i EXCHANGE OF MATERIAL 47 



the inner mechanism of vital activities which was formulated by 

 Liebig, experimentally confirmed by the classical researches of 

 Pfliiger and Hering, and elaborated by Verworn in his hypothesis 

 of biogen ; according to this theory, the most complex molecules 

 constituting the protoplasm of the cells are extremely labile, that 

 is, they are in a constant state of chemical change, undergoing dis- 

 similation and assimilation, being decomposed and oxidised during 

 activity, and reintegrated or built up during rest (Vol. I. Chap. III.). 

 According to this theory, the circulating protein represents the 

 material destined to be partly assimilated and transformed into 

 tissue protein, i.e. living protoplasm, partly stored up as reserve 

 protein, partly transformed into glycogen or fat, and partly 

 oxidised and consumed as a combustible material. Dissimilation 

 or catabolism, equally with assimilation or anabolism, does not 

 depend on the easy oxidation of the dead protein, as was thought 

 by Voit, but upon the extreme mutability of the living protoplasm. 

 In order that it may be elaborated, stored, transformed, or burnt, 

 dead protein must enter the sphere of action of living elements, 

 must, so to speak, pass through the mesh of the tissues. Dead 

 protein, which comes into circulation as the result of protein 

 feeding, will, by stimulating the fundamental trophic activity of 

 the nervous system, accelerate the varied and complex metabolism 

 of the living elements, either for the repair of loss, the increase 

 of reserve protein, the formation of glycogen and fat, or the com- 

 bustion of the surplus. 



It may further be objected that Voit's theory of the com- 

 bustibility of circulating protein presupposes his other and older 

 theory, adopted by Briicke and A. Fick, that only genuine 

 proteins, absorbed and synthetically reconstituted by the intestine 

 and set in circulation, are able to provide for the repair and 

 consumption of the tissues (Vol. II. Chap. V. 6). The latest 

 researches, made after the discovery of erepsin, have proved that 

 the proteolytic process in the intestines does not stop short at the 

 formation of albumose and peptone, but goes on to the formation 

 of amino- acids. Although the circulating protein might be 

 supplied directly by the protein absorbed as such, or indirectly 

 by the protein synthetised from the albumose and peptone, yet 

 observations show that the greater part of the products of the 

 hydrolysis of protein food are absorbed in the form of amino-acids, 

 and are used as such for the repair and functional consumption 

 of the single elements of the tissues. 



If from complex and genuine protein substances, which form 

 a complete nutriment, capable of supporting life without any 

 additions, we pass on to the consideration of the other nutritive 

 substances, the collagens, saccharides, fats, condiments, mineral 

 substances, we find that none of these foods and not even all of 

 them put together can be substituted for protein foods, no matter 



