CHEMICAL CONSTITUENTS OF BODY AND FOOD. 31 



Emulsification is a physical or mechanical rather than a chemical 

 change. Both soaps and emulsions are continually being formed in 

 the body during the digestion of fats. 



3. PROTEIDS, OR PROTEINS. 



The principal constituents forming the muscular, nervous, and 

 glandular tissues, as well as the serum, blood, and lymph, are proteids. 

 In normal urine there are found no proteids, or, if any, only traces. 

 In a great measure the various phenomena of life are present and due 

 to the protoplasm in the cells. On analyzing protoplasm chemically 

 its substance is, of course, killed by the reagents used, but proteids 

 invariably result in the process. Whether the proteids exist as such 

 in the protoplasm, or occur only after the death of the protoplasm, 

 has not been fully established, but they are believed to be the con- 

 stituents of it. However, none of the phenomena of life occur with- 

 out their presence. 



Proteids are very complex, comprising compounds of carbon, 

 hydrogen, nitrogen, oxygen, and sulphur. They may be either solid 

 or liquid, as they are found in the different tissues of the body. The 

 different classes of proteids present both physical and chemical pecu- 

 liarities, although all have certain common reactions. Some are sol- 

 uble, others are insoluble, in water, while nearly all are soluble in 

 ether and alcohol. Strong acids and alkalies are also capable of dis- 

 solving the proteids, but in the process of dissolution decomposition 

 almost invariably occurs. 



The supply of proteids in our bodies is obtained from the vege- 

 table kingdom, being taken in as vegetables directly, or indirectly in 

 the form of meat which is derived from animals that live on vege- 

 tables. Thus the proteids are built up from the simpler inorganic 

 compounds taken from the soil and air and elaborated in plant-struc- 

 ture. 



The chemical composition of the proteids is variable, depending 

 upon the products analyzed by the different investigators, as the 

 purity of the substances cannot be definitely determined. From in- 

 vestigations we have the following average percentages: 0, 21.50 to 

 23.50; H, 6.5 to 7.3; N, 15.0 to 17.6; C, 50.6 to 54.5; S, 0.3 to 2.2. 



The nitrogen and sulphur are each contained in the molecule in 

 two forms, the one loosely combined, the other firmly combined. 

 The basis of construction of all proteids is, according to Kossel, a 

 body called protamin (C 30 H 57 N 17 6 ), which on hydrolysis gives three 

 basic substances each containing six carbon atoms, hence called hexone 



