626 



PHYSIOLOGY OF GONADS 



cases restored lactation for up to 2 days 

 after it had ceased as a result of lesions of 

 the cord which would suggest a primary 

 interference with milk ejection. In the goat, 

 Andersson (1951b) considered that stim- 

 uli may reach the hypothalamus by way of 

 the medial lemniscus in the medulla, but 

 little definite information is available con- 

 cerning the pathways used by the stimuli 

 to reach the hypothalamus and there is here 

 scope for further investigations. (For fur- 

 ther discussion see review by Cross, 1960.) 

 From the hyopthalamus there is little doubt 

 that the route to the posterior lobe is by 

 way of the hypothalamo-hypophyseal tract 

 which receives nerve fibers from the cells in 

 the hypothalamic nuclei, and in the main 

 from the paraventricular and supra-optic 

 nuclei. It was generally assumed that the 

 posterior lobe hormones were secreted in 

 the posterior lobe from the pituicytes in re- 

 sponse to stimuli passing down the hypo- 

 thalamo-hypophyseal tract. In the last dec- 

 ade, however, much evidence has come to 

 light which suggests that the so-called pos- 

 terior lobe hormones are in fact elaborated 

 in the cells of the hypothalamic nuclei and 

 are then transported down the axones as a 

 neurosecretion and stored in the posterior 

 lobe (see Scharrer and Scharrer, 1954). 



Before leaving the neural pathways of the 

 milk-ejection reflex, brief reference must be 

 made to the recent discovery by Soviet phys- 

 iologists that there is also a purely nervous 

 reflex (segmental in nature) involved in the 

 ejection of milk. It is said that within a few 

 seconds of the application of the milking 

 stimulus, reflex contraction of the smooth 

 muscle in the mammary ducts occurs, caus- 

 ing a flow of milk from the ducts into the 

 cistern. This reflex contraction of the smooth 

 muscle is also believed to occur in response 

 to stimuli arising within the gland between 

 milkings thus aiding the redistribution of 

 milk in the udder. This purely nervous reflex 

 is stated to occur some 30 to 60 seconds be- 

 fore the reflex ejection of milk from the alve- 

 oli by oxytocin (for further details sec 

 review by Baryshnikov, 1957). The condi- 

 tioned reflexes associated with suckling and 

 milking have been the subject of numerous 

 investigations l)y Grachev (see Grachev, 



1953, 1958) ; these and other Russian re- 

 searches into the motor apparatus of the ud- 

 der have been fully reviewed by Zaks 

 (1958). 



G. MECHANISM OF SUCKLING 



In the past, various theories have been 

 put forward as to how the suckling obtains 

 milk from its mother's mammary gland. In 

 the human infant some considered that the 

 lips formed an airtight seal around the nip- 

 ple and areola thus allowing the child to 

 suck, whereas others believed that compres- 

 sion of the lacteal sinuses between the gums 

 aided the expulsion of the milk (see Ardran, 

 Kemp and Lind, 1958a, b for review) . In the 

 calf the act of suckling was studied by 

 Krzywanek and Briiggemann (1930) who 

 described how the base of the teat was 

 pinched off between upper and lower jaws 

 and the teat compressed from its base to- 

 wards its tip by a stripping action of the 

 tongue. Smith and Petersen (1945) on the 

 other hand, concluded that the calf wrapped 

 its tongue round the teat and obtained milk 

 by suction. 



Much misunderstanding about the nature 

 of the act of suckling has arisen because the 

 occurrence of milk ejection was overlooked 

 or its significance was not appreciated. As a 

 result, the idea became prevalent that suc- 

 cess or failure in obtaining milk could be 

 reckoned solely in terms of the power behind 

 the baby's suction. This erroneous concept 

 was vigorously attacked by Waller (1938), 

 who pointed out that once the "draught" 

 had occurred the milk at times flowed so 

 freely from the breast that the baby had to 

 break off and turn its head to avoid choking. 

 A similar observation had been made by Sir 

 Astley Cooper in 1840 who in describing the 

 "draught" in nursing women wrote, "If the 

 nipple be not immediately caught by the 

 child, the milk escapes from it, and the child 

 when it receives the nipple is almost choked 

 l)y the rapid and abundant flow of the fluid; 

 if it lets go its hold, the milk spurts into the 

 infant's eyes." An even earlier comment was 

 made by Soranus, a writer on paediatrics in 

 the cai'ly half of the second century A.D., 

 that it was unwise to allow the infant to fall 

 asleep at the breast since the milk some- 



