142 Physiology. 



Kinds of Foodstuffs. i. Proteids (example, casein). 



2. Fats. 4. Water. 



3. Carbohydrates (starch and sugar). 5. Salts. 



The Proteids. The chief substance m 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 connective tissue and bones by prolonged boiling) 

 differs considerably 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 desirable from the fact that it is very easily digested. 



Importance of Proteids. The proteids are of special 

 importance as foods because the most active tissues 

 those of the muscles, nerves, and glands and the most 

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

 tain proteid. Proteid food, therefore, must be taken to 

 make good the losses of these tissues during their oxida- 

 tions. Proteid is the only foodstuff containing nitrogen. 



Proteid-containing Foods. The principal proteid-contain- 

 ing foods are lean meat, fish, eggs, milk, cheese, and some 

 seeds which abound in the vegetable proteids. 



Meat. 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. Pork sometimes con- 

 tains a parasitic worm called trichina, which causes illness, 

 or even death, if eaten. Pork should be thoroughly cooked. 



Fish. Fish, when fresh, is a good food. Although, as 

 a rule, salted meats are less easily digested than fresh, 



