Milk is a yellowish-white, perfectly opaque, sweetish fluid, with an alkaline reaction and a specific gravity of about 1030. When exposed to the air, particularly in warm weather, the milk soon loses its alkalinity, first becoming neutral, and subsequently acid; the milk is then said to have "turned sour," but its appearance is not greatly changed. When it has stood a very long time it may crack or curdle, and separate into two parts - one a thick, white curd and the other a thin, yellowish fluid. This turning sour and ultimate curdling depends upon a change brought about in one of its most important constituents, namely, milk sugar, by means of a process of fermentation. The milk sugar, in the presence of certain forms of bacteria, ferments and gives rise to lactic acid. When the quantity of lactic acid is sufficient, it not only makes the milk sour, but also precipitates another of its important constituents, namely, casein. This albuminous body in its coagulation entangles the fat of the milk, and we have thus formed the curd of cracked milk, while the whey consists of the acid, salts, and remaining milk sugar.
Although the curdling of milk depends on the coagulation of an albuminous body, it is never produced by boiling fresh milk, because the chief proteid is casein, a form of derived albumin (alkali-albumin), which does not coagulate by heat.
When milk is preserved from impurities and kept in a cool place, a thick yellow film soon collects on the top of the fluid; the thickness of this layer - the cream - may be taken as a rough gauge of the richness of the milk. Milk consists of a fine emulsion of fat, the suspended particles of which are kept from running together by a superficial coating of dissolved casein. When left at rest, the light fatty particles float on the top and form cream.
When the mammary glands commence to secrete, the milk contains numerous peculiar structural elements, which subsequently disappear from the secretion, but which are of considerable interest in relation to the physiological process of the secretion. These are the colostrum corpuscles, which consist of large spherical masses of fine fat globules held together by the remains of a gland cell, which encloses the fat globules as a kind of sac or case, and in which at times a nucleus can be made out.
The most remarkable point about the chemical composition of milk is the large proportion of proteid and fat it contains; this renders it unique among the secretions. There are two distinct albuminous bodies present, viz.: casein, which appears identical with alkali-albumin, and another form of albumin allied to serum-albumin. The fats are present in the shape of globules of various sizes, being in the condition of a perfect emulsion, as above stated. They consist of glycerides of palmitic, stearic and oleic acids.
The milk sugar is very like glucose or grape sugar, but not so soluble. It has the peculiarity of undergoing lactic fermentation.
Of the inorganic constituents of milk the most important are phosphates and carbonates of the alkalies, i. e., the salts required to form bone. It is a remarkable fact that the potash compounds, which are the most abundant in the red blood corpuscles, are present in greater quantity than those of soda.
The following table shows the composition of human milk, a comparison of which with that of some domestic animals will be found on page 103: -
Milk sugar ........................
The relative quantity of the several ingredients of milk varies with the kind of diet used. A vegetable diet increases the percentage of sugar, but diminishes that of the other constituents, and also the general quantity of milk. A rich meat diet increases both the general quantity and the percentage of fats and proteids.
The quantity of milk secreted in the twenty-four hours is extremely variable in different individuals, and under different circumstances in the same individual - the average in general being about two pints.
The amount of the different materials in milk varies under the following rules. The proportion of albumin increases as the milk sugar decreases, and the fat remains the same as the period of lactation advances. The portions of milk last drawn are much richer in fats than that which is first taken from the gland. In the evening the milk is richer in fat than in the morning. The general amount of solid constituents falls up to the age of thirty years, then gains slightly until thirty-five, after which age the milk becomes decidedly thinner. These points should be borne in mind, as a knowledge of them may prove useful in the selection of a wet-nurse.
Fig. 168. Section of the Mammary Gland of a Cat in the early stages of lactation. (A) Cavity of alveoli filled with granules and globules of fat. 1,2,3. Epithelium in various stages of milk formation.
Fig. 169. Cells of Mammary Gland during lactation, stained with osmic acid so as to show the various sized oil globules as black masses. (Cadiat).
Although the blood contains albumins, fats, etc., very similar to those which form the solid parts of the milk, we have good reason for thinking that the constituents of milk are not merely extracted from the blood, but that the manufacture of this valuable secretion is due to the activity of the protoplasm of the gland cells, which construct the various ingredients out of their substance.
It has been suggested, as a simple explanation of the formation of milk, that the cells undergo fatty degeneration, and the secretion is then only the debris of the degenerated cells.
Some facts support this view. In the first place, the ingredients one finds in milk are suggestive of, though not identical with, the chemical materials which can be obtained from protoplasm by chemical disintegration rather than of any group of substances found in the blood. Further, we find that the so-called colostrum corpuscles, which appear to be secreting cells filled with fat particles, are thrown off from the gland in the early stages of the secretion, and appear in numbers in the milk.
But these colostrum corpuscles soon cease to be thrown off in the secretion, and the saccules of the glands during active lactation do not contain any signs of the debris of cast-off cells, or any gradations in degeneration. Only one row of finely-granular cells is found lining the saccules, and the cavities are filled with globules of various sizes. From this it would appear that in the earlier stages of the production of the secretion, the mammary cells, after a long period of inactivity, are so unaccustomed to the duty they are called upon to perform, that they succumb in the effort, and, being unable to produce the rich secretion and retain their vitality, they are cast off. Their offspring, however, after a generation or two, acquire the necessary faculty of making within their protoplasm all the necessary ingredients of the milk, and discharge them into the lumen of the saccules without themselves undergoing any destructive change.
The composition of the milk teaches us that the cells of this gland can manufacture from their own protoplasm casein, fat, milk sugar, etc., which fact shows beyond doubt that these complex materials may be made in the body.
The influence of the nervous system on the secretion of the mammary glands is distinctly shown by the wonderful sympathy between the' action of these glands and the conditions of the generative apparatus. Further, different emotions have an effect, not only on the quantity, but also on the quality of the secretion. Local stimulation also promotes the secretion, for the application of the child to the breast at once produces this effect, partly, it may be, through mental influences, but chiefly, no doubt, by reflex excitation of the gland following the local stimulation.
For the details of the dietetic value of milk, see Chapter V (Food), on Food, p. 102.