336 Food as a Motive Poiver. [My, 



thirty-five times as expensive as coal, and it is therefore not won- 

 derful that human labour cannot compete in economy with that 

 of the steam-engine. Food is, indeed, as Donders remarks, a very 

 dear form of fuel. 



TTe have now come to a question which is infinitely more 

 complex than the first, and which embraces the most important cf 

 physiological problems. "We may suppose ourselves acquainted 

 with the composition and force-value of the food, and we have next 

 to inquire what changes it undergoes and what are its functions in 

 the organism. A complete discussion of this subject would be an 

 elaborate treatise on physiology, and even so would unfortunately 

 afford but a very imperfect answer to the questions. But a few 

 points must be briefly glanced at, and they will serve as an intro- 

 duction to some recently acquired theories and facts of great 

 practical importance. In the first place it is essential to notice that 

 all the food which plays any important part in the body is first 

 converted into blood. The processes of digestion, absorption, and 

 sanguification have this for their object, and it is unnecessary to 

 consider them in detail here. It is in the blood that our inquiry 

 practically commences. That the blood consists of two parts, 

 corpuscles and hquor-sanguinis, is fa m iliar to everybody, and it 

 has been clearly demonstrated that the corpuscles are the main 

 agents in animal oxidation, taking up oxygen in the lungs and 

 giving it out again in the course of the circulation. Much of the 

 oxidation so effected undoubtedly takes place in the blood, and to 

 this oxidation is attributed by all physiologists a great part of the 

 heat of the body. According to Liebig, it is non-nitrogenous sub- 

 stances only, substances derived from the starch, fat, and sugar of 

 the food, which are oxidized in this way, and no work, but only 

 heat can result from the oxidation. To the nitrogenous compounds 

 of the blood, the albumen, fibrin, &c, he assigned the exclusive 

 function of nourishing, or, as it were, repairing the solid tissues 

 which are undoubtedly subject to incessant disintegration and decay 

 as a condition of their life. He accordingly divided the constituents 

 of food into respiratory and plastic elements, or, to use a simpler 

 nomenclature, into a heat-givers " and " fiesh-formers." This brilliant 

 generalization has been adopted by the great majority of physio- 

 logists. There can be no doubt that nutritive matter does pass out 

 through the thin walls of the capillaries and irrigate the neigh- 

 bouring tissues. The cells, or elementary parts, of which a tissue 

 consists, develop and grow at the expense of this nutritive fluid. 

 When the cells have completed their term of life they decompose 

 into simpler forms of matter, and pass back, together with the 

 excess of the nutritive fluid, into the blood. This restoration to the 

 blood is no doubt chiefly effected by the agency of the lymphatics, 

 which take their origin, as recent anatomical research has shown, 



