body tissues out into the blood, where- 

 as in cold the reverse reaction occurs and tissue cells swell.'" Thermal con- 

 ductance can change by 5 or 6 times, and skin blood flow varies much more.-^- 

 Loss of heat by radiation exceeds loss by vaporization at low temperatures, but 

 above about 30° loss by vaporization predominates (Fig. 96). Water is vapor- 

 ized over three channels: (1) in the respiratory tract, (2) by the sweat glands, 

 and (3) by insensible loss directly from tissue spaces and out of the skin. The 

 relative contribution of these channels varies with species and conditions of 



