MILK 303 salts in solution) may be separated from the blood by a kind of nitration under blood pressure, as is the case in other secretory processes. The origin of the sugar of milk and of the casein is unknown. (For a description of the minute structure of the milk gland, see Klein s Atlas of Histology, p. 300, and references.) At the beginning of lactation the milk is rich in large irregularly- formed corpuscles (fig. 3, a, a, a) called colostrum corpuscles. These are contractile bodies, slow ly changing their form and squeezing out the oily particles. At first FIG. 3. A drop of milk magnified 300 diameters. they are the only bodies a a colos " " m ^usd.s. present, but they are soon replaced by the ordinary milk globules. Such globules have bright refractive edges, the surface is smooth, they vary in size from ^Vo-th to Winy*! 1 of an inch in diameter, and each consists of a drop of fatty matter surrounded by a layer of albumen ("Ascherson s membrane"). A secretion of milk takes place in newly-born children, from the fourth to the eighth day, and also in rare cases in men (Hermann s Physiology, p. 158). During gestation in the human being the mammary glands increase in size ; immediately after the birth of the child active secretion commences; and usually it is on the stoppage of the secre tion, ten months afterwards, that the process of menstrua tion, which has been arrested by impregnation, again is re-established. The secretion of milk is undoubtedly affected by the nervous system, as is shown by fear or mental distress arresting or injuring the quality of the secretion, and by the " rush " or feeling of fulness in the breast experienced by the mother when the child s mouth touches the nipple, or even when she sees her offspring. The nervous mechanism, however, is unknown, as it has been observed that secretion may continue even after section of all the nerves known to pass to the gland. The nature of the diet has a marked influence on the quality of the secretion. Thus the amount of casein and of fat is greater during an animal than during a vegetable diet. Fatty foods do not seem to increase the amount of fat or butter ; an ample supply of carbo-hydrates (starches and sugars) increases the amount of sugar. These facts indicate that most if not all of the constituents of milk are formed from changes in the pro toplasm of the epithelial cells. In some women the milk is deficient in fat and casein, and consequently is less nutritious. Prolonged lactation diminishes the amount of fat and sugar without materially affecting the amount of albuminous matter ; but the milk is less nutritious and is unfit for the child. The occurrence of menstruation during lactation also deteriorates the milk. (j. G. M.) Milk as Food. The milk of various domesticated animals is more or less used by man for food. The milk of the cow, which may be taken as typical of all others, and is indeed by far the most important and valuable of all, is, when newly drawn, an opaque white fluid, with a yellowish tinge, soft, bland, and sweetish to the taste, and possessed of a faintly animal odour. This odour, according to Schreiner, is due to the presence of sulphuretted hydrogen, and disappears after a siort exposure. The specific gravity of milk ordinarily ranges from 1 029 to 1*033, very seldom reaching 1 035 or falling so low as T027. In chemical constitution it con sists of an emulsion of fatty globules (cream) in a watery alkaline solution of casein, and a variety of sugar, peculiar to milk, called lactose. The fat (which when separated we know as butter) and the lactose constitute the carbonaceous portion of the milk regarded as food. The casein, which forms the principal constituent of cheese, and a certain proportion of albumen which is present, form the nitro genous, while the complex saline substances and water are the mineral constituents. These various substances are present in the proportions which render milk a perfect and typical food suitable to the wants of the young of the various animals for whom it is provided by nature. The milk of all animals, so far as is" known, contains them, although they are present in somewhat different propor tions. It is probable that the milk of ruminants possesses certain physical and physiological distinctions from that of non-ruminant animals, which will account for the virtues attributed to the milk of the ass and mare. The following table exhibits the chemical constitution of the kinds of milk most frequently used by man : Co w. Goat, Ewe. 1 Mare. Ass. Hainan. |S Cameron. Voelcker. Voelcker. Cameron. u ill I Gerber. Water Fat 86-87 3 50 87-00 4-00 84-48 6-11 8370 4-45 90-310 1-055 91-65 0-11 88-02 2-90 Casein and albumin... Sutttir.. j 475 4-00 4-10 4"28 3 94 4 68 5-16 5-73 1-953 6-285 1-82 6-08 1-60 7-03 Ash 070 0-62 079 96 0-369 0-34 0-31 In addition to these constituents milk contains small pro portions of the gases carbonic acid, sulphuretted hydrogen, nitrogen, and oxygen, and minute quantities of other prin ciples, the constant presence and essential conditions of which have not been determined. These consist of galactin and lactochrome, substances peculiar to milk, discovered by Winter Blyth, with certain animal principles such as leucin, pepton, kreatin, tyrosin, &c. The salts in milk consist, according to the average of numerous analyses by Fleischmann, of the following constituents: Phosphoric acid 28 31 Chlorine 16 34 Lime 27 00 Soda... lO OO Potash 17-34 Magnesia 4 07 Ferric oxide ... . 0"62 Milk thus is not to be regarded as a definite chemical compound nor even as a mixture of bodies in fixed and invariable proportions. Not only does the milk of different races and breeds of cows vary within comparatively wide limits ; the milk of the same animal is subject to extensive fluctuation. The principal causes of variation in the individual are age, period of lactation, nature and amount of food, state of health, and treatment, such as frequency of milking, &c. The following table indicates the range of normal variations: Water 90 00 to Fat 2-80 Casein and albumin 3 30 Sugar 3-00 Ash 0-70 The average quantity of milk yielded by cows is also highly variable, both in individuals and breeds. As a rule the smaller breeds of cows yield a small amount of milk rich in cream (butter fat), while the yield of the larger breed is greater in quantity, but comparatively deficient in cream. A good milch cow should yield in a milk-giving period of from eight to nine months about 83-65 4-50 5-55 5-50 0-80 1 Ewe s milk is exceedingly variable, especially in its percentage of fat. The above analysis is one of nine by Dr Voelcker, in which
the fat was found to range from about 2 to 12f per cent.