SOME THEORIES

Waipa Post, Volume 32, Issue 1786, 24 July 1926, Page 7

SOME THEORIES

MILK SECRETION IN DAIRY CATTLE. For long years dairy cattle have been bred for miilk and fat production, and are usually thought of us having been developed to a high state of,efficiency; yet the cows population range* in productiveness all the way from those that will produce only sufficient milk to raise their young to the wonderfujl Jersey, cow that produced over 20,0001b5. of milk in one year. This wide range in productiveness is largely due to three factors —differences in heredity, feeding, and management. The largest milk and fat production results only when there is a favourable combination of th# three factors. A careful study of this subject has been made by Mr Chas. W. Turner, Missouri, who, in explaining the methods employed by cows in making milk, also describefs the processes. The mammary glands of the dairy cow are contained in the udder, which is suspended below the abdomen. The udder is divided, into lateral halves, each of which is encased in a tough, fibrous envelope. Each half is again divided into two glands, the front and rear quarter. The quarters, however, are not separated by partitions similar to the halves, and it i|s believed that there is no connection or passage of milk between the front and rear quarters.

Each quarter of the udder is provided with a milk cistern into which the ducts of the gland, open and carry the milk from the secretory cells. The accumulation of milk in the cistern is then removed by means of the teats. Dairymen often use iflhie expression that a cow i/s “holding up her milk.’’ Cows are believed to be able to do this, due to the muscles located at the branches of the ducts leading into the milk cistern being more or less under the control of the animal. The contraction of these muscles interferes with easy milking.

In structure, the entire milk gland mlay be compared roughly to that of a bunch of grapes. The entire process of milk secretion is carried on in gro.ujps of tiny cells called alveoli. The alveoli are small sac-like bodies, lined with secreting cells, which correspond to a single grape. The fat and other component parts of the milk are secreted through the cell wall into the cavity of the alveolus. Each alveoli is surround,ed by a layer of connective tissues similar to that which holds the muscles of the body together. Capillary blood vessels, nerves, and lymphatics surround this tissue. the milk ia secreted it passes through tiny ducts, which, as they continue, increase in size. Cavities of varying sizes act as storage places for the milk during the interval between milking. The development of the mammary glands begins as the female approaches sexual maturity. The most rfapid development, however, takes place in the latter istages of pregnancy. It was formerly thought that there was a nervous connection between the growth of the mammary glands and the development of the fetus. That such is not the case was shown experimentally by severing all nervous connection between the mammary glands and the pelvic organs. Pregnancy was accompanied as usual by mammary development, and, after parturition, lactation occurred normally. There are many other experiment's which tend to show that the activity of the mammary gland, and more especially the secreting cells, are not under nervous control. Portions of mammary glands have been grafted to abnormal positions in the body, and without the usual nervous connections the glands underwent enlargement djuring pregnancy and later secreted milk. The mammary glands of the Bohemian twins Rosa and Josepha, which had a common circulation of blood, bu(t had entirely separate nervous systems, were said to be similarly affected by the pregnancy of Rosa, and both secreted milk. Fetal extracts injected into virgin animals over a period of time have caused the development of the mammary glands and the secretion of milk. These observations' led Lane-Cl-aypon and Starling to advance the theory that the fetus secretes a chemical stimulus or hormone which causes the development of the mammary gland and the .secretion of milk. Other workers have suggested that a yellow coloured body, called the corpus luteum, which is associated with pregnancy, is the source of the hormone, causing the development o.f the mammary glands and the initiation of milk secretion. ~ . K That other causes, su;ch as mechanical stimulation or removal of the liquid from the mammary glands of heifers, will cause the production of considerable quantities of milk, is a common observation. The .same methods have been known .to.’ cause the. secretion of a milk-like fluid in the rudimentary glands of males. The blood is the "sburce : of 'th'e’'“nutri-' eiits'wiiich' enable 'the alveoli cells to' 's'ecrbte milk. -One of the factors which is thought to regulate the amount of milk secreted) in a given length of time is the volume of blood which

passes through the’ mammary gland. The arteries which carry the blood from the heart to the mammary glands enter at the top of the u|d,der, spreading out into a network of blood vessels which furnish blood to every alveoli. The veins re-comlbine to carry the blood back 'to the heart. There are, however, two sets of veins. One' pair, called the posterior mammary veins, follows the path of the arteries back to the heart, while the other pair, called milk vein's, leaves the front of the udder and runs forward on the surface of the abdomen, finally entering the abdominal cavity through openings commonly called milk wells. It seems reasonable to expect that there should be a relation between the secretion of milk and the volume of blood flowing through the mammary glands at a given time. The volume of blood that can .be carried to the glanfls would depend on the size of the veins and arteries and the rate of flow of the blood. The slight correlation which has been found between the size of the milk veins and milk yield guggests that the relation between the size of the two pairs of veins is not marked. The exact relation between the amount of blood flowing through the mammary gland) during a given period of time and, the secretion of milk during the period would be illuminating. Unfortunately, no experimental data is available. There are sonic indications that during the period of digestion, when more of the blood is undoubtedly diverted to the vafecular system surrounding the digestive organs, that the rate of milk secretion is reduced. Some idea of the enormous 'amount of blood passing through the mammary gland, may be gained from the following estimate. The blood in circulation in a 1000 pound cow is approximately 50 pounds. It is estimated that the blood in man circulates 'throughout the body every minute. If it takes twice that amount of time in the dairy cow the total amount of blood would equal 50 x 30 x 24, or 36,000 pounds per day. The proportion of the blood which passels through the mlammary gland is unknown. If we assume that one-half of this blood flows through the mammary glands, IS.OOO pouncKs of blood would be available for the secreting cells to draw upon for the various constituents of 'which milk is mad,e. The milk producing capacity of cows when all other conditions are the same would depend on the volume of blood which was di* verted to the mammary glands. Milk is a true secretion. It is not merely strained from material in the blood, because it contains ingredients which are not found in the blood. For example, the chief protein of milk, casein, as well as lactose or milk sugar. is not found in blood. These factfe seem to show that the epithelial cells in the alveoli- are the seat of the process of secretion. Marshall states that three different hypotheses have been put forward!' regarding the manner in which-the substances of which the milk is formed, pass out from the secretory cell/s: I —The cells themselves break loose and become disintegrated, setting free their contents in the alveoli of the gland. '2. —The cells slimply excrete the substances into the alveolar lum’ina without becoming detached or destroyed themselves. 3—l Some cells 'simply discharge their contents into the alveoli as suggested by the second theory, while with others, the central part of the cell, containing a degenerate daughter nucleus, breaks away and becomes disintegrated, leaving the basal portion still in position. The latter theory is supported by the investigations of the leading physiologists who have studfied, ’this problem.