MAMMARY GLAND 145 



by a hormone produced in the corpus luteum ("yel- 

 low body" in the ovary that grows for some time 

 after impregnation of the ovum). . . . The sec- 

 ond stage, associated with secretory activity in the 

 later period of pregnancy, is independent of the 

 corpus luteum. It has been shown by Mackenzie 

 that the gland is not under the influence of the 

 nervous system, but that extracts of various or- 

 gans, injected into the blood current of a cat in 

 lactation, cause secretion of milk. 



"The organs found active were the pituitary 

 body, the corpus luteum, the pineal body, the in- 

 voluting uterus (the return of the uterus to normal 

 size after child is born), and the mammary gland 

 itself. The pituitary body is by far the most active. 

 The fetus (the child in the womb after the third 

 month. Before that time it is called the embryo 

 and placenta (organ in uterus that establishes con- 

 nection between mother and child) produce hor- 

 mones which inhibit the gland. 



"Further analysis of the action of pituitary ex- 

 tract was made by Hammond. The effect is said 

 not to be due to pressing out of milk by contraction 

 of muscle in the ducts . . . the daily yield of goats 

 was found to be only slightly increased by injec- 

 tions, so that pituitary extract seems to act by 

 setting free the constituents of milk, rather than 

 by causing increased formation. . . ." 



