SOURCES OF HEAT AND WORK. 919 



cular and nervous substance ; that so long as muscular contractility lasts, 

 so long do oxidation changes go on in a muscle (G. Liebig) ; that a due 

 supply of oxygenated blood is necessary for the continuance of this 

 contractility ; and that the quantity of carbonic acid contained in the 

 venous blood returning from muscles, is in direct proportion to their 

 activity. It is further certain that the muscular and nervous tissues 

 must be largely supplied by the nitrogenous or albuminoid portion of 

 the food. From these facts, it might well be inferred, that the mechan- 

 ical or nervo-muscular work of the body, has its immediate source in 

 the transformation and oxidation of the muscle itself, and, therefore, 

 in the so-called histogenetic, plastic, or flesh-forming nitrogenous food. 

 The opinions and practice of agriculturists, railway contractors, and 

 trainers of men destined for athletic sports, further indicate, that a 

 proportional increase in the quantity of flesh-forming food, is believed 

 to be necessary for animals or men engaged in severe or protracted 

 labor. 



The teaching of Liebig, on these points, is indeed very precise and decided. 

 According to him, the hydrocarbons and carbhydrates are the exclusive heat- 

 formers ; whilst the sole source of mechanical power, is the oxidation of the 

 nitrogenous substance of the muscles and nerves, built up again by the albu- 

 minoid or plastic constituents of the food. These views have been very gener- 

 ally accepted, and have been especially supported, amongst others, by Kanke, 

 Draper, Playfair, and Odling. Draper says of muscular contraction, that it 

 may be supposed to be due to disintegration of the sarcous particles, and that 

 the transformation of muscle by oxidation, may be the condition of muscular 

 action. Odling regards the combustion of the carbon and hydrogen of fat, as 

 liberating a force exhibited solely in the form of heat ; whilst the combustion 

 of an equal quantity of the carbon and hydrogen of voluntary muscle, is ex- 

 pressed chiefly in the form of motion. 



Playfair has endeavored to show, on numerical grounds, that although the 

 chemical combination of the carbon and hydrogen of the albuminoid food, with 

 oxygen, is insufficient to account for the calorific work, yet it is adequate to 

 produce the mechanical work of the body. Hence he concludes that it is the 

 ultimate magazine of force, for the production of its dynamical operations. 

 The following are the principal facts and arguments to which he directs atten- 

 tion. 



From numerous English and foreign sources, he has collected a series of die- 

 taries actually in use, under various conditions of rest or work, of which the 

 annexed Table A, gives only the mean results. The starch equivalent includes 

 the actual starch of the food, together with the carbon and hydrogen of the 

 fat, expressed as starchy matter, 1 part of fat being considered equal to 2.4 

 parts of starch. The subsistence diet, is that which is considered necessary to 

 support life in a condition of rest, or the diet necessary for the vital mechan- 

 ical work of the body ; it is illustrated by the convalescent diet of hospitals, 

 and by the low diets of ill-fed persons. The diet needful for active employment 

 in health, is represented by the diet of soldiers during peace. An improved 

 diet necessary for more arduous work, is that given to soldiers during war. 

 The diet of active laborers, is exemplified in that of the corps of Royal Engi- 

 neers engaged in civil employment. Lastly, a still fuller diet, is that of laborers 

 and others employed in yet more continuous and heavy work. 



