By the nature of their structure the milk-glands belong to compound tubular glands. The peculiar secreting tubular glands are covered with a single layer of epithelium: their appearance, however, varies mark edly with the condition of fulness and the phase of secretion. Sometimes the cells are low and quite flat, With a fairly homogeneous protoplasm and a single nucleus; others are cuboidal, and filled with fine fat glob ules; at other times they are very high, projecting into the lumen of the alveolus, thickly laden with globules of fat of various sizes, and many times possessing two or nuclei (Fig. 52.). In the lumen of the alveolus itself drops of fat are noticed, and also occasionally the re mains of nuclei. The diversity of the histological appearance depends upon the secretory activity of the glands. The alveoli with the flattened epithelium free of fat belong to the resting stage; they have already thrown out their secretion. The others have approached more or less to the acme of their function. It is to be noticed that these various conditions are found close together in a single area, and consequently the same state of secretion is not found throughout the same gland.
Formerly it was held that the microscopical examination of the secreting glands showed that they were undergoing fatty degeneration, and that therefore the milk was produced in reality by the destruction of the gland elements. This theory of the milk being a liluefaction of the breast-gland has, been prominent for several years past and is even found to-day in a number of text books. It must now be abandoned, however, since all recent researches have shown that it neither has au anatomical basis nor is supported by physiological chemistry. The milk is formed, not through a destruction of die gland cells, but through their specific secretory activity. Frequently, perhaps, there is a destruc tion of the superficial portion of the cells: this process, however. is of secondary importance.
The activity of the glands begins with the production of the colos trum, the clear, yellow, mucus-like, tenacious fluid. which is character ized morphologically by the presence of the colostrum bodies, which are fat-laden leucocytes. It is especially distinguishable in its chemical composition by a large amount of free coagulable albuminous bodies, and by a fat which contains an increased amount of unneutralized fatty acids, and which resembles very much the body fat, so far as its con stituents are concerned.
In the last months of pregnancy colostrum can be pressed out with considerable ease. At the time of delivery the production is greater, and in the course of the next few days the secretion gradu ally takes on the characteristics of milk,—there being, however, no particular definite moment at \vhich one can say that the milk comes in.
This usually occurs from the second to the fifth day post partum. The breasts become tense and hard, and subjectively a feeling of tension arises; ordinarily there is a slight rise in temperature (milk fever). With a constantly increasing flow of the milk stream the secretion soon loses completely its colostrum character.
So far as the origin of the milk is concerned, as has already been pointed out, it is formed through the activity and through the de struction of the gland elements. The necessary material must be taken up through the blood. The more definite processes for the production of the principal constituents of milk albumin, fat, and sugar—have not yet been clearly determined. The most important facts and hy potheses arc. here quoted.
Albumin. Casein, the special albuminous body of milk, differs from the albumin of the blood by the presence of phosphorus. One con clusion is that the nucleic acid, derived frotn the cell nuclei of the glands, unites with the blood serum to form casein. In this way casein can be produced by laboratory methods with all its typical characteristics (rennet coagulation).
Fat —Milk fat is formed from previously formed fat. the source of which is, without doubt, the fats taken up in the nourishment of the mother and the deposits of fat already in the body. The part which the fat of nourishment plays in the formation of milk may be proved be yond question; that of the body fat may be determined in an indirect way. The material which has been brought to the glands is there pre pared for the specialized milk fat.
information in regard to this constituent is very un certain. Milk-sugar, the characteristic carbohydrate of the milk, is not found already formed in the body. Probably fermentative processes are concerned in its formation.
Composition of the Mother's composition of the moth er's milk is constant. In the later lactation period the albuminous content is diminished. The fat content. constantly and uniformly in creases during the process of nursing. If the results of systematical analyses of the daily milk are collected, it will be found that the fat con tent approaches a constant quantity. The distribution of the individual constituents of the dry substance is, according to our analyses: Fat .
Sugar. ... 6.9S Ash ... 0.21 The albumin is represented for the most part by the characteristic phosphorus-containing casein, which is distinguished from that of cow's milk by the fine flocculent rennin coagulum. This difference exists only in the test-tube reaction. In the stomach the casein of cow's milk is broken up into a finer coagulum than in the test-tube. Un doubtedly the remainder is made up of the so-called free albuminous bodies, globulin, albumin, opalisin.