MAMMARY GLAND :—SECRETION OF MILK. 315 
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. Whenit 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 
