434 HANDBOOK OF PHYSIOLOGY. 



merely with the slight variations in the amount of capital (weight of 

 body) to which the healthiest subject is liable. 



The chemical composition of the food (p. 209) may be profitably com- 

 pared with that of the excreta, as before mentioned. The greater part 

 of our food is composed of matter which contains much potential energy; 

 and in the chemical changes (combustion and other processes) to which 

 it is subject in the body, active energy is manifested. 



3. The Sources and Objects of Expenditure. The sources of 

 the necessary waste and expenditure in the living body are various and 

 extensive. They may be comprehended under the following heads: 



(1) Commoji wear and tear; such as that to which all structures, liv- 

 ing and not living, are subjected by exposure and work; but which must 

 be especially large in the soft and easily decaying structures of an animal 

 body. 



(2) Manifestations of Force in the form either of Heat or Motion. In 

 the former case (Heat), the combustion must be sufficient to maintain a 

 temperature of about 100 F. (37.8 0.) throughout the whole substance 

 of the body, in all varieties of external temperature, notwithstanding the 

 large amount continually lost in the ways previously enumerated. In 

 the case of Motion, there is the expenditure involved in the (a) Ordinary 

 muscular movements, as in Prehension, Mastication, Locomotion, and 

 numberless other ways: as well as in (b) Various involuntary movements, 

 as in Respiration, Circulation, Digestion, etc. 



(3) Manifestation of Nerve Force; as in the general regulation of all 

 physiological processes, e. g., Respiration, Circulation, Digestion; and in 

 Volition and all other manifestations of cerebral activity. 



(4) The energy expended in all physiological processes, e.g., Nutrition, 

 Secretion, Growth, and the like. 



The total expenditure or total manifestation of energy by an animal 

 body can be measured, with fair accuracy; the terms used being such as 

 are employed in connection with other than vital operations. All state- 

 ments, however, must be considered for the present approximate only, 

 and especially is this the case with respect to the comparative share of 

 expenditure to be assigned to the various objects just enumerated. 



The amount of energy daily manifested by the adult human body in 

 (a) the maintenance of its temperature; (b) in internal mechanical work, 

 as in the movements of the respiratory muscles, the heart, etc. ; and (c) 

 in external mechanical work, as in locomotion and all other voluntary 

 movements, has been reckoned at about 3,400 foot-tons. Of this amount 

 only one-tenth is directly expended in internal and external mechanical 

 work; the remainder being employed in the maintenance of the body's 

 heat. The latter amount represents the heat which would be required 

 to raise 48.4 Ib. of water from the freezing to the boiling point; or if 



