THE MILK. 



335 



The casein is one of the most important ingredients of the milk. 

 It is an extremely nutritious organic substance, which is held in a 

 fluid form by union with the water of the secretion. Casein is not 

 coagulable by heat, and consequently, milk may be boiled without 

 changing its consistency to any considerable extent. It becomes 

 a little thinner and more fluid during ebullition, owing to the melt- 

 ing of its oleaginous ingredients; and a thin, membranous film 

 forms upon its surface, consisting probably of a very little albumen, 

 which the milk contains, mingled with the casein. The addition of 

 any of the acids, however, mineral, animal, or vegetable, at once 

 coagulates the casein, and the milk becomes curdled. Milk is 

 coagulated, furthermore, by the gastric juice in the natural process 

 of digestion, immediately after being taken into the stomach ; and 

 if vomiting occur soon after a meal containing milk, it is thrown 

 off in the form of semi-solid, curd-like flakes. 



The mucous membrane of the calves' stomach, or rennet, also 

 has the power of coagulating casein; and when milk has been 

 curdled in this way, and its watery, saccharine, and inorganic in- 

 gredients separated by mechanical pressure, it is converted into 

 cheese. The peculiar flavor of the different varieties of cheese 

 depends on the quantity and quality of the oleaginous ingredients 

 which have been entangled with the coagulated casein, and on the 

 alterations which these sub- 

 stances have undergone by 

 the lapse of time and ex- 

 posure to the atmosphere. 



The sugar and saline sub- 

 stances of the milk are in 

 solution, together with the 

 casein and water, forming a 

 clear, colorless, homogene- 

 ous, serous fluid. The but- 

 ter, or oleaginous ingredient, 

 however, is suspended in 

 this serous fluid in the form 

 of minute granules and 

 globules, the true "milk- 



fflobules" (Fig" 107 ^ These MILK-GLOBCLKS; from same woman as above, 

 et>. ^r Ig. IU i .) ID four days after delivery> secretion fully established. 



globules are nearly fluid at 



the temperature of the body, and have a perfectly circular out- 

 line. In the perfect milk, they are very much more abundant and 



Fig. 107. 



