MAMMARY GLAND AND LACTATION 



627 



times flowed without suckling and the infant 

 choked. It must thus be emphasized that 

 once milk ejection has occurred the milk in 

 the gland cisterns or sinuses is under con- 

 siderable pressure and the suckling has 

 merely to overcome the resistance of the 

 sphincters in the nipple or teat to obtain the 

 milk. 



Recently the use of cineradiograjihy has 

 allowed a more accurate analysis of the 

 mechanism of suckling. Studies by Ardran, 

 Kemp and Lind (1958b) have shown that 

 the human infant sucks the nipple to the 

 back of the mouth and forms a "teat" from 

 the mother's breast; when the jaw is raised 

 this teat is compressed between the upper 

 gum and the tip of the tongue resting on 

 the lower gum, the tongue is then applied 

 to the lower surface of the "teat" from be- 

 fore backwards pressing it against the hard 

 palate. Suction may assist the flow of milk 

 so expressed from the nipple, but is only of 

 secondary importance. Studies by Ardran, 

 Cowie and Kemp (1957, 1958) in the goat 

 have extended these observations, because 

 it was possible in this species to follow the 

 withdrawal, from the udder, of milk made 

 radiopaque with barium sulfate. As with 

 the infant, the neck of the teat was obliter- 

 ated between the tongue and the palate of 

 the kid and the contents of the teat sinus 

 were displaced into the mouth cavity by a 

 suitable movement of the tongue; while 

 the first mouthful w^as being displaced into 

 the pharynx, the jaw and tongue were low- 

 ered to allow the refilling of the teat sinus. 

 The normal method of obtaining milk is, 

 therefore, for the suckling to occlude the 

 neck of the teat and then to expel the con- 

 tents of the teat sinus by exerting positive 

 pressure on the teat (120 mm. Hg in the 

 goat), so forcing the contents through the 

 teat canal or nipple orifices into the mouth 

 cavity, a process which may be aided by 

 negative pressure created at the tip of the 

 teat. Human infants, goat kids, and calves 

 can obtain milk through rubber teats by 

 suction alone provided the orifice is large 

 enough (see Krzywanek and Briiggemann, 

 1930; Martyugin, 1944; Ardran, Kemp and 

 Lind, 1958a) , but this procedure occurs only 

 w^hen the structure of the rubber teat is such 

 that the suckling is unable to ol)literate the 



neck of the teat and cannot, therefore, strip 

 the contents of the teat by positive pressure. 



V. Relation between the Reflexes Con- 

 cerned in the Maintenance of Milk 

 Secretion and Milk Ejection 



We have seen that the suckling or milk- 

 ing stimulus is responsible for initiating the 

 reflex concerned wath the maintenance of 

 milk secretion and also the milk-ejection re- 

 flex; the question now arises as to what ex- 

 tent their arcs share common paths. It 

 would seem logical to assume that a common 

 path to the hypothalamus exists and parts 

 of this, as we have seen, have been partially 

 elucidated. Although the hypothalamo-hy- 

 pophyseal nerve tracts provide an obvious 

 link between hypothalamus and the pos- 

 terior lobe, the connections between the hy- 

 pothalamus and anterior pituitary are still 

 a matter of some controversy. The possible 

 avenues of communication to the anterior 

 lobe are neural and vascular and these may 

 be subdivided into central and peripheral 

 neural connections and into portal and sys- 

 temic vascular connections. The various ex- 

 perimental findings relating to these routes 

 have recently been critically discussed by 

 Sayers, Redgate and Royce (1958), and by 

 Greep and Everett in their chapters in this 

 book, and it is clear that at present no defi- 

 nite conclusions can be reached concerning 

 their relative importance. So far as the spe- 

 cific question of maintenance of milk secre- 

 tion is concerned, the experiments of Harris 

 and Jacobsohn (1952), which showed that 

 pituitary grafts maintained lactation when 

 implanted adjacent to the median eminence 

 in hypophysectomized rats, were consistent 

 with the existence of a hormonal transmit- 

 ter, passing by w^ay of the hypophyseal por- 

 tal system. On the other hand, transplanta- 

 tion studies by Desclin (1950, 1956) and 

 Everett ( 1954, 1956) have revealed that in 

 the rat the anterior lobe can spontaneously 

 secrete prolactin in situations remote from 

 the median eminence, and Donovan and van 

 der Werff ten Bosch (1957) have reported 

 that milk secretion continued in rabbits in 

 wiiich the pituitary portal vessels had been 

 completely destroyed, although there was, 

 however, an inferred change in milk compo- 

 sition. Evidence has recentlv been obtained 



