METABOLISM. 77 



of the muscular fibre : an interpretation supported by cases in 

 which small rises and falls of temperature cause alternating 

 isomeric changes ; as instance Mensel's salt. 



Ending here this exposition, somewhat too speculative and 

 running into details inappropriate to a work of this kind, it 

 suffices to note the most general facts concerning metabolism. 

 Regarded as a whole it includes, in the first place, those 

 anabolic or building-up processes specially characterizing 

 plants, during which the impacts of ethereal undulations are 

 stored up in compound molecules of unstable kinds; and it 

 includes, in the second place, those katabolic or tumbling- 

 down changes specially characterizing animals, during which 

 this accumulated molecular motion (contained in the food 

 directly or indirectly supplied by plants), is in large measure 

 changed into those molar motions constituting animal activi- 

 ties. There are multitudinous metabolic changes of minor 

 kinds which are ancillary to these many katabolic changes 

 in plants and many anabolic changes in animals but these 

 are the essential ones.* 



* Before leaving the topic let me remark that the doctrine of metabolism 

 is at present in its inchoate stage, and that the prevailing conclusions should 

 be held tentatively. As showing this need an anomalous fact may be named. 

 It was long held that gelatine is of small value as food, and though it is now 

 recognized as valuable because serving the same purposes as fats and carbo- 

 hydrates, it is still held to be valueless for structural purposes (save for some 

 inactive tissue) ; and this estimate agrees with the fact that it is a relatively 

 stable nitrogenous compound, and therefore unfit for those functions per- 

 formed by unstable nitrogenous compounds in the muscular and other tissues. 

 But if this is true, it seems a necessary implication that such substances as 

 hair, wool, feathers, and all dermal growths chemically akin to gelatine, and 

 even more stable, ought to be equally innutritive or more innutritive. In that 

 case, however, what are we to say of the larva of the clothes-moth, which 

 subsists exclusively on one or other of these substances, and out of it forms 

 all those unstable nitrogenous compounds needful for carrying on its life 

 and developing its tissues ? Or again, how are we to understand the nutri- 

 tion of the book-worm, which, in the time-stained leaves through which it 

 burrows, finds no proteid save that contained in the dried-up size, which is a 

 form of gelatine; or, once more, in what form is the requisite amount of 

 nitrogenous substance obtained by the coleopterous larva which eats holes in 

 wood a century old ? 



