PHYSIOLOGICAL 359 



change in body weight, and so on, for from any one the others can 

 be calculated more or less accurately. But most emphasis is placed 

 on the production of heat, as the most direct measurement of energy 

 output and basal metabolism is usually expressed in terms of heat 

 production. The unit is the Calorie, which is the amount of heat 

 required to raise the temperature of a kilogram (nearly a quart) 

 of water from 15° Centigrade to 16°, or, roughly, a third of an ounce 

 of water from freezing-point to boiling-point. An average figure for 

 the basal metaboHsm of a man is 2,000 Calories a day, which repre- 

 sents an output of energy of about one-seventh of a horse-power. 

 If really hard muscular work is done, the daily output of energy 

 may rise to three times that figure or more. 



Naturally the figure of the basal metabolism varies a great deal 

 from one person to another; it bears some relation to the weight of 

 the body, but more to the area of the surface of the body — not, 

 unfortunately, very easy to determine directly. The reason is that 

 the amount of heat lost from the body depends on the amount of 

 surface. In the same way, when different kinds of animals are 

 compared, it is found that smaller animals have usually a higher 

 relative basal metabolism than large ones, because their surface-to- 

 weight ratio is larger. It is not always easy to obtain good figures 

 for animals, however, both because it is not always possible to get 

 them to rest during the experiment, and also because cold-blooded 

 animals have not such a delicately poised physiological balance as 

 mammals and birds; in their case basal metaboUsm has not such a 

 definite, reproducible value, but may depend on their state of 

 nutrition and so forth, so that only average values can be obtained. 



In basal metabolism determinations the subject of the experiment 

 is at rest, and starving. But by simple variations, using the same 

 method, it is possible to determine the cost of various sorts of work, 

 including the work done in keeping warm at various temperatures 

 of the air, the work done in maintaining the raised temperature of 

 fevers, the work done in digestion, and the cost, in terms of work, 

 of growth. The value of different kinds of food as sources of energy, 

 and the effect of stimulants and other drugs, may also be investigated 

 in this way. 



The chief function of the thyroid gland is to regulate the level of 

 metabolism, and whenever a disorder of this gland is suspected, so 

 that it may be functioning too much or too little, basal metabolism 

 determinations are of great value in identifying the condition and 

 watching the progress of the cure. When the active principle of the 

 gland (thyroxin) was prepared synthetically, one of the first tests 

 applied to it was to see whether it raised the basal metabolism of 

 a healthy individual as markedly as an extract from the natural 

 gland does. 



