MAMMARY GLAND AND LACTATION 



611 



mq qro\Om hormone (onthmeTTc scale) 



fa-25 12-5 25-0 50-0 



100-0 



200-0 



S-«^0 



'Zoo-o 



Fig. 10. IL Effect of graded doses of growth hormone on milk yield of row. Upper curve, 

 doses plotted on arithmetic scale. Lower curve, doses plotted on logarithmic scale. (From 

 J. B. Hutton, J. Endocrinol., 16, 115-125, 1957.) 



C)ther hormones of the anterior pituitary 

 in all probability influence milk secretion 

 through their target glands and these will 

 be dealt with later. 



3. Suckling Stimulus and the Maintenance 

 of Lactation 



It has been long believed that regular 

 milking is an important factor in main- 

 taining lactation and that if milk is allowed 

 to accumulate in the gland, as occurs at 

 weaning, atrophy of the alveolar epithelium 

 and glandular involution occur. Evidence 

 in support of this concept was obtained in 

 studies showing that ligature or occlusion of 



the main ducts of some of the mammae of a 

 lactating animal resulted in atrophy of the 

 glands concerned although the other glands 

 were suckled normally (Kuramitsu and 

 Loeb, 1921; Hammond and Marshall, 1925; 

 Fauvet, 1941a). Studies by Selye and his 

 colleagues, however, revealed that such 

 occluded glands did not atrophy as quickly 

 as did glands of animals in which the suck- 

 ling stimulus was no longer maintained 

 (Selye, 1934; Selye, Collip and Thomson, 

 1934) and it was postulated that the suck- 

 ling stimulus evoked from the anterior 

 pituitary the secretion of prolactin which 

 maintained the secretory activity of the 

 gland. This theory has been widely accepted 



