THE PHYSIOLOGY OF LACTATION 37 



There are evidently many links missing in the chain of our 

 knowledge. Milk secretion is a complicated process, and the 

 small positive knowledge we possess may be summed up in the 

 following manner: Epithelial cells take up material from the cir- 

 culation and transform it into milk constituents, then discharge 

 the altered substances into the lumina of alveoli, and here the 

 secretion is worked over into the finished product. The cells con- 

 tribute from the chromatin and protoplasm part of their sub- 

 stance to the secretion and ultimately disappear. Part of the body 

 fluid enters the gland ducts, which become distended and attract 

 leukocytes, which, in turn, aid in the regeneration of cells partly 

 by furnishing food and partly by removing degenerated cell sub- 

 stance. With the progress of lactation the leukocytic infiltration 

 increases again, and at the end of lactation cell products are reab- 

 sorbed. 



Theories of Lactation 



The appearance of milk in the mammary glands has been as- 

 cribed to two influences. The first of these is the increased amount 

 of blood in the circulation available after the discharge of the 

 fetus. The second is the presence in the blood of particular sub- 

 stances which favor milk formation. 



The first influence is due to the fact that a large amount of 

 blood is diverted into normal channels after the expulsion of the 

 placenta. It is commonly assumed that increased blood-supply 

 augments secretion from all glands, including the mammary glands. 

 However, the importance of this influence is diminished when 

 we remember that the blood diverted from large tumors after 

 removal by operation has not been known to increase glandular 

 secretions. Furthermore, milk has been actually secreted after 

 premature birth when the fetus was not sufficiently developed to 

 demand a large blood-supply, although in such cases the amount 

 of milk secreted is smaller than under normal conditions. 



The particular substances in the blood which favor milk forma- 

 tion may be of a stimulating nature or they may be converted by 

 the mammary gland into milk. The theory is sometimes sug- 

 gested that the mammary gland is endowed with the ability to 

 secrete at all times. It has never been possible, however, to 

 demonstrate a secretion in the rudimentary gland of a fetus. 

 This gland is the so-called milk line. Gaines reported postmortem 

 examinations of two kids whose mother died two weeks before 

 term, and although he found the mammary tissue plainly devel- 

 oped, no milk could be detected. He adds, however, that chem- 

 ical examination for milk-sugar was not made, and that this might 

 have shown the presence of milk in an amount too small to be dis- 



