20 HOMOIOTHERMISM 



loss. The subcutaneous tissues often cause a tempera- 

 ture lag or thermal gradient, which in a large animal will 

 be several degrees. This factor is of most importance 

 in the homoiotherm but nevertheless must not be con- 

 sidered as a negligible factor in the poikilotherms. 



The heat lost by a body in a unit of time is propor- 

 tional to the difference between its temperature and that 

 of the surrounding medium. A warm body loses heat 

 and becomes colder, while the environment becomes 

 warmer. This transference of heat goes on until the 

 organism and the environment are at the same tempera- 

 ture. The time required for this equilibrium to become 

 established depends mainly upon the gradient between 

 the integument of the animal and the immediately sur- 

 rounding medium. The circulation of the blood tends 

 to make the gradient greater and consequently a living 

 poikilotherm will come more quickly to the temperature 

 of its surroundings than will a dead one. This we have 

 shown to be true of a turtle immersed in a water bath of 

 constant temperature. The temperature was taken by 

 a thermocouple placed in the coelomic cavity very near 

 the center of the body. The surrounding water was kept 

 in rapid circulation and the turtle was held stationary. 

 The turtle weighed 278 grams and at the beginning of 

 the experiment was transferred from a water bath which 

 was 10 deg. C. higher than the experimental bath. The 

 absolute temperatures were read with a standard mer- 

 curial thermometer calibrated to one-hundredth of a 

 degree centigrade. The temperature differences were 

 determined with copper-constantan thermo-couples. 

 After one determination was made of the rate of heat 

 transference from the living turtle to the water bath, 

 the experiment was repeated with the same turtle killed 

 by ether anesthesia. The experiment was then reversed 



