Cti. XL.] REGULATION OF TEMPERATURE 663 



to Rubner in particular that we owe the experimental demonstration of the law of 

 the conservation of energy in the living organism. 



The various tissues of the living body in the performance of their several 

 functions break down and oxidise the proteids, fats, carbohydrates, and other 

 materials of which they are composed, and seize upon the energy previously stored 

 and thus liberated, converting it here into the invisible molecular motion of heat, 

 there into the motion of visible masses of matter in the performance of work, and 

 again into the energy of chemical change as the needs of the organism demand. 

 Of these the liberation of heat is by far the greatest in amount, and for this reason, 

 as well as to simplify calculations, it Jias become customary to express the available 

 energy in terms of units of heat. The energy expended as work may be divided 

 into (1) external work, i.e. , the work done on masses outside the body ; and (2) internal 

 work, i.e., the physical and chemical changes produced within the body in the 

 processes of breathing, digestion, circulation, and the like. 



If the law of the conservation of energy applies to the animal organism, the 

 following are its necessary consequences : 



1. If an animal is doing no external work, and is neither gaining nSr losing 

 substance, the potential energy of the food (expressed as its heat of combustion} 

 will be equal to that of the excreta, plus that given off as heat, plus that of internal 

 M'ork. 



2. If an animal is doing external work, and is neither gaining nor losing 

 substance, the potential energy of the food will be equal to the potential energy of 

 the excreta, plus that given off as heat, plus that of the internal work, plus that of 

 the external work. 



3. If an animal is doing no external work, but gaining or losing body substance, 

 the potential energy of the food will equal the potential energy of the excreta, plus 

 that given off as heat, plus that of the internal work, plus that of the gain by the 

 body-substance (a loss by the body being regarded as a negative gain). 



4. In an animal doing external work, and gaining or losing body-substance, 

 the potential energy of the food will equal the potential energy of the excreta, plus 

 that given off as heat, plus that of the internal and external work, plus that of the 

 gain (positive or negative) of the body-substance. 



In actual experimentation it is practically impossible to adjust the food so that 

 there is no gain or loss of body-substance, hence experiments necessarily fall under 

 (3) or (4) ; and the majority under (3). 



The quantities to be determined, then, are : 



i. Potential energy of the food. 



ii. Potential energy of excreta (faeces, urine, etc). 



iii. The heat produced (including that into which any mechanical work is 

 converted). 



iv. The potential energy of the gain or loss of body-substance. 



If, then, the equality stated under (3) and (4) is found to exist, we shall be 

 justified in the conclusion that the law of the conservation of energy applies to the 

 animal body. This is what the painstaking work of Rubner, Laulanie, Atwater, 

 and others has succeeded in showing is actually the case. 



Regulation of the Temperature of Warm-blooded Animals. 



We have seen that heat is produced by combustion processes, 

 and lost in various ways. In order to maintain a normal tempera- 

 ture, both sides of the balance-sheet must be equal. This equalisation 

 may be produced by the production of heat adapting itself to 

 variations in discharge, or by the discharge of heat adapting itself 

 to variations in production, or lastly, and more probably, both sets 

 of processes may adapt themselves mutually to one another. We 

 have, therefore, to consider regulation (1) by variations in loss, and 

 (2) by variations in production. 



