198 PHYSIOLOGY FOR DENTAL STUDENTS. 



hydrogen and carbon of the foodstuffs to form water and carbon 

 dioxide gas. 



Oxidation in the Tissues. The actual mechanism which 

 unites the oxygen with the carbon and hydrogen of the food- 

 stuffs within the tissue cells, is not entirely known. In spite of 

 the fact that the processes of combustion of hydrocarbon matter 

 outside the body yields the same end products as the oxidations 

 taking place within it, the two processes are not strictly analo- 

 gous. An important point of difference lies in the fact that the 

 intracellular materials fats, proteins, and carbohydrates are 

 oxidized with relatively great rapidity at low temperatures 

 (88), whereas the same reactions outside the body require a 

 very high temperature. 



Let us take as an example the cell of the yeast plant, in which 

 there is a substance, under the influence of which, the sugar 

 molecule becomes split up, at a temperature below that of the 

 body, to produce carbon dioxide and water. Similar substances 

 are present in the tissue cells of plants and animals; they are 

 the ferments or enzymes (see p. 34), and they act as catalytic 

 agents. The function of these bodies is to increase the velocity 

 of many chemical reactions which otherwise proceed so slowly 

 that they may be said in some cases not to exist. A class of 

 these substances is present within the tissue cells, which at the 

 demand of the tissues control the extent and the velocity of the 

 union of oxygen with the hydrocarbons of the food. Such en- 

 zymes are known as oxidases. 



What evidence have we, however, that this oxidation takes 

 place within the tissues and not within the blood itself? It is 

 conceivable that the substances that are to be oxidized are col- 

 lected from the tissues by the blood, and that the oxygen combines 

 with them in this fluid. It is quite possible that some oxidation 

 takes place in the bood, for it is essentially a tissue and has 

 a metabolism of its own, but this is not true for the oxidation 

 which concerns the tissues, since this takes place in the tissues 

 themselves, as can be shown by the following fact : The blood of 

 a frog may be replaced with saline solution, in which oxygen is 

 dissolved under pressure, without killing the animal. It is 



