Temperature: Metaholic Aspects and Perception 361 



In turtles and frogs the arterial blood pressure is directly related to blood 

 temperature, and when the brain is pithed the blood pressure is no longer 

 responsive to temperature.'^'' Local warming or cooling in the hypothalamus 

 of the turtle but not in other regions of the brain has an effect on blood 

 pressure. In birds and mammals, also, a drop in body temperature is accom- 

 panied by a decrease in blood pressure. Rodbard'^*' has suggested that the 

 temperature-regulating center of homoiotherms evolved from a region of the 

 hypothalamus responsible for circulatory adjustment. Migration of vertebrates 

 from water to land has been accompanied by development of a tolerance for 

 greater and more rapid changes of external temperature than occur in water. 

 Reptiles tolerate a wide range of body temperature and a corresponding range 

 of autonomic function; birds and mammals tolerate a narrower range of body 

 temperature. It would be of interest to know whether fish have a temperature- 

 sensitive autonomic center comparable to that in turtles. 



TEMPERATURE RELATIONS IN HOMOIOTHERMIC 

 AND HETEROTHERMIC ANIMALS 



Birds and mammals differ from all other animals in having a temperature- 

 sensitive center in the brain which, by altering heat retention and heat pro- 

 duction, acts as a thermostat for the body. Cooling mechanisms come into more 

 prominent action in high environmental temperatures, and warming mechan- 

 isms in low external temperatures, so that over a limited environmental 

 range body temperature is relatively constant. However, body temperature 

 shows diurnal fluctuations associated particularly with muscular activity, 

 feeding, and digestion. The diurnal temperature variation is less in large 

 birds than in small birds, and nocturnal birds such as the owl have their 

 temperature maximum during the night. '•''' Similar maxima at night occur in 

 nocturnal mammals, such as the opossum.^-' When the activity rhythm is 

 reversed by changing the illumination, the body temperature rhythm is also 

 reversed. Ability to regulate body temperature varies ako with age, maturity, 

 metabolic state, and other factors. The temperature in the interior of the body 

 is higher than at the body surface. In a nude man at an air temperature of 34° 

 C. various body regions were within 2.5 degrees of one another in temperature, 

 but at an air temperature of 22° C. the rectal temperature was 36.5° C, 

 while that of the hands was 30°, and that of the feet 27°.^*' In a dog at 37.2° 

 C. (per rectum), the temperature of thigh muscle was 36.4°, and of thigh skin 

 33.4°.^'' At an air temperature of 100° F. man gains when clothed 200 cal./hr., 

 and, since the skin absorbs nearly all impinging rays (as a black body), the 

 absorption is twice as great in the sun as in the shade.'* In exercise the skin 

 temperature first rises, then falls as sweating begins. 



Degrees of Homoiothermy. Some representative body temperatures of 

 birds and mammals are given in Table 61. Birds are higher in body tempera- 

 ture than mammals, and "higher" mammals have higher temperature and 

 better regulation than more primitive orders. ^"^ Extensive tables of body 

 temperatures are given by Kanitz-'^ and by VVislocki;'"' homoiothermism is 

 discussed by Pearse and Hall,^'"* and temperature regulation in man is pre- 

 sented by Burton,^^ and Dubois.^^' It has been suggested^-'' that since sperma- 

 togenesis is temperature sensitive, the presence of a scrotum keeps the testes 

 at lower than body temperature and favors survival. Sterility occurs in rodents, 



