THE INTESTINAL JUICE 95 



stated (Chap. VII), the very complex protein molecules 

 are built up by combining the molecules of certain nitro- 

 gen-containing organic acids to which are given the name 

 of amino-acids. There are twenty different amino-acids 

 now known as constituents of proteins. One protein may 

 differ from another in the actual amino-acids which are 

 present, or in the relative proportions of the different 

 amino-acids, or in the way in which the amino-acids are 

 put together in its molecules. A simple mathematical 

 calculation will show that by starting with as many as 

 twenty different amino-acids and granting that they may 

 be put together in different ways or in different propor- 

 tions, or that one or more may be left out, a great variety 

 of proteins can exist. Since we are naturally inclined 

 to believe that the differences which occur among animals 

 and plants are, in many cases at least, bound up with 

 chemical differences in their proteins, we can thus con- 

 ceive how the differences are possible. In fact, it is hard 

 for us to imagine how living protoplasm, which has many 

 features in common wherever it is found and yet which 

 can take this infinite variety of forms, could exist were 

 it not that its chief constituent is a substance having the 

 peculiar complexity of protein. The reason why protein 

 must be digested into its constituent amino-acids becomes 

 clear from the facts which have just been set down ; for, 

 while in one or all of the three respects outlined above 

 the protein of the food is always certain to differ from the 

 protein within the body of the organism which eats it, 

 the amino-acids in the food protein are exactly the same 

 as the amino-acids in the body protein. Hence, it follows 

 that if the food proteins are broken up into their consti- 

 tuent amino-acids, these can then be recombined in 

 proper proportion to form the body proteins. 



The Intestinal Juice. — The food which has been 

 acted upon by the pancreatic juice will have all its starch 

 converted into malt-sugar, all or most of its proteins 

 decomposed into amino-acids, and its fats digested by 



