LACTATION 



793 



presence of the fetus and a change in the number or quantity of hormones 

 developed during gestation. Full-term pregnancy occurs when the uterus is 

 isolated from the nervous system, hence it has been inferred that there is 

 some sort of special secretion or hormone possibly of the embryo itself, that 

 makes its way into the blood and influences the organs of the mother. 



At the end of the period of pregnancy the strong uterine walls begin 

 periodic contractions which ultimately result in the delivery of the fetus. 

 These contractions are at first weak and at long intervals, but gradually 

 become very strong and even violent and follow each other in rapid succes- 

 sion. The uterine contractions are supported by reflex contractions of the 

 abdominal and thoracic muscles. After the fetus is delivered the uterine 

 contractions become milder, but still continue until the placenta is finally 

 expelled. 



FIG. 508. 



FIG. 509. 



FIG. 508. Section of Mammary Gland of Bitch, Showing Acini, Lined with Epithelial 

 Cells of a Polyhedral or Short Columnar Form. X 200. (V. D. Harris.) 

 FIG. 509. Globules and Molecules of Cow's Milk. X 400. 



The initiation of the contractions of the uterus at delivery probably de- 

 pends on the chemical stimulation of some substance or substances produced 

 in the uterus itself or in the fetus; substances that react on the nervous mech- 

 anism and on the uterine muscles themselves. This view cannot be said to 

 be proven, but it is supported by certain observed facts and experiments. 



Lactation. There is a marked development of the mammary glands, 

 during the period of gestation, especially in the later weeks. No milk is 

 secreted during this time and the change is sharply limited to the processes of 

 growth in the glands. Upon delivery of the fetus the gland immediately 

 enlarges very sharply and an abundant secretion is formed. During gesta- 

 tion it is thought that an inhibitive hormone prevents actual secretion. 

 After parturition these hormones are removed and secretion begins under the 

 stimulative influence of definite hormones from the corpus luteum, the invo- 

 luting uterus, and the mammary gland tissue, see Chapter on ductless glands. 



