40 MILK 



Hormones differ from food substances in not being assimilable. 

 They are specific and consequently exert their stimulating influence 

 on particular glands. They do not provoke antibody formation 

 and are very stable, remaining iminjured by boiling, drying, or the 

 addition of alcohol. 



Theories Depending Upon the Presence of Food Substances in 

 the Blood. — Rauber observed that leukocytes pass from the 

 maternal circulation into the fetus, and since milk — especially 

 colostrum — contains large numbers of leukocytes, he concluded 

 that the food of the suckling is a direct continuation of the proc- 

 ess of feeding the fetus. In other words, the food of the fetus after 

 expulsion is diverted to the breast and there transformed into 

 milk. Therefore, the author reasons that the food of fetus and 

 suckling come from the same source. This theory explains in a 

 measure the fact that milk flow commences with the birth of the 

 child, that is to say, the food substances that were used by the 

 fetus before birth become available after birth for milk secretion. 

 There is a gradual adaptation of the mother to the needs of the 

 infant, and this adaptation results in the formation of food sub- 

 stances. The mammary gland has the ability to produce mi lk 

 even before birth, but lacks the necessary material. Schein holds 

 that the hormones of other authors are really the substances the 

 transformation of which results in secretion of the mammary 

 glands. Furthermore, the quantity of food needed by the fetus is 

 constantly increasing in proportion to its growth, and the mother 

 has to furnish this increase. But after birth the direct influence 

 of the productiveness of the mother ceases, and consequently there 

 is no increase after the flow of milk has commenced. 



This theory does not presume the presence of hormones. It is 

 conceivable, however, that the food substances stimulate the 

 gland to action by their presence, and on this assumption the two 

 theories are not necessarily contradictory. The second theory 

 does not explain the growth of the mammary gland previous to 

 birth, and by combining both ideas the facts find a harmonious 

 explanation. 



The Secretion of Milk 



Sucking of the infant is a downward movement of the lower 

 jaw, followed by closing the jaws. Suction is thus produced and 

 the milk is forced from the breast by negative pressure. The 

 pressure caused by the first sucking movements is not suflficient 

 to release much milk, but with each movement the pressure in- 

 creases, until the milk flows more freely. Gaines has shown that 

 m a dog the milk flow increased for the first three minutes, then 

 declined, and was exhausted after six minutes. With bottle-fed 



