ANIMAL HEAT 3 f$ 



of animals under cover. Other causes of variation in tempera- 

 ture will be considered presently. The thermometer does not 

 tell us the amount of heat formed in the body, it only indicates 

 the outcome of a difference between the heat produced and 

 the heat lost. These important points must now be studied. 



Heat Production. — The oxidations occurring in tissues, and 

 leading to the production of heat, have previously engaged our 

 attention ; the bulk of these changes occur in the skeletal muscles, 

 in which four-fifths of the daily heat are generated, and in 

 active glands such as the liver. The heat furnished by glandular 

 activity is amply demonstrated in the liver, though certainly 

 not in all secreting glands. The temperature of the blood in 

 the hepatic veins is higher than in the portal, higher even than 

 in the aorta. It was shown by Bernard that in the dog, while 

 the portal vein was registering 39-6° C. (103-5° F.), the blood 

 in the hepatic veins was 41-2° C. (106*3° F.). Heat is formed 

 during muscular contraction. Experiments carried out on the 

 external masseter muscle of the horse showed that during con- 

 tractions the thermometer registered 2-8° C. (5-0° F.) higher 

 than in the same muscle at rest. As the blood streams out of 

 the muscle its temperature is higher than that in the corre- 

 sponding artery, and in this way the whole mass of blood would 

 have its temperature raised were it not for mechanisms by which 

 the heat is dissipated. But the excessive production of heat is 

 not always compensated by a sufficiently rapid loss, and a high 

 temperature may in consequence be produced as the result. 

 This is a most important point in connection with working 

 horses. In the case of man a rise of i° to 1-5° C. in body tem- 

 perature may occur as the result of work. In the horse half 

 an hour's trotting may raise the temperature from 0-4° to 1-4° C. 

 (07° to 27° F.) above the normal ; the amount of rise is largely 

 a question of ' condition '; temperatures of 40° to 41° C. (104° to 

 105° F.), and even higher, after hard work, especially in a hot 

 sun, are not uncommon. With rest the temperature falls in 

 the course of a few hours, the mechanism for getting rid of 

 heat being able to cope with it ; as heavy a fall as 4 F. in 

 two hours has been recorded (Willis). With animals unfit for 

 work through want of condition the temperature may take longer 

 to fall, or even remain above the normal sufficiently long to be 

 designated febrile ; ' fatigue fever ' is not unknown in man. 

 Fever may be due either to excessive production of heat or 

 defective dissipation. In the above case it is probable that both 

 factors are at work. 



The act of feeding, which involves increased muscular activity, 

 not only immediately, but subsequently in the muscles of the 

 whole alimentary canal, raises the temperature of the body. 



