THERMOTAXIS 673 



culation through the skin. This greater cutaneous circulation means 

 the permanent withdrawal of blood from the rest of the body, which 

 c.m be compensated by a corresponding increase in the total circulating 

 mass. The fact that the blood contained in the living skin must 

 form a substantial fraction of the total blood, and a fraction rela- 

 tively more important in the smaller than in the larger animals, 

 would of itself help to establish a relation between surface area 

 and total quantity of blood. The greater volume of the blood 

 will contribute to the increased output of the heart in the smaller 

 animals. In this connection the fact already alluded to more than 

 once may be again emphasized, that it is not the quantity of blood con- 

 tained in an organ or an organism, but the quantity passing through 

 the capillaries in a given time, which is the important thing for its 

 function, in the case of the lungs for the function of the gaseous ex- 

 change, in the case of the skin for the regulation of the heat-loss. 



There is reason to suppose that the average amount of blood contained 

 in the cutaneous vessels, as well as the average amount flowing through 

 them per unit of time, is greater in proportion to the total area of the 

 skin in animals like man, with a skin devoid of fur and well supplied 

 with sweat glands, than in animals well protected by hair and with 

 few sweat glands. For in the latter the cutaneous circulation cannot 

 take as great a share in the regulation of the heat-loss as in the former, 

 although, of course, the nutrition of the hair itself requires a certain 

 supply of blood. The relatively insignificant proportion of the total 

 blood usually assigned to the skin of such an animal as the rabbit 

 (p. 56) probably gives quite an erroneous idea of the proportion in 

 the skin of a living man. It must be remembered that such determin- 

 ations as have been carried out have been made on dead animals whose 

 skin under ordinary conditions contains less blood than during life. 



Rubner has found that animals abundantly fed do not show so much 

 change in the production of heat when the external temperature is 

 varied as starving animate, perhaps because the thicker coat of sub- 

 cutaneous fat so steadies the rate at which heat is lost that it becomes 

 easy for the vaso-motor mechanism alone to hold the balance between 

 loss and production. In well-fed animals it is the heat-loss which is 

 chiefly affected, and it may be that this has something to do with the 

 explanation of Loewy's results on man (p. 669). 



Lorrain Smith discovered the interesting fact that after removal of 

 the thyroid glands (in cats), the heat-production, as measured by the 

 amount of carbon dioxide given off, is more sensitive to changes of 

 external temperature than in the normal animal. 



Effect of Excessive Loss of Heat- Varnishing of the Skin. It 

 must not be imagined that the production of heat can be in- 

 creased indefinitely to meet an increased heat-loss. The organism 

 can make considerable efforts to protect itself, but the loss of heat may 

 easily become so great that the increase of metabolism fails to keep 

 pace with it. The internal temperature then falls, and if the fall be 

 not checked, the animal dies. A mammal, when cooled artificially 

 to the temperature of an ordinary room (15 to 20 C.), does not recover 

 of itself, but may be revived by the employment of artificial respiration 

 and hot baths, even when the rectal temperature has sunk to 5 to 

 10 C. If the skin of a rabbit be varnished, and the air which it is 

 the function of the fur to maintain at rest around it be thus expelled, 

 the animal dies of cold, unless the loss of heat is artificially prevented. 

 If, without varnishing at all, the greater portion of the skin of a rabbit 

 or guinea-pig be closely clipped or shaved, similar phenomena are 

 observed. Prevented from covering itself with straw, the animal dies, 



43 



