ANIMAL FOODS AND FEEDING 299 



it in a certain condition. It may be divided somewhat as 

 follows, in the case of man : 



Maintaining body heat and muscular tone . . 40 per cent 



Respiration 20 per cent 



Circulation 10 per cent 



Digestion and assimilation 12 per cent 



Other involuntary muscular work . . . . 18 per cent 



The total amount of energy thus required has been deter- 

 mined in the case of both man and domestic animals. The 

 following may be taken as average results. 



A man of 150 lbs. at rest requires approximately 2000 Cal. 

 per day. 



A steer of iioo lbs. (500 Kg.) requires for maintenance 

 approximately 12,000 Cal. per day. 



All of the energy of the food in excess of this maintenance 

 requirement is utilized by the body in (i) voluntary muscular 

 work, (2) production of body substance (increase in weight). 



Respiration Calorimeter. — Experiments to carefully deter- 

 mine the relation between the energy of the food and the energy 

 utilized by the body in maintenance and in doing work or 

 increasing weight have been carried out in what is known as 

 the respiration calorimeter. This consists of a large box in which 

 a man or animal may live under very accurately controlled 

 conditions such that all food and air used and all excretion 

 products, heat produced and muscular w^ork performed are 

 determined. As a result of these experiments, performed on 

 man in this country by Atwater and Benedict, and on animals 

 by Armsby, and in Europe by Rubner, Zuntz, Kellner and 

 others, it has been fully established that the law of the con- 

 servation of energy applies to the utilization of food in the 

 animal body, i.e. all of the energy of the food actually metab- 

 olized is utilized in one of the ways mentioned. 



Production Value. — The energy that is in excess of the 

 maintenance requirement is thus productive in that it produces 



