266 Original Articles. | April, 
merely to extend themselves almost without limit, but also to accu- 
mulate in their substance a store of Organic Compounds for the con- 
sumption of animals—the ultimate source not only of the materials 
required by animals for their nutrition, but also of the forces of various 
kinds which these exert. 
Recent investigations have rendered it doubtful, however, whether 
the doctrine that every exertion of the functional power of the nervo- 
muscular apparatus involves the disintegration of a certain equivalent 
amount of tissue, really expresses the whole truth. It has been main- 
tained, on the basis of carefully conducted experiments, in the first 
place, that the amount of work done by an animal may be greater than 
can be accounted for by the ultimate metamorphosis of the azotized 
constituents of its food, their mechanical equivalent being estimated by 
the heat producible by the combustion of the carbon and oxygen which 
they contain ;* and secondly, that whilst there is not a constant re- 
lation (as affirmed by Liebig) between the amount of motor force 
produced and the amount of disintegration of muscular tissue repre- 
sented by the appearance of urea in the urine, such a constant relation 
does exist between the development of motor force and the increase of 
carbonic acid in the expired air, as shows that between these two phe- 
nomena there is a most intimate relationship.| And the concurrence 
of these independent indications seems to justify the inference that 
motor force may be developed, like Heat, by the metamorphosis of con- 
stituents of food which are not converted into living tissue ;—an in- 
ference which so fully harmonizes with the doctrine of the direct 
convertibility of these two forces, now established as one of the surest 
results of Physical investigation, as to have in itself no inherent im- 
probability. Of the conditions which determine the generation of 
motor force, on the one hand, from the disintegration of muscular 
tissue, on the other from the metamorphosis of the components of the 
food, nothing definite can at present be stated ; but we seem to have a 
typical example of the former in the parturient action of the Uterus, 
whose muscular substance, built up for this one effort, forthwith 
undergoes a rapid retrograde metamorphosis ; whilst it can scarcely 
be regarded as improbable that the constant activity of the Heart 
and of the Respiratory muscles, which gives them no opportunity of 
renovation by rest, is sustained not so much by the continual renewal 
of their substance (of which renewal there is no histological evidence 
whatever) as by a metamorphosis of matters external to themselves, 
supplying a force which is manifested through their instrumentality. 
To sum up: The Life of Man, or of any of the higher Animals, 
essentially consists in the manifestation of Forces of various kinds, of 
which the organism is the instrument ; and these Forces are developed 
* This view has been expressed to the author by two very high authorities, 
Prof. Helmholtz and Prof. William Thomson, independently of each other, as 
an almost necessary inference from the data furnished by the experiments of 
Dr. Joule. 
+ On these last points reference is especially made to the recent experiments 
of Dr. Edward Smith. 
