THE STORY OF NINETEENTH-CENTURY SCIENCE 



that the liver and the intestinal glands have each an im- 

 portant share in the work of preparing foods for absorp- 

 tion, as also has the saliva that, in short, a coalition of 

 forces is necessary for the digestion of all ordinary foods 

 taken into the stomach. 



And the chemists soon discovered that in each one of 

 the essential digestive juices there is at least one sub- 

 stance having certain resemblances to pepsin, though 

 acting on different kinds of food. The point of resem- 

 blance between all these essential digestive agents is 

 that each has the remarkable property of acting on 

 relatively enormous quantities of the substance which 

 it can digest without itself being destroyed or apparent- 

 ly even altered. In virtue of this strange property, 

 pepsin and the allied substances were spoken of as fer- 

 ments, but more recently it is customary to distingush 

 them from such organized ferments as yeast by desig- 

 nating them enzymes. The isolation of these enzymes, 

 and an appreciation of their mode of action, mark a 

 long step towards the solution of the riddle of digestion, 

 but it must be added that we are still quite in the dark 

 as to the real ultimate nature of their strange activity. 



In a comprehensive view, the digestive organs, taken 

 as a whole, are a gateway between the outside world 

 and the more intimate cells of the organism. Another 

 equally important gateway is furnished by the lungs, 

 and here also there was much obscurity about the exact 

 method of functioning at the time of the revival of phys- 

 iological chemistry. That oxygen is consumed and 

 carbonic acid given off during respiration the chemists 

 of the age of Priestley and Lavoisier had indeed made 

 clear, but the mistaken notion prevailed that it was in 

 the lungs themselves that the important burning of fuel 



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