PHYSIOLOGICAL OXIDATIONS. 975 



facts indicate that when a sufficient supply of oxygen is provided 

 any further increase has no immediate effect in aiding or hastening 

 the oxidations. The intensity of the process is conditioned by the 

 tissue itself. The initial stimulus or substance that sets going the 

 whole reaction arises within the tissues. The second generalization 

 that seems to be accepted more and more of recent years is that the 

 oxidations of the body, those reactions that give rise to much heat, 

 do not affect the living tissue itself. They take place under the 

 influence of the living matter, or by the aid of substances (enzymes) 

 formed by the living matter, but the material actually burnt is not 

 organized living substance. As the living yeast cells break down 

 sugar in the liquid surrounding them, so the living tissue cells metab- 

 olize and oxidize the dead food material contained in the lymph 

 and tissue liquid in which they are bathed. The opposite point of 

 view was ably advocated by Pfluger. This observer, in fact, ex- 

 plained the mystery of physiological oxidations by assuming that 

 the oxygen together with the food material is synthesized into the 

 highly complex and unstable living molecules. The active intra- 

 molecular movement within these molecules leads constantly to a 

 breaking down, a splitting off of simpler molecules which consti- 

 tute the products of physiological oxidation. The instability of 

 the molecule is due to its size and the activity of the intramolecular 

 movements, or, as Pfluger expressed it, "The intramolecular heat 

 of the cell is its life." This point of view, however, has not found 

 acceptance. It is implied or stated by most recent authors that 

 the food material is attacked and oxidized outside the living 

 molecule, in the form of fat, sugar, protein, or rather in the form 

 of the intermediary products arising in the metabolism of these 

 substances. The tendency for many years has been to show that 

 these processes in the body are chemical changes that do not differ 

 fundamentally from similar processes outside the body. The 

 point of view actually adopted by most workers is that the living 

 matter effects its wonderful changes in the food material by making 

 use of intracellular ferments or enzymes (endo-enzymes).* That 

 such enzymes are formed, one may say, generally in the tissues 

 of the body, has been brought out in the preceding chapters upon 

 Digestion and Nutrition. It is necessary only to recall the facts 

 that lipase, the fat-splitting enzyme, has been isolated from many 

 tissues, and that in the liver and muscles and probably other tissues 

 there exist enzymes capable of converting glycogen to sugar or 

 the reverse, and of oxidizing the sugar completely by the serial 

 action of several intracellular enzymes. Finally, with regard to 

 the protein material, it is now recognized that proteolytic enzymes 

 exist in many, if not all, of the living tissues. This point is demon- 

 * For literature, see Vernon, "Intracellular Enzymes," London, 1908. 



