764 A MANUAL OF VETERINARY PHYSIOLOGY 



finally and irretrievably destroyed. The most favourable working 

 temperatures for the ferments of the animal body are from 40 to 

 45 C. (104 F. to 113 F.). This fact is necessarily ascertained by 

 observations outside the body. It is fair to assume that within the 

 body the normal internal temperature of the animal is that which 

 is most favourable to their activity. There are some ferments which 

 act best in acid, others in alkaline media. Most 'enzymes of the 

 body are soluble in water, glycerine, or in salt solutions. They may 

 be precipitated from solution by an excess of alcohol, or in the case 

 of the digestive ferments, by saturation with ammonium sulphate. 

 It is doubtful whether any of the ferments have been obtained in a 

 pure condition on precipitation. They adhere strongly to proteins, 

 which are also thrown down by the above reagents. In fact, for 

 long it was usual to regard the ferments as protein in nature, but 

 belief in this view is shaken by the fact that some body ferments have 

 been prepared which are apparently free from protein, though 

 all contain nitrogen. 



Some body ferments are secreted in an inactive form as a pro- 

 ferment or zymogen (p. 171), and stored up in the cell ready for use. 

 They may be activated at the moment they are required, or the 

 presence of another substance is necessary to activation. This 

 may be of either organic or inorganic origin. In the former case 

 the activating substance is spoken of as a kinase, of which the best 

 example is that described at p. 254, in which enterokinase activates 

 the trypsin of the pancreatic juice. 



The specific action of ferments is a characteristic feature. No 

 ferment can do the work of another. In the case of a secretion in 

 which more than one ferment is present — as, for example, in the 

 pancreatic juice — it is assumed that each of the characteristic 

 fermentations of which it is capable — viz., fat-splitting, starch-con- 

 verting, protein - splitting, and milk-curdling — are produced by 

 separate enzymes. This characteristic is especially well marked 

 in sugars, which, in their chemical nature, are closely related, yet 

 each requires its own enzyme. For example, lactase will only 

 act upon lactose ; it effects no change in dextrose or maltose. 



The oxidations occurring in the tissues, the nature of which has 

 always been a physiological puzzle, are brought about by ferment 

 activity; through the agency of oxidases, already spoken of at 

 p. 326. Such oxidases have been prepared from various tissues, 

 and are found widely distributed in the animal body. It is sup- 

 posed that the oxygen produced is obtained by the splitting off of 

 oxygen from hydrogen peroxide. From liver, lung, muscle, and 

 spleen, an oxidase has been obtained which is capable of oxidising 

 hypoxanthin into xanthin, and xanthin into uric acid ; another 

 animal oxidase is capable of oxidising aldehydes ; a third oxidises 

 tyrosine ; while the oxidation of sugar can be effected by tissue 

 juices or extracts of the various organs. 



Frequent reference has been made throughout these pages to 

 oxidations occurring in the tissues (see pp. 129, 130, 326, 365, 372, 

 408, 411), and the curious story of oxidation in muscle is related in 

 detail at p. 358. The mind is liable to visualise these oxidations as 

 if the body were a lamp. Such a temperature in the tissues would 

 insure their destruction, while the carbon in the cells is not in the 

 form of a wick charged with oil. The protoplasm is three-fourths 

 water, and the problem presented in physiological oxidations is to 

 understand the production of heat and energy by the oxidation of 



