526 THE SECRETIONS. 



CHAPTER XXXVIII. 



THE SECRETIONS. 

 SECTION I. MILK. 



178. Characters of Milk. Newly-drawn milk is an 

 opaque fluid of a white or yellowish-white color. Its color 

 and opacity are due to its being an emulsion, i. e., to its con- 

 sisting of little globules of fat suspended in a solution of 

 albumin, milk, sugar, and organic salts. Each globule of fat 

 is covered by a thin coating of casein. When the milk is 

 allowed to stand, the fat globules, being lighter than the fluid 

 in which they swim, rise in great part to the top, and form 

 cream, and the lower part of the fluid often acquires a bluish 

 tinge. A similar separation also takes place in the milk gland 

 itself, so that the milk last drawn is richest in cream. The 

 globules of fat are prevented from uniting by the thin albumi- 

 nous coating which surrounds each ; but when this is broken 

 latiou, they coalesce, forming butter. Changes also 

 occur in the milk, sugar, casein, and fats of the milk, more or 

 , 'liekly. according to the higher or lower temperature to 

 which it is exposed. The milk-sugar becomes converted, ap- 

 parently through the agency of a ferment, into lactic acid. 

 This gives the milk an acid reaction, and precipitates the case- 

 in, causing the milk to curdle. The coagulum, or curd, in- 

 l the fat globules. The liquid from which it is separated, 

 a sul ut ion of milk, sugar, and salts, is known as whey. The 

 curd, when completely separated from the whey, is called 



che. 



.J//r/-Msr,,/,/r,// /'J.m ID i t)tif i<m. Examine milk under the mi- 

 >pe. It will be seen to consist of a colorless fluid, con- 

 taining large numbers of minute fat globules. Add a drop of 

 acetic acid, so as to dissolve the coating of casein : the globules 

 will coalesce. Besides these globules, cells containing much 

 fat may be. seen, and also masses of fat similar to those within 

 the cells, but destitute of an envelope. These cells are found 

 much more frequently in the milk (called colostrum) which is 

 secreted for the first lew days after parturition, and they have, 

 therefore, received the name of colostrum corpuscles.' They 

 sometimes exhibit contractile movements. 



/!,-u rfjini. The reaction of human milk is always alkaline,- 

 and that of cows' milk is generally so. Free lactic acid always 



