430 THE GLANDULAR ORGANS. 



dissolved by acids and alkalies, but which normally prevents the 

 solvent action of the ether upon the contained fat. Later investi- 

 gations have rendered this view improbable, however, and, as a 

 matter of fact, no one has ever succeeded in demonstrating the pres- 

 ence of a special membrane. The normal resistance to the action of 

 ether is now explained upon the assumption that each globule is 

 surrounded by a delicate layer of albumin, which does not consti- 

 tute a true membrane, however, but is formed as a result of molecu- 

 lar attraction. It is possible, indeed, to prepare emulsions of fat 

 artificially by shaking with albuminous solutions, which in their 

 behavior to ether are quite similar to milk. As regards the char- 

 acter of the particular albumin which forms this layer, our knowl- 

 edge is not complete. It has been supposed by some that it is 

 formed by casein, but there are reasons for believing that the 

 albumins of the milk in general may here be concerned. 



On standing, the greater portion of the fat rises to the surface of 

 the milk and forms its cream. On beating the milk for some 

 time, the individual fat-globules are caused to coalesce, and sepa- 

 rate out as a semisolid mass, which constitutes the butter. The 

 remaining liquid is termed buttermilk, and still contains some fat 

 which has remained in emulsion. 



Besides the fat-globules the milk contains also innumerable gran- 

 ules of calcium phosphate (probably a mixture of diphosphates and 

 triphosphates) in suspension, which are visible only on microscopical 

 examination and are said to number about 4,000,000 per cbmm. 

 On filtration through a Chamberlain filter, under pressure, these 

 remain behind together with the fat. But we then also find that 

 one of the most important albuminous constituents of the milk, 

 viz., casein, which is found in combination with lime, is likewise not 

 present in solution, and is thus obtained in the form of a thin, jelly- 

 like material. The filtrate constitutes the milk-serum and contains 

 those components of the fluid which are present in a state of actual 

 solution. 



Upon the addition of chymosin to fresh milk, at the temperature 

 of the body, it coagulates almost at once. The resulting clot, 

 which constitutes cheese, then contracts and a yellowish fluid gradu- 

 ally appears, which is termed sweet whey. During this process the 

 reaction of the milk is not changed. A similar coagulation is noted 

 when fresh milk is allowed to stand exposed to the air. In this case, 

 however, the reaction of the whey is acid, owing to the formation of 

 lactic acid from lactose in consequence of the activity of certain 

 micro-organisms. 



Perfectly fresh milk does not coagulate on boiling, but it will be 

 noted that a skin forms on the surface of the milk, which is rapidly 

 reformed when removed. This consists of coagulated casein in com- 

 bination with mineral salts, and especially phosphates of calcium. 

 Actual coagulation does not occur, even if a current of carbon dioxide 

 has previously been passed through the liquid. If the milk has 



