656 ANIMAL HEAT 



The differential micro -calorimeter of A. V. Hill is the most accurate 

 apparatus for measuring very small quantities of heat e.g., the heat 

 given off by resting muscles, or by muscles during the process of heat 

 rigor, or in such reactions as the souring of milk by the lactic acid 

 bacillus. It consists of a pair of Dewar flasks (ordinary thermos 01 

 vacuum bottles), in which loss of heat to the surroundings is greatly 

 hindered by exhausting the air from the space enclosed by the double 

 wall of the flask. The bottles are packed in sawdust in cylindrical tins. 

 The tissue or liquid experimented with is put into one flask, and water 

 into the other (control) flask, and the amount of the water is adjusted 

 so that the change of temperature of each flask due to conduction and 

 radiation to or from the outside is the same. Loss of heat is thus 

 automatically eliminated, and from the excess of temperature of the 

 experimental over that of the control flask the heat production in the 

 former can be calculated. The temperatures are measured by thermo- 

 electric junctions of an alloy called constantan and copper, one junction 

 being in each flask. To economize time in making such observations, 

 a number of pairs of flasks can be simultaneously employed. The 

 apparatus is so sensitive that a liberation in ten hours of 4 gramme- 

 calorie per gramme of contents of the flask can be followed and esti- 

 mated with an error not exceeding 3 per cent. 



Body-Temperature. All the higher animals (mammals and birds) 

 have a practically constant internal temperature (fowl 41 to 44 C., 

 mouse 37 to 38, dog 38 to 39, man 37 in the rectum), but a few 

 hibernating mammals, such as the marmot, are homoiothermal in 

 summer, poikilothermal during their winter sleep. In the lower 

 forms the body-temperature follows closely the temperature of the 

 environment, and is never very much above it (frog 0-5 to 2 above 

 external temperature). Both in a frog and in a pigeon heat is 

 evolved as long as life lasts ; but per unit of weight the amphibian 

 produces far less than the bird, and loses far more readily what it 

 does produce. The temperature of the frog may be 25 C. in June 

 and 5 in January. The structure of its tissues is unaltered, and 

 their vitality unimpaired by such violent fluctuations. But it is 

 necessary, not only for health, but even for life, that the internal 

 temperature (the temperature of the blood) of a man should vary 

 only within relatively narrow limits around the mean of 37 to 

 38 C. 



Why it is that a comparatively high temperature should be 

 needed for the full physiological activity of the tissues of a mammal, 

 while the, in many respects, similar tissues of a fish work perfectly, 

 although perhaps more sluggishly, at a much lower temperature, is 

 not quite clear. Nor do we know the precise significance of that 

 relative constancy of temperature in the warm-blooded animal, 

 which is as important and peculiar as its absolute height. The 

 higher animals must possess a superior delicacy of organization, 

 hardly revealed by structure, which makes it necessary that they 

 should be shielded from the shocks and jars of varying temperature 

 that less highly endowed organisms endure with impunity. Leaving 

 the discussion of the local differences and periodic variations of the 



