CHAPTER XII 



ANIMAL HEAT 



Oxidations. — In dealing with internal respiration on p. 129, 

 we learnt the fundamental fact that the oxidations of the body 

 do not occur in the blood, but in the tissues. By means of these 

 oxidations heat is produced, and the substances which are 

 oxidised — viz., protein, fat, and carbohydrate — have already 

 been studied in the chapters on digestion and nutrition. The 

 manner in which oxidations are carried out in the tissues is not 

 clearly understood — in fact, it is by no means decided how 

 oxidations occur outside the body. The view that oxygen 

 directly unites with the substances oxidised is no longer accepted, 

 for it is known that oxidations do not occur in the absence of 

 watery vapour. In spite of the fact that oxidations within and 

 without the body are very similar, and in their results practically 

 identical, the conditions under which each is produced are not 

 the same, the great dividing line being the relatively low tem- 

 perature at which oxidations in the body are effected. It is 

 probable that oxidations in the tissues occur under the influence 

 of enzymes and not directly by the presence of oxygen, for it 

 can be shown that, provided sufficient oxygen be supplied, 

 any further increase does not affect the rate of oxidation. We 

 have had before us evidence showing the existence of tissue 

 ferments capable of splitting fat, of oxidising sugar, of converting 

 sugar into glycogen, glycogen into sugar, and of acting on 

 proteins ; all of these may be isolated from the body tissues, 

 and are known as intracellular enzymes. Other evidence can 

 also be adduced of the existence of tissue ferments, by the fact 

 that living substance removed from the body with suitable pre- 

 cautions will be found to digest itself ; this is known as autolysis. 

 Tt is supposed that the enzymes of the body stimulate the oxygen 

 to activity; such enzymes have been called oxidases, and have 

 been found both in plants and in the animal body. They have 

 not, however, been found in connection with the oxidation of 

 protein, fat, or carbohydrate, though this may yet be demon- 



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