8 August, 1907.] Elements of Animal Physiology. 463 



sluggish on a cold one. The vitality of such a creature is constantly 

 changing with the weather and the animal has but feeble powers of 

 resisting extremes of temperature. A homoiothermal animal on the other 

 hand can remain uniformly active despite extensive changes in climate 

 or season and is not compelled on the onset of cold to abate one jot of its 

 activity ; on the contrary, for a reason we shall shortly see, it is actually 

 stimulated by cold to more energetic movements. 



The temperature of a mammal or bird is never absolutely constant ; 

 it shows distinct oscillations, but these oscillations are small and within 

 fairly fixed degrees of temperature. The temperature of a man at 6 

 o'clock in the evening is generally about if degrees* F. higher than that 

 taken shortly after midnight, but the average temperature remains remark- 

 ably constant. When we speak therefore of the normal temperature of a 

 man or any other mammal we mean a temperature which lies well within 

 the daily range of oscillation in health. Thus the normal temperature of 

 a man is given as 98.4 F. in England, but as 98.9 F. in Germany; 

 both are right for both are within the daily range of rise and fall. 



Another difficulty in giving a fixed value to the temperature of a homoio- 

 thermal animal is the fact that all parts of the body of the same animal 

 have not the same temperature. Stated generally the more central organs 

 are warmer than those nearer the surface, the maximum being found in 

 the blood flowing from the fiver. As a rule the readings of a thermometer 

 placed in the rectum may be accepted as giving a good idea of the internal 

 temperature of an animal. The following values may be taken as approxi- 

 mately correct for the normal rectal temperatures of various domestic 



101.7 F. 



107 — 109 



108 — no 



107 



109 



It is interesting to note that the temperature of most homoiothermal 

 animals is onlv a little l>e]ow that temperature at which the enzymes act 

 most rapidly ; this is doubtless the chief reason why we find such a 

 temperature in the highest evolved animals. 



As man and most domestic animals belong to the homoiothermal group 

 attention mav be confined exclusively to this class and the methods by 

 which a practically constant temperature is maintained, may be discussed 

 under the headings, the source of heat, the distribution of heat, the loss of 

 heat, and finally the physiological variations in an animal's temperature. 



The Source of Heat. — In every living cell of the body heat is pro- 

 duced, but the amounts formed in the smaller organs, such as the salivary 

 glands, may be very minute and scarcely detectable. It is to the skeletal 

 muscles that we must look for the main source of animal heat. In an 

 animal apparently quite at rest the breathing muscles are active and 

 moveover the other skeletal muscles are mostly in a state of tautness or 

 "tone" which implies chemical change and heat production. During 

 violent exercise the heat produced mav be so great that the animal cannot 

 get rid of it quick enough and a passing rise in temperature may be noted. 

 On the other hand if an animal is drugged with chloroform, ether, or 

 alcohol, muscle tone is greatly diminished and the animal's temperature 

 wall fall unless the surrounding air be kept sufficiently warm. An animal 



* Throughout this chapter temperature is measured in degrees Fahrenheit, as this 

 system, for some inexplicable reason, is more popular in English-speaking countries. 



