MAMMAKY GLAND : SECRETION OF MILK. 315 



poured into the canal leading to the internal ear, for the pur- 

 pose (it would seem) of preventing the entrance of insects. 



376. The secretion of Milk is important, not so much to 

 the parent who forms it, as to the offspring for whose nourish- 

 ment it is destined. It does not seem to carry off from the 

 system any injurious product of its decomposition ; for it bears 

 a remarkable analogy to blood in the combination of substances 

 which it contains ; nevertheless it is found that, when this 

 secretion is once fully established, it cannot be suddenly 

 checked, without producing considerable disturbance of the 

 general system. The structure of the Mammary gland closely 

 resembles that of the parotid already described (fig. 165). It 

 consists of a number of lobules, or small divisions, closely 

 bound together by fibrous and areolar tissue ; to each of these 

 proceeds a branch of the milk-ducts, together with numerous 

 blood-vessels ; and the ultimate ramifications of these ducts 

 terminate in a multitude of little follicles, about the size (when 

 distended with milk) of a hole pricked in paper by the point 

 of a very fine pin. 



377. The nature of the composition of Milk is made evident 

 by the processes to which we commonly subject it. When it is 

 allowed to stand for some time, its oleaginous part, forming the 

 cream, rises to the top. This is still combined, however, with 

 a certain quantity of albuminous matter, which forms a kind 

 of envelope round each of the oil-globules ; but in the process 

 of churning, these envelopes are broken, and the oil-globules 

 run together into a mass, forming butter. In ordinary butter 

 a certain quantity of albuminous matter remains, which, from 

 its tendency to decomposition, is liable to render the butter 

 rancid ; this may be got rid of by melting the butter at the 

 temperature of 180, when the albumen will fall to the bottom, 

 leaving the butter pure and much less liable to change. In 

 making cheese, we separate the albuminous portion, or casein, 

 by adding an acid which coagulates it. The buttermilk and 

 whey left behind after the separation of the other ingredients, 

 contain a considerable quantity of sugar, and some saline 

 matter. The proportion of these ingredients varies in different 

 animals ; and also in the same animal, according to the sub- 

 stances upon which it is fed, and the quantity of exercise it 

 takes. The amount of casein seems to be greatest in the milk 

 of the Cow, Goat, and Sheep ; that of oleaginous matter in the 



