1322 



HORMONAL REGULATION OF BEHAVIOR 



by Cowie and Folley, and we need not con- 

 sider them exhaustively here. However, it 

 will be useful to consider certain aspects of 

 the physiology of milk production which 

 directly influence, or are influenced by, the 

 behavioral interaction between mother and 

 young. 



The viUk-eiection reflex. When a young 

 mammal suckles, and thus withdraws milk 

 from the mammary gland of its mother, 

 only a small portion of the milk present in 

 the mammary gland at the beginning of 

 the nursing episode is available near the 

 nipple so that it can be withdrawn by the 

 young without active participation on the 

 part of the mother. Most of the milk can- 

 not be extracted without some active par- 

 ticipation by the mammary gland, which 

 normally occurs in response to the suckling 

 stimulus (Folley, 1956; Harris, 1958). This 

 active participation of the mammary gland 

 in the ejection of milk, in response to a 

 suckling stimulus, is called "the milk-ejec- 

 tion reflex" (Cowie, Folley, Cross, Harris, 

 Jacobsohn and Richardson, 1951). 



The phenomenon of milk ejection has 

 long been known among dairy workers, by 

 whom it has been called "milk let-down" 

 (Ely and Petersen, 1941), a term referring 

 to the sudden flow of milk which occurs 30 

 to 90 seconds after the beginning of suckling 

 (Harris, 1958). This is commonly observed 

 in the milking of dairy cows, and has been 

 described as occurring in the domestic rab- 

 bit (Cross, 1952), in which the suckling 

 young are veiy active for 30 to 90 seconds 

 after the beginning of suckling, then sud- 

 denly become motionless, simultaneously 

 with the beginning of a characteristic gulp- 

 ing sound which indicates that substantial 

 milk flow has begun. By removing the pup- 

 pies from the nipples of a lactating dog once 

 every minute during a nursing episode, re- 

 placing them with other puppies, and then 

 weighing the puppies which had just been 

 allowed to suckle, Gaines (1915) was able 

 to estimate the amount of milk produced 

 by the lactating bitch during each minute 

 of the suckling episode. He found that the 

 flow of milk was very slow during the first 

 minute or so, but then quickly increased. 

 These observations make it clear that the 

 flow of milk during nursing is in part an 

 active response of the mammary gland to 



the suckling stimulus, with a latency of 30 

 to 90 seconds. 



Gaines (1915) allowed puppies to suckle 

 an anesthetized lactating bitch, and found 

 that they could not withdraw milk from 

 the mammary gland. Similarly, Cross and 

 Harris (1952) anesthetized a lactating fe- 

 male rabbit, and found that suckling young 

 could obtain no milk from her even after 6 

 minutes of vigorous suckling, although in 

 normal nursing most of the milk is with- 

 drawn within the first 5 to 6 minutes (Cross, 

 1952). The milking of amputed cow udders 

 yields considerably less than half the 

 amount which can be got from the same 

 udders, containing the same amount of 

 milk, in the intact cow (Hammond, quoted 

 by Harris, 1958) . It is clear that not much 

 milk can be withdrawn, even by normal 

 suckling behavior, from an inactive, non- 

 participating mammary gland. 



Tgetgel (1926) measured the pressure of 

 milk in the cistern of the mammary gland 

 of a cow (see chapter by Cowie and Folley) , 

 and showed that it rose gradually between 

 one milking episode and the next. This is, 

 of course, a consequence of the gradual in- 

 crease in the amount of milk present in the 

 gland as milk is produced in the alveoli 

 (milk-secreting tubules) to replace that 

 withdrawn at the last milking episode. 

 However, in addition to this gradual rise in 

 pressure between milking episodes, Tgetgel 

 noted an abrupt increase in intramammary 

 pressure as a response to the application of 

 the milking stimulus, even when it was ap- 

 plied shortly after milking when the over- 

 all pressure in the gland was low. Gaines 

 (1915) had earlier made similar observa- 

 tions on the goat. This rise in milk pressure 

 tends to squeeze the milk out through the 

 nipple. Normally, the nipple's sphincter 

 offers resistence to this pressure, which must 

 be overcome by the mouth of the suckling. 

 However, if a cannula is inserted through 

 the nipple opening, the milk can actually 

 l)e seen to spurt out of the nipple (Usuelli 

 and Plana, quoted by Folley, 1956). 



This increase in intramammary pressure 

 no doubt constitutes the milk-ejection re- 

 flex. 



Mechanism of milk ejection. What stim- 

 ulates the mammary gland to eject milk 



