MAMMARY GLAND AND LACTATION 



621 



was involved in the discharge of preformed 

 milk from the mammary gland had, how- 

 ever, been recognized. Schafer (1898) con- 

 sidered that milk discharge was aided by 

 contraction of plain muscle w^ithin the 

 gland and pressure on the alveoli produced 

 by vasodilation. 



The first full investigation of the physi- 

 ology of milk removal was that by Gaines 

 in 1915. Unfortunately, his remarkably ac- 

 curate observations and perspicacious 

 conclusions aroused little general interest 

 and were almost wholly overlooked for 

 more than quarter of a century. It is now 

 of interest to recall the more important of 

 Gaines' observations. First, he made a clear 

 distinction between milk ejection and milk 

 secretion — "Milk secretion, in the sense 

 of the formation of the milk constituents, 

 is one thing; the ejection of the milk from 

 the gland after it is formed is quite another 

 thing. The one is probably continuous; the 

 other, certainly discontinuous." Secondly, 

 he concluded that "Nursing, milking and the 

 insertion of a cannula in the teat, excite a 

 reflex contraction of the gland musculature 

 and expression of milk. There is a latent 

 period of 35 to 65 seconds. . . . Removal of 

 milk from the gland is dependent on this 

 reflex, and it may be completely inhibited 

 l)y anaesthesia. The conduction in the reflex 

 arc is dependent upon the psychic condition 

 of the mother." He also observed that the 

 increased flow of milk following the latent 

 period after stimulation was associated wath 

 a steep rise in pressure within the gland 

 cistern and that the reflex could be condi- 

 tioned. Thirdly, with reference to the gland 

 capacity, he reported that "the indication 

 is that practically the entire quantity of 

 milk obtained at any one time is present 

 as such in the udder at the beginning of 

 milking." Lastl3^ he confirmed earlier ob- 

 servations that injections of posterior pitui- 

 tary extract caused a flow of milk in the 

 lactating animal and he postulated that 

 "pituitrin has a muscular action on the ac- 

 tive mammary gland causing a constriction 

 of the milk ducts and alveoli with a con- 

 sequent expression of milk. This action 

 holds, also, on the excised gland in the 

 absence of any true secretory action." Gaines 

 regarded the milk-ejection reflex as a 



l)urely neural arc although he emphasized 

 that the effect was "very similar to that 

 produced by pituitrin." All that is required 

 to bring these views of milk ejection in line 

 with present day concepts is to recognize 

 that the reflex arc is neurohormonal in char- 

 acter, the efferent component of which is 

 a hormone released from the neurohypoph- 

 ysis. When Gaines was carrying out these 

 experiments hardly anything was known of 

 neuro-endocrine relationships and there was 

 no background of knowledge to lead anyone 

 to conceive that the effects of the posterior 

 pituitary extract might represent a physio- 

 logic rather than a pharmacologic effect. 

 In 1930 Turner and Slaughter hinted at 

 a possible physiologic role of the posterior 

 pituitary in milk ejection and, as we have 

 noted (page 610), Gomez (1939) used pos- 

 terior pituitary extract in replacement ther- 

 apy given to hypophysectomized lactating 

 rats. It was not until 1941, however, that 

 the role of the posterior pituitary in milk 

 ejection was seriously postulated by Ely 

 and Petersen (1941) who, having shown in 

 the cow that milk ejection occurred in the 

 mammary gland to which all efferent nerve 

 fibers had been cut, suggested that the reflex 

 was neurohormonal, the hormonal compo- 

 nent being derived from the posterior pitui- 

 tary, and being, in all likelihood, oxytocin. 

 The neurohormonal theory of Ely and Peter- 

 sen and the subsequent work of Petersen and 

 his colleagues (see reviews by Petersen, 

 1948; and Harris, 1958), unlike the earlier 

 work of Gaines, aroused wide interest and its 

 practical applications permitted rationaliza- 

 tion of milking techniques in the cowshed 

 thereby improving milk yields. Despite the 

 attractiveness of the concept, however, a 

 further 10 years were to elapse before un- 

 equivocal evidence of the correctness of the 

 theory was forthcoming and this evidence 

 we shall now briefly review. 



B. ROLE OF THE NEUROHYPOPHYSIS 



The first reliable indication that the 

 suckling or milking stimulus does in fact 

 cause an outpouring of neurohypophyseal 

 hormones were the observations that in- 

 hibition of diuresis occurred following the 

 application of the milking or suckling 

 stimulus (Cross, 1950; Peeters and Cous- 



