PARENTAL BEHAVIOR 



1323 



and how is this stimidation conveyed to the 

 ghxnd? 



The now generally accepted interpreta- 

 tion of the physiologic basis of the milk- 

 ejection reflex was first suggested by Ely 

 and Petersen (1941j. They found that mo- 

 tor denervation of one half of the udder of 

 a lactating cow had no effect on either lac- 

 tation or milk ejection. Further, injection 

 of oxytocin, which is normally produced by 

 the posterior lobe of the pituitary gland, 

 caused milk ejection in cows that were not 

 being milked or suckled. They therefore 

 suggested that the sensory side of the milk 

 ejection reflex consists of stimulation of the 

 nipple by the suckling stimulus, whereas 

 the motor side consists of the release of oxy- 

 tocin by the posterior lobe of the pituitary 

 gland, which, when it reaches the mammary 

 gland in the circulating blood, directly 

 causes an increase in intramammary pres- 

 sure and the subsequent ejection of milk. 



Andersson (1951a, b) demonstrated that 

 milk ejection in sheep and goats can be in- 

 duced by direct stimulation of the anterior 

 hypothalamus by implanted electrodes. 

 Cross and Harris (1951, 1952) obtained 

 similar results in the rabbit. They showed 

 that milk ejection occurred in response to 

 electrical stimulation of the supra-optico- 

 hypophyseal tract, and that electrolytic le- 

 sions in this region interfered with its oc- 

 currence. Lesions in the dorsal and posterior 

 hypothalamus did not have this effect (see 

 also Shimizu, Ban and Kurotsu, 1956 ». 

 Cross and Harris (1951) also found milk 

 ejection following electrical stimulation of 

 the pituitary stalk. Benson and Cowie 

 ( 1956) found that removal of the posterior 

 lobe of the pituitary gland abolished the 

 milk ejection response, and that later, after 

 hypertrophy and reorganization of the cut 

 end of the neural stalk, the response was 

 restored. These observations provide con- 

 firmation of Ely and Petersen's (1941) hy- 

 pothesis that the ejection of milk is stimu- 

 lated in the mammary gland by a hormone 

 secreted by the posterior pituitary. Further 

 evidence is found in the fact that direct 

 stimulation of the hypothalamus increases 

 milk ejection in denervated udders as well 

 as in udders whose neural connection to the 

 central nervous system is intact (Anders- 

 son, 1951a). Even more dramatic is the fact 



that an isolated perfused mammary gland 

 ejects milk when perfused with blood from 

 a cow that has just been suckled, and does 

 not do so when perfused with blood from a 

 cow that was not ejecting milk at the time 

 the blood was withdrawn (Petersen and 

 Ludwick, 1942). 



Cross (1950, 1951) found that suckling 

 in rabbits caused an inhibition of urine flow 

 similar to that caused by the injection of 

 posterior pituitary extracts. Andersson and 

 McCann (1955) observed that milk ejec- 

 tion elicited by hypothalamic stimulation 

 in the goat is accompanied by this anti- 

 diuretic effect. Hawker and Roberts 

 (quoted by Harris, 1958) assayed the blood 

 in the jugular veins of goats and cows be- 

 fore, during, and after milking. They found 

 the amount of oxytocic hormone in the 

 blood of goats to be higher during milking 

 than before or after. In cows the level of 

 oxytocic hormone activity rises some min- 

 utes before the beginning of milking, pre- 

 sumably in response to stimuli associated 

 with the preparations for milking. 



It is abundantly clear that milk ejection 

 is stimulated by posterior pituitary sub- 

 stances secreted into the blood in response 

 to suckling stimulation. It is beyond the 

 scope of this chapter to consider in detail 

 which of the substances secreted by this 

 gland is the milk ejection hormone, but 

 overwhelming evidence suggests that it is 

 oxytocin (see chapter by Cowie and Folley; 

 Folley, 1956; Harris, 1958). We may ask 

 how this hormone induces milk ejection. 



The walls of the milk-secreting alveoli in 

 the mammary gland of the goat contain a 

 network of contractile myo-epithelial fibers, 

 and Richardson (1949) demonstrated that 

 these contract in response to local electrical 

 stimulation, resulting in contraction of the 

 alveoli. Cross (1954) found that a mechani- 

 cal tap on the mammary gland of a rabbit 

 in the neighborhood of a milk-distended 

 lobule causes a sharp rise in the intralobu- 

 lar pressure. The latency of this rise in 

 pressure is very short, on the order of 1 

 second compared with 30 to 90 seconds for 

 the milk-ejection reflex. Gaines (1915) had 

 found that the butting of the mammary 

 gland by the kid contributed to the effi- 

 ciency of milk ejection in the goat, and it 

 may be that the local response of the myo- 



