144 PHYSIOLOGY FOR NURSES 



words the fetus is, practically, a part of the mother's 

 body. 



Parturition is the act of bringing the child into the 

 world. The "term of pregnancy" is about ten lunar 

 months (280 days) from the time of the last menstrua- 

 tion. Delivery is accomplished by involuntary contrac- 

 tion of the uterine muscle fiber aided by voluntary con- 

 traction of the diaphragm and the muscles of the 

 abdominal wall. The cause of the uterine contractions 

 is unknown, though possibly a hormone produced by 

 the mammary gland may excite them. 



The mammary gland begins to enlarge and form 

 secreting alveoli shortly after pregnancy is established 

 and towards the end secretes a small amount of a fluid 

 called colostrum. After delivery the gland is actively 

 stimulated and rapidly enlarges. For a few days the 

 secretion retains the character of colostrum, but, usually 

 on the third or fourth day, the flow of milk has become 

 well established. 



There is some evidence that the fetus forms a hormone 

 which stimulates growth of the mammary gland, but 

 inhibits its secretion. This substance is withdrawn when 

 the fetus is born and its removal permits the functional 

 activity of the gland. 



That the mammary gland is under the control of the 

 central nervous system is indicated by the effects of 

 various emotions upon its secretion; but there are no 

 known secretory nerves, though there are vasoconstric- 

 tor and vasodilator fibers as in the other glands. 



The male cell which fertilizes the ovum is secreted by 

 the testicle, an organ analogous to the ovary of the 

 female. 



