Section 2. 

 The Secretion of Milk. 



As the period of parturition approaches, the mammary glands 

 become swollen owing to active changes occurring in them, 

 and at, or shortly after, the birth of the animal milk is formed. 

 The nature of the sympathy existing between the generative 

 apparatus and the mammary glands was, until recent years, 

 quite unknown. The experiments of Lane-Claypon and Starling* 

 have, however, indicated that the body of the foetus contains 

 a specific chemical substance (hormone), which, after absorption 

 into the maternal blood, stimulates the growth of the mammary 

 gland. This substance does not produce a secretion of milk ; 

 on the other hand, it inhibits it, and it is not until the womb is 

 empty — i.e., until after the birth of the animal — that, in conse- 

 quence of no further hormones being produced, the secretion of 

 milk begins. Other observers locate the seat of the hormone 

 responsible for the growth of the mammary glands in the corpus 

 luteum of the ovary, which, it will be remembered, grows to a 

 considerable size during the first part of pregnancy. The 

 essential features, however, are that a chemical substance is 

 responsible for the growth of the mammary glands during 

 pregnancy, and the withdrawal of this chemical substance from 

 the body at parturition is followed by a secretion of milk. 



Some evidence has been brought forward to show that the 

 mammary gland is under the control of the nervous system, but 

 the severance of all nervous connections does not stop secretion. 

 The gland has even been transplanted in the guinea-pig to the 

 neighbourhood of the ear, and functioned at the next parturition. 

 Nevertheless, in the absence of a nervous connection it is difficult 

 to explain the contractions which occur in a recently emptied 

 uterus during suckling. The normal growth of the gland, apart 

 from pregnancy, is regulated by the ovaries. These produce a 

 substance which is carried by the blood, and leads to mammary 

 growth. When the ovaries are removed, the glands atrophy. 

 If the ovaries be removed from the cow when in full milk, lacta- 

 tion is maintained for a considerable period — it is said for as long 

 as two or three years. 



The stimulus to milk secretion, when once established, is 

 suckling. In the absence of this, the glands after a day or two 



* Proceedings of the Royal Society, B, vol. lxxvii., 1906. 

 725 



