ORGANIC AND INORGANIC FORCES. 913 



so that their mutual equivalents, when they are transformed, one into 

 the other, can be determined. The introduction of this method into 

 the domain of physiology, necessitates the determination of the quantity 

 of matter undergoing change by oxidation, and of the work performed 

 by it, in the living body. The results, however incomplete, are full of 

 interest and promise. 



In comparing the animal body with a machine having its source of 

 power in combustion or chemical changes, it is usual to make this dis- 

 tinction: in the latter, the force is entirely derived from the combus- 

 tion of substances introduced into the machine, and acts upon parts of 

 the machine, themselves passive in the work; whereas in the former, 

 the parts of the machine not only perform the work, but, to a certain 

 extent, their very matter undergoes the changes by which the force is 

 produced. In the steam engine, the heat and the mechanical work 

 are produced by the direct transformation of fuel distinct from the 

 machine itself; in the animal frame, the warmth and motor force are 

 evolved from the direct transformation of the fluids and solids of the 

 living apparatus. As will be hereafter seen, the quantity of work 

 accomplished, in proportion to the amount of chemical change which 

 takes place, is far greater in the animal body than in the most eco- 

 nomical steam engine. 



But, although the solids or fluids of the animal machine undergo 

 chemical metamorphoses, as the indispensable condition of its action, 

 the waste occasioned by those changes is, necessarily, ultimately sup- 

 plied from the food. If food be taken in excess, as in the luxus con- 

 sumption, it undergoes oxidation in the blood, without passing into 

 tissue; if the quantity be normal, it enters both the blood and the tis- 

 sues, and then becomes oxidized; lastly, if the food be withheld, the 

 blood and the tissues undergo oxidation, they having been themselves 

 derived from previously assimilated food. The food is, in the last 

 resort, the source and measure of the power engendered, as a conse- 

 quence of oxidation in the body. Accordingly, exact ^numerical esti- 

 mates of the work accomplished in the human body, must refer both 

 to the amount of combustible or oxidizable material in the food, and 

 to that of the products of its oxidation found in the excretions. 



The two most obvious forms of work performed in the living human 

 body, are the proper dynamic or mechanical work, and the calorific 

 work. Besides these, however, there are the nutritive work, and the 

 mental work. The mechanical work is nervo-muscular, and is associ- 

 ated with electric work. Some of it is internal, such as that of the 

 respiratory muscular apparatus, especially of the diaphragm and inter- 

 costal muscles, of the organs of circulation, the heart and arteries, and 

 that of the pharynx, oesophagus, stomach, intestines, and other inter- 

 nal muscular organs. Other internal mechanical work is that per- 

 formed by the muscles which maintain the position of the body, by the 

 muscles of mastication, and by those of the organs of speech and sense, 

 and also the tonic contraction of the whole muscular system. A very 

 large part of the mechanical work is, however, ordinarily external, such 

 as is manifested in the movements of locomotion and labor. The pro- 

 portion of the internal to the external work, in a laboring man, is as 



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