MAMMARY SECRETION. 



611 



entirely of fat, and are composed of combinations of gtycerin with 

 oleic, palmitic, and stearic acids. They are surrounded by a thin layer of 

 casein, not in the form of a solid deposition, but the casein probably 

 exists in a condition of high imbibition rather than in a state of solidity 

 or of true solution. The laj'er of casein cannot be regarded as consti- 

 tuting a membrane, although it, to a certain extent, fulfills the same 

 function. If acetic acid is added to a preparation of milk, under the 

 microscope it will be seen that the caseous envelope is dissolved and the 

 oil-globules run together and form irregular masses of oil. 



When cows' milk is shaken up with caustic potash and then 

 agitated with ether the oil passes into solution in the ether. The 

 previous subjection to the action of caustic potash is, however, essential, 

 since ether will not dissolve the oil from cows' milk unless the casein 

 envelopes be previously dissolved by acetic acid or potash. 



FIG. 258. MICROSCOPIC APPEARANCE OP MILK AND COL,OSTRTTM. (Landois.) 



The upper half of the figure represents milk ; the lower half colostrum. 



If milk be allowed to stand for some time the oil-globules, which in 

 freshly secreted milk are uniformly distributed through the milk, now 

 rise to the surface and form a layer largely composed of fat, or the 

 so-called cream. 



The reaction of freshly secreted milk is alkaline in the herbivora and 

 in the human female, while that of the carnivora is acid. 



The milk of herbivora frequently will exhibit both an alkaline and 

 an acid reaction, due to the presence of an acid sodium phosphate 

 (H 2 NAP0 4 ) and of an alkaline disodic phosphate (NA a HP0 4 ). Such 

 a reaction is spoken of as amphioter. 



When milk is allowed to stand, the alkaline reaction, when present, 

 gives place to an acid reaction, which is due to the fermentation of the 

 milk-sugar and its conversion into lactic acid. When the cream is 



