350 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



glands. The ovarian cycle comes to a climax in ovulation, when one or more 

 eggs leave the ovaries. In rats and mice, the interval between ovulations is 

 four and a half or five days; in cattle, horses, and pigs, 25 days. Dogs breed in 

 early spring and fall, irregularly; cats in spring and early fall, sometimes more 

 often. In rabbits, cats, and dogs ovulation occurs only when induced by 

 copulation. 



The events of the 21 day estrous cycle in the pig may be taken as an exam- 

 ple of a cycle essentially similar to others. For two and a half weeks, the extent 

 of the diestrous period, the pig moves about, eats, and sleeps in apparent 

 satisfaction. Then, in the last three days of the cycle, the estrous or "heat" 

 period, she becomes restless and sexually excited. At the same time, special 

 activity is going on in the ovary. About two days before estrus begins a certain 

 few ovarian follicles grow rapidly and their cavities fill with fluid containing 

 estrogen. On the first day of estrus, they are fully mature. By the second day 

 the eggs have been forced out of the follicles and are in the oviducts, due to 

 meet the sperm cells. 



During the reproductive cycle there is a seesaw influence between the an- 

 terior lobe of the pituitary gland and the ovary. The follicle-stimulating hor- 

 mone of the pituitary excites the maturing ovarian follicles and their produc- 

 tion of estrogen. Estrogen stimulates the glands in the walls of the uterus and 

 regulates their blood supply, effects changes in the walls of the vagina and 

 mammary glands, and brings about the characteristic behavior of estrus. When 

 it reaches a certain level, it also inhibits the production of the follicle-stimu- 

 lating hormone and stimulates the production of the luteinizing hormone by 

 the pars anterior of the pituitary. Under the influence of these pituitary hor- 

 mones, ovulation occurs. Aided by another hormone of the pars anterior, 

 luteotrophin, the corpora • lutea, made from the emptied ovarian follicles, 

 secrete progesterone which causes further uterine secretion and growth. By 

 about the sixth day after ovulation, the corpora lutea produce their full quota 

 of progesterone. They continue for a time to make this secretion which further 

 stimulates the uteri (two uteri in the pig), provided embryos are developing 

 in them. Evidently the developing embryos contribute substances to the 

 mother's blood that support the corpora lutea. The placenta (Chap. 19) asso- 

 ciated with each embryo produces hormones that help to maintain the embryos 

 in the uteri and prevent more new eggs from maturing in the ovary. 



If the eggs are not fertilized, they degenerate, and phagocytic cells consume 

 them as in all mammals. On the fifteenth day after the last ovulations the 

 corpora lutea also degenerate and in consequence the activity and preparations 

 which they stimulated in the uterus likewise subside. Their control of young 

 ovarian follicles is lifted and on the nineteenth day after the ovulations, an- 

 other group of these enlarges, and another reproductive cycle is about to begin. 



