Significance of the Chemical Composition of the Secreting and Dry Mammary Gland to Milk Secretion

The results herein presented furnish exact critical evidence for one more stage in milk secretion. Cows producing up to 30 pounds of milk at one milking are shown to have the lactose equivalent of all this milk in the udder when milking commences. The average excess of lactose found in the udder after subtracting the amount necessary for the contained milk is equal to 2.1 pounds. This represents the milk retained in the udder when the cow is believed to be dry. These conclusions are further supported by the fact that no sugar is found in the udder in the quiescent state. The study of the total composition of the udder as fat, ash, nitrogen, and lactose, and of the contained milk shows that there is a large excess of fat, ash, and nitrogen in proportion to that necessary for milk formation. The excess of udder lactose over the milk lactose is much less. The lactose would therefore appear to be formed from some element in the blood, probably dextrose, only as needed for the formation of milk. The composition of the dry udder is quite different in certain respects from that of the actively secreting gland. It builds up a fat reserve of a quite different Reichert-Meissl number from that of butter-fat. It has no sugar, its ash content is reduced, and the nitrogen content is like that of the secreting gland.