A Study of the Influence of Thyroxin on Milk Secretion

A knowledge of the physiological factors affecting milk secretion is essential to the study of the causes of variation in milk yield among dairy cows.It is reasonable to suppose that the activity of the mammary gland is a direct function of the general metabolism of the animal.If this supposition is correct, those factors that affect the metabolism should have a similar influence on milk yield.The stimulating effect of thyroxin and thryoid extracts upon the metabolic rate of animals has been well estabfished (1, 10, 11).Henderson (9) has studied the effects of variou~ stimulants and galactogogues over two day test periods, lVicCandlish (13, 14) studied the effect of drug administration on milk yield in a two day period.Ott and Scott (15, 16) injected animal extracts and observed the number of drops of milk secreted in a five minute period following injection.Changes brought about by the various treatments were not significantly large or of long enough duration that the data could be applied to the study of the problem of milk secretion.Turner (19) has shown that galactin, a hormone elaborated in the anterior pituitary gland, is able to initiate milk secretion in the developed mammary gland.Corner (2) has shown the positive effects of hypophyseal extracts in the hormonal control of lactation.Gowen and Tobey (5) found that injections of insulin and phloridzin caused a marked decline in milk yield.Grimmer and Wentzel (8) claim that thyroideetomised goats secreted milk at a rate comparable to that before thyroideetomy.Graham (6) has studied the effect of thyroidectomy and thyroid feeding on milk secretion and milk fat production of cows.He shows that thyroidectomy and a similar control operation without the thryoidectomy caused a rapid decline in milk secretion and that the effects of the thyroidectomy could not be distinguished ~rom those of the control operation.Thyroid feeding to thyroidectomised cows brought the milk yield level back to that before the operation.Thyroid feeding to normal cows caused a rise in