24 



KNOWLEDGE 



[Nov. 11, 1881. 



OlS j;raiiinip, or at tho rate of OOt grainine per hour. 

 TJK'ri' was thus a positive reduction in the amount of urea 

 oliniiiiated, which wa.s prol>al)iy attributable to the teni- 

 porarv alistincnce from nitro^^enous aliment; since tlie re- 

 sults of subsequent observations carried-on for a much longer 

 period upon men going througli severe exertion upon an 

 ordinary diet (as those made liy Dr. Austin Flint, of New 

 York, lipon Weston, the pedestrian, during a five days' walk 

 of 310 miles), show a slight total increase in the elimination, 

 which is fairly attributable to the general " wear and 

 tear " produced by the excessive strain put upon the 

 machine. There is, then, no foundation whatever for 

 the assumption of Liebig, that evei-y exertion of muscular 

 energy involves the death and disintegration of an equi- 

 valent amount of muscle-substance. 



4. It now appears certain that the chemical change 

 which is the source of Muscular energy occurs in the 

 Muscle itself, not in the stream of Blood that courses 

 through it For the muscular tissue nourishes itself 

 at the expense, not merely of the protoplasmic con- 

 stituents of the food brought to it by the blood-current, 

 but also of the saccharine ; and, in addition, takes in 

 oxygen which the red corpuscles of arterial blood bring to 

 it from the lungs. A sort of explosive mixture is thus 

 formed, which is " fired " (so to speak) by the nerve- 

 discharge ; a certain quantity of the saocharoid being thus 

 caused to unite with the oxygen contained in the tissue, 

 producing at the same time tlie leat which raises the 

 temperature of the muscle, and the motor force exerted in 

 its contraction ; while the carbonic acid, which is the product 

 of this oxidation, together with the residual water,* is 

 conveyed away in the return - current of venous blood. 

 But that this " explosion " (tlie term is only used figura- 

 tively, to indicate the suddenness of the chemical change, 

 and its excitement by nerve-discharge) does not involve 

 the destruction of the tissues, is evident from the con- 

 sideration stated by Mayer, that the amount of work 

 done by the heart (which is capable of very exact 

 measurement) would require, ^if it involved the dis- 

 integration of the muscular tissue, a renewal of it at the 

 rate of « Jieart per vxek. But muscle-substance constitutes 

 no exception to the general rule that every tissue in the 

 body has a tenn of life of its o^ti ; as we see in the rapid 

 ■waste it undergoes when entirely thrown out of use. And 

 it is the need of renovation thence arising, that gives rise 

 to a demand for 7dfro;/etious aliment ; this being used to 

 keep the machine, so to speak, in working order, not to 

 serve as its fuel. 



Again, it was formerly supposed by jihyKiologists that 

 the conversion of ai-terial into venous blood (which chiefly 

 coiisists in the replacement of a portion of its oxygen liy 

 carbonic acid) takes place in the capillary network of the 

 system generally ; but we now know that it goes on at very 

 diverse rates in fliflerent part.s, and varies in tlie same part 

 according to its functional acti\"ity. Now, this \ariation espe- 

 cially shows itself in the blood tliat passes through the 

 Muscular substance ; for when a muscle is at rest, the blood 



♦ The mcc)iarni(U may be regarded as consisting of carbon plus 

 the components of water, so that tho amount of energy (whether 

 manifested in Heat or in Work) produced by their o.^idation, is 

 proportional simply to the quantity of carbonic acid generated. 

 In oleaginous compounds, on tho other hand, the number of oxygen 

 atoms is not nearly ct|ual to that of tho liydrogen atoms ; so that 

 their oxidation generates not only carbonic acid, but also water. 

 It is pretty certain that this motamorpho.sis, like that of saccharoid, 

 takes place in tho substance of tho muscle, since it must be the 

 chief source of energy in carnivorous animals, whoso food contains 

 no saccharoid. But we have no means of distinguisliing tho water 

 thus generated from that which is otherwise present. 



returned by its veins retains almost completely the cha- 

 ract<'r of that lirouglit by its arteries ; whilst, if the muscle 

 l)e thrown into contraction by nervous or electric stimula- 

 tion, the returning bloo<l at once assumes the ordinary 

 venous character — thus showing the dependence of its con- 

 version upon the action of the muscle. We get a corre- 

 sponding result by experimenting on a separated mu.scle ; 

 that of a cold-blooded animal being most suitable, as longes-t 

 retaining its vitality when removed from the body. If the 

 muscle of a Frog, placed in a closed chamber e'xhau.sted of 

 all save w'atery vapour, be repeatedly called into contrac- 

 tion by electric stimulation, the chamber is found, after a 

 time, to contain carbonic acid in a quantity proportionate to 

 the number of such contractions, showing that some compo- 

 nent of the muscle-substance has undergone oxidation ; and 

 since no other product of chemical action is discoverable, it 

 may fairly be concluded that what has been given off from 

 the muscle is part of the saccharoid matter which chemical 

 analysis proves to have been previously stored up in its 

 substance. The union of the saccharoid with the oxygen 

 also stored up in the muscle, produces an amount of energy 

 that can be determined (in the form of units of heat) by the 

 quantity of carlionic acid found in the vessel. This energy, 

 however, may express itself in motion as well as in heat ; 

 and the amounts of both may be determined with consider- 

 able precision — the former as units of work done, the 

 latter as units of /leat by which the tempei-ature of the 

 muscle is raised. Now, if the units of work be turned into 

 their equivalent units of heat, and the two amounts be 

 added together, they give a total so closely accordant with 

 that deduced from the quantity of carbonic acid produced, 

 as to afford the most striking confirmation of Mayer's 

 prediction : — " Convert into heat," he said, '■ the mechanical 

 product yielded by an animal in a given time, add thereto 

 the heat directly produced in the body during the same 

 period, and you will have the total quantity- of heat* which 

 corresponds to the chemical processes." 



To sum up : — The mechanical working of the body of a 

 living animal is as directly dependent as its heating upon 

 the oxidation of the hydiy-carbons of its food ; and these 

 may be most economically supplied by non-nitrogenous sub- 

 stances. On the other hand, the mechanism can oidy be 

 kept in working order by the continual renovation of its 

 substance (its very existence as a living whole involving the 

 continual death and decay of its component parts) ; and for 

 this renovation a supply of proteids is essential, with a cer- 

 tain admixture olfal to serve as material for protoplasm. 



I have tliought it worth while to enter somewhat fully 

 into the particulars of this inquiry, since it aflbrds an 

 excellent example of the truly scientific methods on which 

 Plnsiology is now being studied, and of the value of the 

 results that are being obtained \>y their use. It is by such 

 methods alone that the Physical and Chemical actions 

 taking place within the li\-ing body can be so determined 

 as to give to Physiology that place among the exactsciences 

 which its earlier cultivators could scarcely venture to hope 

 it would ever attain. 



• It would appear from the experiments of Fiok upon frog's 

 muscles, that of the whole energy generated by the oxidation 

 process, about five-sevenths show themselves as lieat, whilst the 

 other two-sevenths do the mechanical work. In the bodies of Man 

 and other warm-blooded animals, however, the ju-oportion of heat to 

 work is ordinarily much larger ; the former constituting about five- 

 sixths of the total energy generated by the oxidation of the food, 

 wliilc the latter is only about one-sixth. Still, considered simply as a 

 contrivance for doing mechanical work, the human body compares 

 favourably with a steam-engine ; the very best form of that machine 

 only exerting about one-eighth of the power which is generated by 

 the combustion of the coalit consumes, the remaining seven-eighths 

 being wasted. 



