MANUAL OF CxiTTLE-FEEDI]S'G. 417 



being wholly completed in tlie glands, so that the resulting 

 milk shows no trace of the process, hut appears like a 

 homogeneous fluid. 



Composition of Milk. — Under the microscope, milk is 

 seen to consist of a fluid, in which are suspended tlie above 

 mentioned milk-globules, which render the fluid opaque. 

 On standing, these milk-globules rise to the surface and 

 form the cream, while the liquid portion, more or less free 

 from the fat-globules, forms skimmed milk- 

 It is in the composition of the fluid portion that milk 

 shows most plainly that it is not simply a filtrate from the 

 blood. It contains — in the case of the cow, e. g, — fi*om 

 two to five per cent, of protein ; but while the protein of 

 the blood exists as albunnn or fibrin, only a very small 

 part of the protein of milk consists of albumin, most of it 

 existing as casein (see p. 17), a substance not found else- 

 where in the body. A small amount of peptones is also 

 found in milk. 



Moreover, milk contains, in addition to the casein, from 

 three to five per cent, of a peculiar sugar — milk-sugar, or 

 lactose — which also has never yet been met with elsewhere 

 in the organism. These two substances, together with the 

 composition of its ash, stamp milk with a peculiar charac- 

 ter, and are sufficient of themselves to show that it is not 

 a secretion in the common sense of the word. 



The milk-globules have given rise to much discussion. 

 They consist essentially of a mixture of several fats, which, 

 when separated from the milk or cream by churning, con- 

 stitute butter. 



The milk-globules are generally described as surrounded- 



by a membrane consisting of some variety of protein. 



This membrane is not visible under the microscope, but 



several facts have been brought forward as proofs of its 



18* 



