COMPOUND ALIMENTARY SUBSTANCES. 77 



Animal Products used as Food Eggs. The eggs of 

 various birds are frequently used as food, but those of the 

 common fowl are by far the most esteemed. They contain, 

 necessarily, all the principles which are demanded in the 

 development of the chick, and are exceedingly nutritious. 

 They are taken sometimes raw, but most frequently cooked, 

 and in a great variety of forms which it would be out of 

 place even to enumerate. The white of egg uncooked, or 

 imperfectly coagulated, is ordinarily more digestible than 

 when it has been rendered solid by prolonged boiling. 



It is only necessary to refer to the composition of eggs to 

 appreciate their nutritive value. According to Payen, the 

 egg contains " nitrogenized substances (membranes, albu- 

 men, vitelline, extract of meat, yellow coloring matter) ; 

 fatty matters (margarine, oleine, cholesterine, etc., margaric, 

 oleic, and phospho-glyceric acids) ; a saccharine matter, sul- 

 phur, phosphorus, and mineral salts ; phosphates of lime and 

 magnesia, chlorides of sodium and potassium, carbonate of 

 soda." * In addition, the important principle, iron, is always 

 found in the ash after incineration. The white is composed 

 chiefly of albumen with inorganic salts ; and the yolk, of 

 vitelline, with a large quantity of oil in the condition of an 

 emulsion. 



Milk. At certain periods of life milk is the sole article 

 of food, and it must be regarded as at all times one of the most 

 nutritious and important of the animal products, being used 

 largely in its natural form, and furnishing butter and the 

 numerous varieties of cheese which are so largely consumed. 

 The composition of milk affords an explanation of its high- 

 ly nutritive properties. It contains a large amount of ni- 

 trogenized material, composed in greatest part of caseine, 

 but containing albumen, and another principle not very well 

 determined, called lacto-proteine ; fatty matter in abundance, 

 composed of a variety of principles of this class (oleine, 

 1 Op. dt., p. 130. 



