334 



SECRETION. 



On being allowed to stand for a short time, the milk separates, without 

 coagulating, into two tolerably distinct portions. A large proportion of the 

 globules rises to the top, forming a yellowish- white and very opaque fluid, 

 called cream, leaving the lower portion poorer in globules and of a decidedly 

 bluish tint. In healthy milk the stratum of cream forms one-fifth to one- 

 third of the entire mass of the milk. In the human subject the skim-milk is 

 not white and opaque, but it is nearly as transparent as the whey. The spe- 

 cific gravity of the cream from milk of the average specific gravity of 1032 is 

 about 1024. The specific gravity of skim-milk is about 1034. 



Microscopical Characters of the Milk. Milk contains an immense num- 

 ber of minute, spherical globules, of highly refractive power, held in sus- 

 pension in a clear fluid. These are known under the name of milk-glob- 

 ules and are composed of palmitine, oleine, and fatty matters peculiar to 

 milk. 



The human milk-globules are -g-^-J-oT to T ^g~o ^ an i nc ^ (1 to 20 ft) in 

 diameter. They usually are distinct from each other, but they may occasion- 

 ally become collected into groups, without indicating any thing abnormal. 

 In a perfectly normal condition of the glands, when the lacteal secretion has 



become fully established, the milk con- 

 tains nothing but a clear fluid with 

 these globules in suspension. The pro- 

 portion of fatty matters in the milk is 

 twenty -five to thirty- eight parts per 

 thousand ; and this gives an idea of the 

 proportion of globules which are seen 

 on microscopical examination. 



In some regards milk does not pre- 

 sent the characters of a simple emulsion. 

 If it be shaken with ether, the mixture 

 remains opaque ; but the fatty matters 

 are dissolved on the addition of potassi- 

 um hydrate. Dilute acetic acid added 

 to milk causes the globules to run to- 

 gether. These reactions have led to the 

 view that the milk-globules have a membrane which is dissolved by potassi- 

 um hydrate and by acetic acid. It is probable that the butter in normal 

 milk does not exist precisely in the form of a simple emulsion, but that the 

 globules have a very thin, caseous coating. In view of the action of reagents 

 upon the globules, the only alternative, if the existence of a caseous coating 

 be denied, is the opinion that the addition of potassium hydrate or of acetic 

 acid renders the caseine incapable of holding the fat in the condition of an 

 emulsion. There is actually little more than a verbal difference between 

 these two opinions. 



Composition of the Milk. The following table, compiled by Robin 

 from the analyses of various chemists, gives the constituents of human 

 milk : 



FIG. 103. Human milk-globules, from a healthy 

 ing-in 

 unke). 



lying-in woman, eight days 'after delivery 

 (Fi ' 



