THE MILK. 429 



gland, while perfectly fresh, is ground to a pulp and kept in normal 

 salt solution at the temperature of the body. An intermediary prod- 

 uct is then also apparently formed, which is of a colloid nature, but 

 not identical with glycogen. In view of recent researches, which 

 tend to show that the reducing group which is present in the gluco- 

 proteids is, in the case of the mucins at least, not a true carbohydrate, 

 but of the nature of chondroitin-sulphuric acid or an allied substance, 

 it would be exceedingly interesting to ascertain whether the reduc- 

 ing substance in the case of the mammary nucleo-glucoproteid also 

 may not be of this order. 



Of other constituents of the gland, we find various xanthin- 

 bases, and in the functionally active organ also a certain amount of 

 fat which is present in the form of globules of variable size, in the 

 bodies of the cells. 



The specific secretory product of the mammary glands is the milk. 



The Milk. The milk is the specific secretory product of the 

 mammary glands, and constitutes the natural food of all mammals 

 in the early stages of their extra-uterine existence. It contains all 

 those food-stuffs which are necessary for the maintenance of life, 

 viz., albumins, carbohydrates, and fats. The nutrient components 

 of the milk, however, are more or less specific of the secretion 

 in question, and are not found elsewhere in the body as such. 

 They are produced in the gland itself from the common constituents 

 of the blood. Among these the albumins are the most important, 

 and there can be little doubt at the present time that the fats of 

 the milk also are largely referable to this source. This is appar- 

 ent from the fact that in the bitch, for example, the amount of fat 

 increases with an increased ingestion of meat that is free from fats, 

 Avhile it is diminished when the animal is fed on fats only. The 

 so-called milk-sugar also is apparently derived from albumins, as 

 the substance continues to be formed although no carbohydrates are 

 ingested. Its amount, however, is then somewhat smaller, and 

 increases if cane-sugar or starch is added to the diet. 



General Characteristics. Fresh milk is an opaque, white, yellow- 

 ish-white, or bluish-white liquid, of a somewhat creamy consistence, 

 a more or less sweetish taste, and an insipid odor which is peculiar 

 to the particular animal from which the milk has been obtained. 

 The opacity is largely due to the presence of calcium caseinogen. 



On microscopic examination innumerable fat globules are seen, 

 which vary from 0.0024 to 0.0046 mm. in diameter, and num- 

 ber from 200,000 to 5,000,000 per cbmm., with an average of 

 about 1,050,000. The fat is thus present in a state of fine 

 emulsion, but, in contradistinction to other emulsions, in feebly 

 alkaline media, it cannot be extracted by shaking with ether 

 directly, or at least only with much difficulty. If, on the other 

 hand, an acid or a caustic alkali is previously added to the milk, 

 this is readily accomplished. From this observation it has been 

 concluded that each fat-globule is surrounded by an albumin- 

 ous membrane, the haptogenic membrane of Ascherson, which is 



