544 OF SECRETION. 



has been separated, contains a large proportion of the saccharine and saline 

 matter, entering into the original composition of the milk. This may be readily 

 separated by evaporation.* 



685. When Milk is examined with the microscope, it is seen to contain a 

 large number of particles of irregular size and form, suspended in a somewhat 

 turbid fluid ; these particles (according to the measurement of Donnet) vary 

 in size from about the l-12,700th to the l-3040th of an inch ; and they are 

 termed milk-globules. They are not affected by the mere contact of ether or 

 alkalies ; but if these reagents are shaken with them, an immediate solution is 

 the result. The same effect happens, if they are first treated with acetic 

 acid. Hence it is evident, that the globules consist of oily matter, enclosed in 

 an envelop of some kind : and an extremely delicate pellicle may, in fact, be 

 distinguished after the removal of the oily matter by ether ; or after the glo- 

 bules have been ruptured, and their contents pressed out, by rubbing a drop 

 of milk between two plates of glass. No proof of the organization of this pel- 

 licle has, however, been detected ; and it can scarcely be regarded in the light 

 of a true cell. Besides these milk-globules, other globules of much smaller 

 size are seen in milk ; and these present the peculiar movement which is ex- 

 hibited by molecules in general. Most of them seem to consist of oily matter, 

 not enclosed in an envelop ; as they are at once dissolved when the fluid is 

 treated with ether : but, according to the statements of Donne, it would seem 

 that a portion of them are composed of casein, suspended, not dissolved, in the 

 fluid. It may be reasonably doubted, however, whether these were not in a 

 state of change ; whether from their own ^decomposition, or from incipient 

 coagulation ; either of which might have taken place during the processes of 

 filtration, &c., that were required to determine their nature. Besides the 

 foregoing particles, there are found in the Colostrum, or milk first secreted 

 after delivery, large yellow granulated corpuscles, which are described by 

 Donne as composed of a multitude of small grains aggregated together, and 

 frequently including a true globule of milk in their centre : these are for the 

 most part soluble in ether ; but traces of some adhesive matter, probably mucus, 

 holding together the particles, are then seen. Lamellae of epithelium are also 

 found in the milk. All the larger globules may be removed by repeated 

 filtration; and the fluid is then nearly transparent. This, in fact, is the 

 simplest way of separating the oleaginous from the other constituents of the 

 milk ; as little casein then adheres to the former. That, the transparent fluid 

 which has passed through the filter contains nearly the whole amount of the 

 casein of the milk, appears a sufficient proof that this is, for the most part, 

 truly dissolved in the fluid. We shall now consider the chemical characters 

 of each of the foregoing ingredients. 



686. The Oleaginous matter of milk principally consists, like fatty matter 

 in general, of the two substances, elaine and stearine ; which are converted in 

 the process of saponification into the elaic, stearic, and margaric acids : but it 

 also contains another substance peculiar to it, which yields in saponification 

 three volatile acids, of strong animal odour, to which Chevreul has given the 

 names of butyric, caproic, and capric acids ; whilst the fatty substance itself, 

 to which the peculiar smell and taste of butter are due, is designated as 

 butyrine. The peculiar acids are not only formed when the butyrine is treated 

 with alkalies ; but are produced by the ordinary decomposition of this prin- 

 ciple, which is favoured by time and moderate warmth. The Casein, or 

 cheesy matter of milk, which is obtained with some slight admixture of fatty 

 matter in the production of cheese from skimmed milk, is chiefly distinguished 



* A considerable quantity is thus obtained for household purposes in Switzerland. 

 f Cours de Microscopic, Douzieme Le9on. 



