MILK SECRETION

Timaru Herald, Volume CXLVIII, Issue 21821, 26 November 1940, Page 3

MILK SECRETION

Research In America Interesting Facts It is not entirely understood yet as to how milk is forced out of the udder. It has been proved, without any question that the time of milk secretion is all during the interval between milking. There is no sudden, rapid secretion during the milking act. Experimental work at Washington and at the University of Minnesota has shown that all of the milk that is drawn at a milking is present in the gland at the beginning of the milking. One interesting thing that has been found out by slaughtering cows at the regular milking time, then milking out the glands after they were removed, was that these post mortem milks were all normal for the constituents except the milk fat, and, according to Petersen, invariably the milk drawn from the udder after it was cut off from the cow just slaughtered was lower in fat than normally, and this is laid to the hypothesis that milk fat is secreted independently of the other ingredients of the milk. Apparently a cow does not have any voluntary control over whether she will let down her milk or not. The mammary nerve trunk has been cut off before it entered the udder and, apparently, this has no appreciable effect on the letting down of the milk. If the cow could control this voluntarily, then the milking of an udder after the mammary nerve trunk was cut off would be profoundly affected by such an operation. Petersen advances the theory that the “letting down” of the milk is due to the action of some hormone upon the muscle fibres of the gland. The theory that has been advanced is that the handling of the udder, or any of the other things that excite the production of milk at milking time, sends a stimulant to the pituitary gland, and that the pituitary gland produces a hormone that has an action upon the muscle fibres of the udder and that the muscle fibres express the milk out of the cells in which it is secreted. Milk True Secretion In the studies that have been made of milk secretion, four theories have been advanced: (1) filtration theory; (2) cell degeneration theory; (3) true secretion theory; (4) a combination of two of the above. Petersen in his book has come to the conclusion that milk is secreted in the cell, for the most part, and that the constituents from which milk is secreted are brought to the cells by the blood. Some things are evidently filtered directly from the blood into the cells lining the cavities of the udder, which are called the alveolar cells. For example, one of the proteins of the milk, the globulin of the milk, is identical with the globulin of the blood. Pressure will cause a decrease in the secretion of the milk. When a cow is giving a large amount of milk and it is all secreted between milkings, if the milk is not removed from the udder it will cause a slight pressure and then it will be reabsorbed. This probably does not happen, except in cases of very large production, and this accounts for the fact that milking high producing cows more than twice a day will often bring about an increase in production. According to Petersen, in addition to the amount of water for maintenance of a cow, the additional water needed daily for milk production varies from 3.5 to 5.5 pounds of water for each pound of milk produced. Therefore, if a cow is giving 30 pounds of milk a day, just to manufacture the milk itself she must have from 100 to 150 pounds of water, in addition to what she needs for maintenance.