68 SECRETION. 



phatics, and nerves, and during pregnancy and lactation the ves- 

 sels, as well as the gland, undergo considerable increase in size. 



What is milk? 



The secretion of the mammary gland is a bluish-white fluid. It 

 is opaque, and this opacity is caused by the presence of minute fat- 

 globules which are held suspended an emulsion. Besides the fat- 

 globules the microscope shows in milk from a newly-active gland 

 certain albuminous bodies which are known as colostrum-corpus- 

 cles. They are probably cells from the gland which are under- 

 going fatty degeneration. Milk is alkaline in reaction, and has a 

 sp. gr. of about 1030. 



Describe the composition of milk. 



The ingredients of milk are water, containing in suspension fats 

 and in solution casein, serum, albumin, milk-sugar, and salts. Hu- 

 man milk differs from cow's milk in containing less proteids and fats 

 and more sugar : 



Human Milk. Cow's Milk. 



Water 890 858 



Proteids .... 35 68 



Fats 25 38 



Sugar 48 30 



Salts 2 6 



1000 1000 



What is cream ? 



The fat-globules rise to the top if milk is allowed to stand, and 

 are called cream. By agitating o'r " churning'' the cream the albu- 

 minous envelopes of the fat-globules are broken, and they coalesce 

 to form a fat mass known as " butter." 



Why does milk "turn sour"? 



The addition of an acid to milk causes the casein, which is held 

 in solution by the alkaline sodium phosphate present, to be precip- 

 itated. The precipitation of the casein leaves behind the watery 

 and soluble ingredients, but includes with it the fat-globules. The 

 precipitate is known as the " curd," and the watery residuum as 

 '' whey." This process is imitated in nature by the breaking up 

 of milk-sugar (lactose) to form lactic acid through the action of a 

 micro-organism, bactfriinn lactis. 



