650 OF SECRETION. 



lieulty was experienced in arresting the secretion. His genital organs were fully deve 

 loped." Corresponding facts are also recorded of the male of several of the lower animals 



854. The secretion of Milk consists of Water holding in solution Sugar, 

 various Saline ingredients, and a peculiar albuminous substance termed 

 Caseine; and having Oleaginous particles suspended in it. The constitution 

 of this fluid is made evident by the ordinary processes, to which it is sub- 

 jected in domestic economy. If it be allowed to stand for some time, exposed 

 to the air, a large part of the oleaginous globules come to the surface, being 

 of less specific gravity than the fluid through which they are diffused. At 

 the same time there is reason to believe that they undergo a change which 

 will be presently described. The cream thus formed does not, however, con- 

 sist of oily particles alone; but includes a considerable amount of caseine, 

 with the sugar and salts of the milk. These are further separated by the 

 continued agitation of the cream ; which, by rupturing the envelopes of the 

 oil-globules, separates it into butter, formed by their aggregation, and butter- 

 milk, containing the caseine, sugar, &c. A considerable quantity of caseine, 

 however, is entangled with the oleaginous matter ; and this has a tendency to 

 decompose, so as to render the butter rancid. It may be separated by melting 

 the butter at the temperature of 180; when the caseine will fall to the bot- 

 tom, leaving the butter pure, and much less liable to change. The milk, after 

 the cream has been removed, still contains the greatest part of its caseine and 

 sugar. If it be kept long enough, spontaneous change takes place in its com- 

 position; the sugar is converted into lactic acid, and this coagulates the caseine, 

 precipitating it in small flakes. The same precipitation may be accomplished 

 at any time, by the addition of an acid; all the acids, however, which act 

 upon albumen, do not precipitate caseine, as will presently be pointed out in 

 detail; the most effectual is that contained in the dried stomach of a calf, 

 known as rennet. This exerts so powerful an influence over it, that, accord- 

 ing to the experiments of Berzelius, a piece of the membrane coagulated the 

 caseine of 1800 times its weight of milk, with the loss of only l'17th part of 

 its own weight; so that the active principle, dissolved from the rennet, must 

 have collected the caseine of about 30,000 times its weight of milk. The 

 whey left after the curd 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.* 



a. 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 iluid; these particles 

 (according to the measurement of Donnef) vary in size from about the 1-12, 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 solu- 

 tion 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, inclosed in an envelope 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 globules 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 pellicle has, however, been detected ; and it is probably to be regarded as the simple 

 result of the contact of oil with albuminous matter, which is known to give rise to a mem- 

 branous film. Besides these milk-globules, other globules of much smaller >i/e are seen in 

 milk; and these present the peculiar movement, which is exhibited by molecules in gene- 

 ral. Most of them seem to consist of oily matter, not inclosed in an envelope, 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 caseine, suspended, not dis- 

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



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

 j- Cours de Microscopic, Douzieme Legon. 



