168 FOODSTUFFS. 



3. Carbohydrates (example, sugar). 



4. Water. 



5. Salts. 



(6. Oxygen is by some authors called a food, but it is 

 more convenient to treat of it elsewhere.) 



The Proteids. The chief substance in the white of an 

 egg is Albumen, a typical proteid. Of the many proteids, 

 some of the more commonly known are Casein (the curd of 

 milk), Gluten (in grains), Legumin (in peas and beans), Fibrin 

 (in blood), Myosin (in muscles). Gelatin (obtained from con- 

 nective tissue and bones by prolonged boiling) differs con- 

 siderably from the proteids in composition, but may be 

 counted in with them. It is less valuable as a food than the 

 true proteids, although in certain circumstances more desi- 

 rable from the fact that it is very easily digested. 



The proteids are 



1. Composed of carbon, hydrogen, oxygen, and nitrogen, 

 with a little sulphur, and, in some, traces of phosphorus. 



2. Jelly-like, and do not easily diffuse through animal 

 membranes (a characteristic to be kept in mind when study- 

 ing digestion). 



3. Coagulable (usually) by heat, acids, alcohol, etc. 



4. Easily putrefy when moist and warm. 



The proteids are of special importance as foods because 

 the most active tissues, muscular, nervous, and glandular, and 

 the most important liquids of the body, e.g., blood and lymph, 

 have proteid as a chief constituent. Proteid food, therefore, 

 must be taken to make good the losses of these tissues during 

 their oxidations. 



Proteid-Containing Foods. Lean meat has about 

 twenty per cent of proteid, the rest being chiefly water. 

 Beef and mutton are more easily digested than veal and pork. 

 (For a comparison of fat and lean meat, see Appendix.) 



