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276 COMPARATIVE PHYSIOLOGY. 



1. The proteids of milk are : 



(a.) An albumin very like serum-albumin. 



(6.) Casein, normally in suspension, in tbe form of extremely 

 minute particles, wMch contributes to the opacity of milk. 



It can be removed by filtration through porcelain ; and pre- 

 cipitated or coagulated by acids and by rennet, an extract of 

 the mucus membrane of the calf's stomach. After this coagu- 

 lation, whey, a fluid more or less clear, separates, which con- 

 tains the salts and sugar of milk and most of the water. Much 

 of the fat is entangled with the casein. 



Casein, with some fat, makes up the greater part of cheese. 



2. Fats. — Milk is an emulsion — i. e., contains fat suspended 

 in a fine state of division. The globules, which vary greatly in 

 size, are surrounded by an envelope of proteid matter. This 

 covering is broken up by churning, allowing the. fatty globules 

 to run together and form butter. 



Butter consists chiefly of olein, palmitin, and stearin, but 

 contains in smaller quantity a variety of other fats. The ran- 

 cidity of butter is due to the presence of free fatty acids, espe- 

 cially butyric. 



) The fat of milk usually rises to the siwface as cream when 

 /mUk is allowed to stand. 



3. Milk-sugar, which is converted into lactic acid, probably 

 by the agency of some form of micro-organism, thus furnish- 

 ing acid sufficient to cause the precipitation or coagulation of 

 the casein. 



^Milk, when fresh, should be neutral or faintly alkaline. 



4. Salts (and other extractives), consisting of phosphates of 

 calcium, potassium, and magnesium, potassium chloride, with 

 traces, of iron and other substances. 



it can be readily understood why animals fed on milk 

 rarely suffer from that deficiency of calcium salts in the bones 

 leading to rickets, so common in the ill-fed. It thus appears 

 that milk contains all the constituents requisite for the building 

 up of the healthy mammalian body; and experiments prove 

 that these exist in proper proportions and in a readily digestible 

 form. The author has found that a large number of animals, 

 into the usual food of which, in the adult form, milk does not 

 enter, like most of our wild mammals, as well as most bh-ds, 

 will not only take milk but soon learn to like it, and thrive well 

 upon it. Since the embryo chick lives upon the egg, it might 

 have been supposed that eggs would form excellent food for 



