432 CORRELATION OF PHYSICAL AND VITAL FORCES. 



tionship.* And the concurrence of these independent indica- 

 tions seems to justify the inference that motor force may be 

 developed, like heat, by the metamorphosis of constituents 

 of food which are not converted into living tissue ; an infer- 

 ence 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 improbability. Of the conditions which deter- 

 mine the generation of motor force, on the one hand, from the 

 disintegration of muscular tissue, on the other from the meta- 

 morphosis 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 by the retrograde metamor- 

 phosis of the organic compounds generated by the instru- 

 mentality of the plant, whereby they ultimately return to the 

 simple binary forms (water, carbonic acid, and ammonia), 

 which serve as the essential food of vegetables. Of these 

 organic compounds, one portion (a) is converted into the 



* On these last points reference is especially made to the recent experi- 

 ments of Dr. Edward Smith. 



