ANIMAL HEAT 583 



In any case it must be carefully remembered that the question 

 of the minimum amount of protein necessary in a permanent diet 

 is quite distinct from the question of the minimum heat value of the 

 diet for a man of given body- weight doing a definite amount of work 

 under definite conditions. Whether the protein allowance be scanty 

 or liberal, the total heat value cannot be permanently reduced below 

 a certain minimum depending on the work done, the climate, and 

 other conditions. McCay points out that while Bengalis subsist on 

 food containing only about one-third the amount of protein in such 

 a ' standard ' diet as Voit's, and may therefore be supposed to be 

 immune from the dangers of an excessive protein metabolism, the 

 large intake of carbo-hydrate rendered necessary by the poverty of 

 the food in protein is associated with perhaps greater evils, among 

 them a marked predisposition to diabetes and renal troubles. Their 

 weight, chest measurement, and muscular development are inferior 

 to those of other Asiatics living in the same climate but with dietetic 

 habits which ensure them a larger supply of protein. 



The Seats of Heat-production. We have already recognised 

 the skeletal muscles as important seats of heat -production. A 

 frog's muscle, - contracting under the most favourable con- 

 ditions, does not convert at most more than one-fourth or 

 one-fifth of the energy it expends into mechanical work ; at 

 least three-fourths or four-fifths of the energy appears as heat. 

 The muscles of mammals and of man in the intact body work, 

 upon the whole, more economically than the excised frog's 

 muscles at their maximum efficiency. Under the best con- 

 ditions that is, when the work is moderate and not too rapidly 

 done about one-third of the chemical energy expended may be 

 transformed into mechanical work, and only two-thirds into heat 

 (Zuntz). In hard work three-quarters of the energy may be 

 changed into heat ; but even then the efficiency of the muscles 

 far outstrips that of the best steam-engines, which convert only 

 an eighth of the total energy into work. 



Notwithstanding the splendid efficiency of the muscular 

 machine, the gaseous metabolism easily rises during muscular 

 work to five times, and in severe labour to nine times its resting 

 value, although persons inured to toil work more economically 

 than amateurs. In one of Atwater's ' severe work ' experi- 

 ments the work done in twenty-four hours had a heat-equivalent 

 of 1,482 calories (equal to over 630,000 kilogramme-metres). The 

 total heat-production (including the equivalent of the work) 

 was 9,314 calories. It is not difficult to show that the greater 

 part of the metabolism and heat-production of a man doing 

 ordinary work is accounted for by the contraction of the voluntary 

 and involuntary muscles. 



Even in muscles completely at rest metabolism goes on, and some 

 heat is produced. By analyzing the gases of the arterial and venous 

 blood Zuntz compared the oxygen consumption and carbon dioxide 

 production in the hind-legs of dogs when the sciatic and anterior 



