TEST OF ANIMAL MOTOR FOKOE. 431 



expense of which the animal organism is constructed ; for 

 besides supplying the material of the tissues, a portion of 

 that food (as already shown) becomes the source, in its retro 

 grade metamorphosis, of the production of the heat which 

 supplies the constructive power, whilst another portion may 

 afford, by a like descent, a yet more direct supply of organ 

 izing force. And thus we find in the action of solar light 

 and heat upon plants whereby they are enabled not merely 

 to extend themselves almost without limit, but also to accu 

 mulate in their substance a store of organic compounds for 

 the consumption 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 disinte 

 gration of a certain equivalent amount of tissue, really ex 

 presses the whole truth. It has been maintained, 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 azo- 

 tized 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 relation (as affirmed by 

 Liebig) between the amount of motor force produced and the 

 amount of disintegration of muscular tissue represented 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 phenomena there is a most intimate rela- 



* This view has been expressed to the author by two very high author- 

 ities, Prof. Helnaholtz and Prof. William Thomson, independently of each 

 other, as an almost necessary inference from the data furnished by tha 

 experiments of Dr. Joule. 



