288 Biological Chemistry. 



a narrow range of temperature (at about 37*5 C. in man). 

 Degradative changes of the extent to which they occur in 

 the body can often be brought about in the laboratory, 

 when no extracts of organs or secretions are used, only 

 when powerful chemical reagents such as strong solutions 

 of acids or alkalis or high temperatures are employed. 

 Thus, for example, it is not easy to break down starch or 

 proteins to the extent to which they are degraded by saliva 

 or gastric juice at a temperature of 37, unless powerful 

 hydrolytic reagents, such as strong solutions of mineral 

 acids, are used. The action of the saliva or gastric juice 

 is due to the presence of certain substances known as 

 " enzymes " or " ferments," to the general action of which 

 some attention must now be directed. 



The Nature of Enzyme or Ferment Action. The 

 word fermentation has for ages been associated with the 

 phenomenon of the formation of alcohol from sugar and 

 carbon dioxide, although it is only within comparatively 

 recent times that the nature of the process which takes 

 place has been understood. Lavoisier, towards the end of 

 the eighteenth century, obtained an approximately correct 

 idea as to the nature of the chemical process involved, 

 and the results obtained were amplified and corrected 

 by later investigators when the fundamental principles 

 governing the changes of organic substances came to be 

 more fully understood. For a long time, however, the 

 character of the agency producing the change was a matter 

 of considerable controversy ; the nature of the yeast cells 

 and other bodies which produced fermentative action was 

 the subject of various theories, none of which received 

 universal acceptance. About the year 1838, three ob- 

 servers, almost simultaneously and independently of one 

 another, claimed that the fermentation was the result of 

 the activity of a living organism. These investigators, 



