416 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXV III. 



with the fatty acid which is called a soap. Suppose, for instance, that 

 potassium hydrate is used ; we get 



C 3 H 5 (O.C 15 H 31 CO) 3 + 3KHO = C 3 H 5 (OH) 3 + 3C 15 H 31 CO.OK. 



[Tripalmitin a fat.] [Glycerin.] [Potassium palmitate 



a soap.] 



Enmlsification. Another change that fats undergo in the body 

 is very different from saponification. It is a physical not a chemical 

 change; the fat is broken up into very small globules, such as are 

 seen in the natural emulsion milk. 



The Proteins. 



The proteins are the most important substances that occur in 

 animal and vegetable organisms, and protein metabolism is, as already 

 noted (p. 6), the most characteristic sign of life. 



They are highly complex compounds of carbon, hydrogen, oxygen, 

 nitrogen, and sulphur, occurring in a solid viscous condition or in 

 solution in nearly all parts of the body. The different members of 

 the group present great similarities, for instance, in the heaviness of 

 their molecules, and in giving certain colour tests we shall be describ- 

 ing presently ; there are, on the other hand, considerable differences 

 between the various proteins. 



The proteins in the food form the source of the proteins in the 

 body tissues, but the latter are usually different in composition from 

 the former. The food proteins are in the process of digestion broken 

 up into simpler substances, usually called cleavage products, and it is 

 from these that the body cells reconstruct the proteins peculiar to 

 themselves. As a result of katabolic processes in the body, the 

 proteins are finally again broken down, carbonic acid, water, 

 sulphuric acid (combined as sulphates), urea, and creatinine being 

 the principal final products which are discharged in the urine and 

 other excretions. The substances intermediate between the proteins 

 and these final katabolites will be discussed under urine. 



The following figures will show how different the proteins are 

 even in elementary composition. Hoppe-Seyler many years ago gave 

 the variations in percentage composition as follows : 



C H N S O 



From 51-5 6'9 15'2 0'3 20'9 



To 54-5 7'3 17'0 2-0 23'5 



Eecent research has since shown that the variations are even greater 

 than those given by Hoppe-Seyler. 



Differences are also seen when the cleavage products are separ- 

 ated and estimated. These differ both in kind and in amount, but 

 nearly all of them are substances which are termed amino-acids. 

 Emil Fischer to whom we owe so much of our knowledge in this 



