3 86 THE POPULAR SCIENCE MONTHLY. 



the material which by its decomposition produces the force for mus- 

 cular work is finally decomposed, with evolution mainly of carbonic 

 acid and water. They differ in their views of the nature of the process 

 and the steps by which these ultimate products are obtained. 



We have here endeavored to show briefly what has been gained in 

 comparatively recent times by the growth of knowledge in regard to 

 the source of muscular power. Let us attempt a brief summary of the 

 main points brought forward in the preceding discussion : 1. The 

 source of muscular energy is in the chemical decomposition of certain 

 substances, which is accompanied with a release of energy. 2. The 

 muscular contraction produces a greatly increased production of car- 

 bonic acid and water, and an increased consumption of oxygen, in the 

 general respiration. To what extent this is due to the mere muscular 

 contraction, to what extent to the influence of muscular exercise on 

 other functions, is difficult to estimate with certainty. 3. The ex- 

 cised muscle, when caused to contract, gives off carbonic acid, and this 

 action is in great part independent of a simultaneous absorption of 

 oxygen. 4. The blood coming from the contracting muscle con- 

 tains more carbonic acid and less oxygen than that coming from the 

 resting muscle, and less oxygen than that coming to the contracting 

 muscle. 5. The ratio of carbonic acid given off to oxygen taken 

 up is increased by muscular exertion. 6. The nitrogen elimination 

 is but slightly increased during muscular exertion. No considerable 

 amount of nitrogenous muscular tissue is consumed. 7. The imme- 

 diate fuel-material is mainly non-nitrogenous and carbohydrate in its 

 character, probably in part at least derived from the muscle-glyco- 

 gen, and perhaps from some other substances stored in some manner 

 in the muscular tissue, possibly also to some extent from sugars con- 

 veyed to the tissues by the blood. 8. It is not certain to what 

 extent this glycogen or other non-nitrogenous fuel-material is derived 

 from nitrogenous or albuminoid material during rest or repose of the 

 muscles, but such an origin, for a portion at least of the fuel-material, 

 has some evidence in its favor. 9. The nature of the decomposi- 

 tion of this fuel-material is as yet an unsettled matter. The older 

 theory of direct oxidation has been to a great extent replaced by the 

 more modern theory of fermentative decomposition, i. e., splitting up 

 by combination with water into simpler products with an accompany- 

 ing release of energy, and this process followed by secondary oxida- 

 tions exerted by the oxygen of the blood. Satisfactory experimental 

 evidence for deciding with respect to these theories as yet fails us. 



In conclusion, it is well, however, to recollect that at best the ques- 

 tions touched upon are but secondary to the more fundamental question 

 upon which no investigation has as yet thrown even the most dim and 

 feeble light, viz., " What is muscular force ? " It seems impossible 

 to conceive how a collection of cells with thin, elastic walls, and filled 

 with a fluid or semi-fluid mass, can contract in such a way as to mani- 



