FOODS. 145 



Kinds of Foodstuffs. 



1. Proteids (example, casein). 4. Water. 



2. Fats. 5. Salts. 



3. Carbohydrates (example, sugar). 6. Oxygen. 

 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 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 circum- 

 stances more desirable from the fact that it is very easily 

 digested. 



Characteristics of Proteids. The proteids are 



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

 studying digestion). 



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



4. Easily putrefy when moist and warm. 



Importance of Proteids. The proteids are of special 

 importance as foods because the most active tissues, muscle, 

 nerve, and gland, and the most important liquids of the 

 body, e.g. blood and lymph, have proteid as a chief con- 

 stituent. Proteid food, therefore, must be taken to make 

 good the losses of these tissues during their oxidations. 



