994 THE PHYSIOLOGY OF REPRODUCTION. 



through the blood rather than through the nervous system. Some 

 direct evidence for this point of view is furnished by the interesting 

 experiments of Starling and Lane-Claypon.* These authors found 

 that extracts made from the body of the fetus, or rather from the 

 bodies of many fetuses, when injected repeatedly into a virgin rabbit 

 caused a genuine development of the mammary glands closely 

 simulating the growth that normally occurs during pregnancy. 

 Since similar extracts made from ovaries, placental and uterine 

 tissues had no effect, they conclude that a specific chemical sub- 

 stance (a hormone) is produced in the fetus itself and, after absorp- 

 tion into the maternal blood, acts upon the mammary gland, stim- 

 ulating it to growth. Since the birth of the fetus is followed by 

 active secretion in the mammary glands they adopt further the 

 view that this substance, while promoting the growth of the gland 

 tissue, inhibits the catabolic processes which lead to the formation 

 of the secretion. With the birth of the fetus this substance is 

 withdrawn and secretion begins, and, on the contrary, the secretion 

 is suspended when a new pregnancy is well advanced. Further 

 evidence of the same kind is furnished by the interesting case of the 

 Blazek sisters.f These twins had a common circulation but sepa- 

 rate nervous systems. Pregnancy and parturition in one was 

 followed by a secretion of the mammary glands of both. 



As was said in speaking of the histology of the gland, the se- 

 creting alveoli are not fully formed until the first pregnancy. Dur- 

 ing the period of gestation the epithelial cells multiply, the alveoli 

 are formed, and after parturition secretion begins. As the liquid 

 is formed it accumulates in the enlarged galactophorous ducts, and 

 after the tension has reached a certain point further secretion 

 is apparently inhibited. If the ducts are emptied, by the infant 

 or otherwise, a new secretion begins. The emptying of the ducts, 

 in fact, seems to constitute the normal physiological stimulus to 

 the gland-cells, but how this act affects the secreting cells, whether 

 reflexly or directly, is not known. 



Composition of the Milk. — The composition of milk is com- 

 plex and variable.* The important constituents are the fats, held 

 in emulsion as minute oil droplets, and consisting chiefly of olein 

 and palmitin; casein, a nucleo-albumin which clots under the in- 

 fluence of rennin; milk-albumin or lactalbumin, a protein resem- 

 bling serum-albumin; lactoglobulin ; lactose or milk-sugar; lecithin, 

 cholesterin, phosphocarnic acid, tirea, creatin, citric acid, enzymes, 



* Lane-Claypon and Starling, "Proceedings of the Royal Society," 1906, 

 B. Ixxvii.; see also Starling in "Lancet," 1905. 



t Basch, "Deut. Med. Wochenschrift," 36, 981, 1910. 



* For data as to composition and hygienic relations, see Bulletin 41, 

 "Hygienic Laboratory," PubUc Health and Marine Hospital Service, U. S., 

 Washington, 1908. 



