MAMMARY GLAND AND LACTATION 



625 



mother. Many years later Ely and Petersen 

 (1941) confirmed this and, having shown 

 that injections of adrenaline blocked the 

 milk-ejection reflex, postulated that the in- 

 creased blood level of adrenaline in emo- 

 tionally disturbed cows interfered with the 

 action of oxytocin. In the last few years, the 

 nature of the inhibitory mechanisms has 

 been more fully investigated. Braude and 

 Mitchell (1952) showed in the sow that 

 adrenaline exerts at least part of its inhibi- 

 tory effect at the level of the mammary 

 gland and that, whereas the injection of 

 adrenaline before the injection of oxytocin 

 blocked milk ejection, less inhibition oc- 

 curred if both were given together. Cross 

 (1953, 1955a) confirmed these observations 

 in the rabbit and demonstrated that electri- 

 cal stimulation of the posterior hypothala- 

 mus (sympathetic centers) inhibited the 

 milk-ejection response to injected oxytocin, 

 an effect which was abolished after adrenal- 

 ectomy. Cross concluded from his experi- 

 ments that any central stimulation causing 

 sympathetico-adrenal activity inhibits the 

 milk-ejection response and that the effect 

 appears to depend on a constriction of the 

 mammary blood vessels resulting from the 

 release of adrenaline and excitation of the 

 sympathetic fibers to the mammary glands. 

 Whereas such a mechanism could account 

 for the emotional disturbance of the reflex. 

 Cross was careful to point out that there 

 was no direct proof that this was so and he 

 later demonstrated (Cross, 1955b) that in 

 rabbits in which emotional inhibition of 

 milk ejection was present, milk ejection 

 could be effected by the injection of oxy- 

 tocin (Fig. 10.19). In such cases there was 

 clearly no peripheral inhibitory effect of 

 milk ejection. Cross concluded that the main 

 factor in emotional disturbance of the milk- 

 ejection reflex is a partial or complete in- 

 hibition of oxytocin release from the pos- 

 terior pituitary gland. At present nothing is 

 known of the nature of this central inhibi- 

 tory mechanism.^ 



^ A curious form of the suckling stimulus is il- 

 lustrated in carvings which siumount the main door 

 of the church of Sainte Croix in Bordeaux. The 

 carvings illustrate penances prescribed for wrong 

 doers who have committed one of the seven deadly 

 sins. The penance for indulgence in the sin of luxiu y 

 is the application to the breasts of serpents or toads. 



Inhibition of the milk ejection reflex may 

 also occur when the mammary gland be- 

 comes engorged with secretion to such an 

 extent that the capillary circulation is so re- 

 duced that oxytocin can no longer reach the 

 myoepithelium (Cross and Silver, 1956; 

 Cross, Goodwin and Silver, 1958). 



F. NEURAL PATHWAYS OF THE 

 MILK-EJECTION REFLEX 



Interpretation of some of the earlier 

 studies on neural pathways is difficult be- 

 cause investigators did not realize that, al- 

 though the milk ejection reflex normally 

 occurs in response to the suckling stimulus, 

 it can become conditioned and can then oc- 

 cur in response to visual or auditory stimuli 

 associated with the act of nursing. In such 

 cases an apparent lack of effect on milk 

 ejection of section of nerves or nerve tracts 

 would not necessarily imply that the nerves 

 normally carrying the stimuli arising from 

 the suckling had not been cut. Studies on 

 the effects of hemisection of the spinal cord 

 in a few goats led Tsakhaev (1953) to the 

 conclusion that the apparent pathway used 

 by the milk-ejection stimulus was un- 

 crossed. More recently pathways within the 

 spinal cord have been investigated by Eayrs 

 and Baddeley (1956) who found inter alia 

 that lactation in the rat was inhibited by 

 lesions to the lateral funiculi, and by section 

 of the dorsal roots of nerves supplying the 

 segments in which the suckled nipples were 

 situated. With few exceptions hemisection 

 of the spinal cord abolished lactation when 

 the only nipples available for suckling 

 were on the same side as the lesion, but not 

 when the contralateral nipples were avail- 

 able. It was concluded that the pathway 

 used by the suckling stimulus enters the 

 central nervous system by the dorsal routes 

 and ascends the cord deep in the lateral 

 funiculus of the same side. Inasmuch as in 

 these experiments lactation was assessed 

 from the growth curve of the pups, it is not 

 always clear whether the failure of lactation 

 was due to a cessation of milk secretion or to 

 loss of the milk-ejection reflex. It was noted, 

 however, that injections of oxytocin in some 



It may be questioned whether this unusual form of 

 the suckling stimulus would not inhibit rather than 

 evoke the milk-ejection reflex. 



