620 



PHYSIOLOGY OF GONADS 



15 30 



60 90 120 



Time (min) 



150 



Fig. 10.17. Effect of various concentrations of 

 insulin on the respiratory metabolism of slices 

 of rat mammarj' glands. (From J. H. Balmain, C. P. 

 Cox, S J. Folley and M. L. McNaught, J. Endo- 

 crinol., 11, 269-276, 1954.) 



portion of the milk secreted by the alveohir 

 cells in the intervals between suckling or 

 milking remains within the alveoli and the 

 fine ducts. Only a small portion passes into 

 the larger ducts and cisterns or sinuses from 

 which it can be immediately removed by 

 suckling, milking, or cannulation; its re- 

 moval requires no maternal participation 

 and has been termed passive withdrawal 

 (see Cowie, Folley, Cross, Harris, Jacob- 

 sohn and Richardson, 1951, and page 612). 

 The larger portion of the milk in the alveoli 

 and fine ducts becomes available only with 

 the active participation of the mother and 

 requires the reflex contraction of special cells 

 (see page 623) surrounding the alveoli in re- 

 sponse to the milking or suckling stimulus 

 to eject the milk from the alveoli and fine 

 ducts into the cistern and sinuses of the 

 gland. The occurrence of this reflex has long 

 been known, although its true nature has 

 only recently been generally recognized.^ 



-H. K. Waller {Clinical Slujlits un Lnrfallon, 

 London: Heinemann, 1938), and later one of us 

 (S. J. Folley, Physiology and Biochemistry of Lac- 

 tation, London and Edinburgh: Oliver & Boyd, 

 1956) have drawn attention to the fact that the 

 theme of the "milk-ejection reflex" was the inspira- 

 tion of a paiming by II Tintoretto entitled "The 

 Origin of the Milky Way" which hangs in the 



111 the past it has been termed the "draught" 

 in lactating women (see Isbister, 1954) and 

 the "let-down" of milk in the cow. The 

 latter term is particularly misleading since 

 it implies the release of some restraint, 

 whereas there is, in fact, an active and 

 forceful expulsion of milk from the alveoli 

 and we have, therefore, urged that this term 

 be no longer used in scientific literature and 

 that it be replaced by the term "milk ejec- 

 tion" (Folley, 1947; Cowie, Folley, Cross, 

 Harris, Jacobsohn and Richardson, 1951), 

 a term, incidentally, which was used by 

 Gaines in 1915 in his classical researches 

 on the phenomenon (see below j. 



The true nature of the milk removal pro- 

 cess was for many years not recognized, 

 probably because it was assumed that the 

 mammary gland could not contain all the 

 milk obtainable at a milking, and this as- 

 sumption made it necessary to postulate a 

 very active secretion of milk during suckling 

 or milking. Even as late as 1926 two phases 

 of milk secretion were described in the cow ; 

 the first phase was one of slow secretion 

 occurring between milkings, the second 

 phase was one of very active secretion oc- 

 curring in response to the milking stimulus 

 when a volume of milk about equal to that 

 produced in the first phase was secreted in 

 a matter of a few minutes (Zietzschmann, 

 1926). That some physiologic mechanism 



National Gallery, London. Both authors point out 

 tliat the picture shows evidence of a considerable 

 intuiti^■e understanding of the physiologic nature 

 of the milk-ejection reflex. Thus, it illustrates, first, 

 that the application of the suckling stimulus causes 

 a considerable increase in intranianiinai >• jiressure 

 resulting, in this instance, in a sjnni cii' milk from 

 the nipples, and second, that ihv Muklmg stimulus 

 applied to one nipple gives rise to a systemic rather 

 than a localized effect, for the milk is forcibly 

 ejected from the suckled and unsuckled breasts 

 ahke. The same theme was also treatetl by Rubens 

 in a picture called "The Birth of the Milky Way" 

 which can be seen in the Prado Museum, Madrid. 

 This picture differs from Tintoretto's in one im- 

 portant detail, the stream of milk coming only from 

 one breast. 



The forcible ejection of milk from the nipple has 

 doubtless been the subject of many statues. An ex- 

 ample known to the authors is the fountain in the 

 Sfiuare at Palos Verdes, near Los Angeles, Cali- 

 fornia. The center piece of this fountain has a nude 

 female torso at each of its four corners from whose 

 nipples spurt streams of water. 



