fluid-filled vesicular follicles. As the sexual cycle continues, one of the follicles grows much larger than the rest. Ovulation occurs, when this enlarged follicle releases its ovum into the pelvic cavity (1:609).
The length of the human sexual cycle is approximately 28 days. Traditionally, the first day of menstrual bleeding is designated as the first day of the cycle. Moreover, the cycle is subdivided into the follicular phase (prior to ovulation) and the luteal phase (after ovulation). These two phases are of approximately equal length (5:1971-1980). Ovulation normally occurs around the fourteenth day (1:609).
The central nervous system influences female reproductive system function. Through the secretion of gonadotropin-releasing hormones into the pituitary portal circulation, the hypothalamus plays "a limiting role in the control of gonadotropin secretion (5:1971-1980)." The anterior pituitary secretes both FSH and LH. During the first half of the sexual cycle, FSH initiates the growth of ovarian follicles. These growing cells then begin to secrete estrogens. The estrogen has multiple effects. For one, it has a negative feedback effect on the anterior pituitary. It inhibits the secretion of both FSH and LH such that their levels decline around the tenth day of the cycle (1:622-623). Second, estrogen also influences the uterus. By stimulating cellular proliferation and growth, the hormone initiates a severalfold thickening of the uterine lining, or endometrium.
Once the follicles grow to about half their maximum size, estrogen ceases to have a negative feedback effect on the pituitary. In c
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