MILK SECRETION.

King Country Chronicle, Volume XXVII, Issue 4459, 19 October 1933, Page 3

MILK SECRETION.

PROCESS EXPLAINED. STRUCTURE OF THE UDDER. A knowledge of the composition of milk and of the physiology of its production is not by any means an essential to successful dairy farming but at the same time an understanding of the principles underlying this important function of the animal body should help dairy farmers to a clearer realisation of the need foi some of the practices they are constantly being recommneded to adopt. The milk secreting structure of a cow’s udder may be regarded as a process of evolution from simple surface skin. The milk-producing glands are similar in part to the skin sweat glands, and in part to the skin oilproducing glands. It is well-known that many things other than milk are excreted through the udder for example, medicines given to the cow frequently are excreted in the milk; indeed, it is a fairly common practice to give medicines in this way for the benefit of the nursing calf. The udder itself consists of gland tissues proper —i.e., the milk-secret-ing tissues —and a connective frame work, together with blood vessels, lymph vessels and nerves. The udder is divided into lateral halves, the partition extending lengthwise of the body. Each of these lateral halves, is encased in a tough fibrous sack, and each side has two glands—the front and back quarters, which are not separated by partitions like the two side halves. Each of the quarters is divided internally into distinct masses of gland tissue, known as lobes, which in turn are divided into smaller lobes or lobules. There is one outlet milk duct, or tube, for each lobule. It is from these small milk ducts that the milk from each quarter passes o'n its way to the milk cistern at the base of the teat. These mammary glands are undeveloped and rudimentary until maturity, and become active, as a rule, in the manufacture of milk towards the close of the first pregnancy. Size of Udders. From the fibrous sack which surrounds each side-half of the udder layers of this same tissue branch off and extend into and through each quarter, constituting the coarser framework of the quarter. The relative amount of this tissue, which has nothing to do with milk production, varies greatly in different udders and at different periods of life in the same udder. This is the simple explanation of why some cows with large udders produce little milk, and other cows with comparatively small udders give substantial yields of milk. The blood vessels in the udder act as general distributors, or “wholesalers,” of food material, as well as wholesale collectors of waste matter.

The lymph vessels, on the other hand, serve as “retailers” and small collectors, and are immediately concerned in delivery to the manufacturing cells of food material to be utilised in the manufacturing processes. The lymph vessels also collect the waste matter which is passed on to the blood for excretion. There are two sets of blood vessels in the udder —the arteries through which flows the pure blood and the veins which take the impure blood back to the heart and lungs. There are two sets of lymph vessels also. The first of these are the superficial lymph vessels distributed to the sheath around the udder and to the skin, while the second ramify through the udder and supply the little bottle-shaped groups of milk-producing cells called the alveoli. The blood vessels split up into finer and finer branches until ultimately they are microscopic in size. The tiniest blood vessels, the capillaries, form net works around the little milk tubes and alveoli, or milk-secreting cells. The lymph vessels divide into tangled masses of minute lymph tubes in and between the gland cells. From these vessels even more minute lymph tubes extend to the lymph spaces surrounding the little groups of milk cells, which are arranged in hollow masses somewhat like a round water bottle with a long neck. The nerve fibres are distributed in part direct to the blood vessels controlling the amount of blood passing through the udder, thus controlling the amount of raw material brought to the udder and, therefore, the quantity of finished product. Composition of Milk. Before studying the process of manufacture it might be well for us to understand clearly the composition of milk. Milk may be .said to consist of milk globules (the butter-fat) and milk plasma, the latter being made up of water, casein, albumen, milk sugar and ash. The proteins (casein and albumen) make up on the average about 4 per cent, of the total while about 4 y 2 per cent, consists of milk sugar and about 4 per cent, of butter-fat. Usually there is less than 1 per cent, of mineral matter or ash, and about 87 per cent, of water. The fat globules are in the form of an emulsion, with a film of the liquid portion of the milk between the globules. Let us considere now how these materials are brought together to form milk. They do not exist as such in the blood stream, which carries to the udder only the products of digestion. The manufacturing processes actually take place in the udder itself. As previously explained, the milksecreting cells are arranged in hollow groups—the alveloi —the necks of which open into a commo'n outlet tube. Before pregnacncy these little alveoli and their outlet tubes are blocked with masses of cells. As the period of pregnancy develops these cells divide. Some continue attached to the basement membrane which surrounds each alveolus, while others remain free in the cavities of the alveoli, also in their necks and in their outlet tubes. These loose cells undergo a fatty degeneration, and pass out in the first milk as colostrum with which every dairy farmer is familiar. The cells which remain attached to the basement membrane carry on the business of manufacturing the fat and other normal constituents of milk. It is probable that this process of milk formation is under nervous control, and it is well known that it may be seriously interfered with by disturbances of the nervous system.

The production of the fat, casein, milk sugar, etc., is an actual manufacturing - process, and not merely filtration from the blood for none of these things are in the blood when it reaches the udder. The milk cells simply take certain elements from the blood, and put those elements together in such ways as to form the fat, casein, sugar, etc. In the first portion of the process the milk cells lining the alveoli undergo fatty changes. The cell manufactures the fat which accumulates in the free end of the cell. This free end changes into a tiny fat globule, and separates from the parent cell, which lengthens again, and develops another fat cell, which, in turn, separates from the parent cell. Thus butter-fat production is a process of moulting or shedding, just as the hair sacs produce and shed hair. The fat globules escape into tiny cavities within the udder, and thence to the milk cistern at the base of the teats. The second portion of the process is the formation of the proteins, sugar, etc., from food materials in the lymph fluid which surrounds each cluster of milk cells. The lymph fluid receives raw material from the blood, and simply passes it on to the milk-forming cells, which convert it to the various

substances found in milk. The quantity and quality of the milk depend, first, upon hereditary peculiarities of the individual cow. After that they depend upon the food, the time the cow has been milking, the time she has been carrying her calf, the condition of her nervous system, and, to a very great extent, upon the quantity of blood passing through the udder.