The Hypothalamus

The hypothalamus, located in the brain directly above the pituitary, is known to exert control over it by means of neural connections and hormone-like substances that are called releasing factors, the means by which the nervous system controls sexual behavior via the endocrine system.
Sexual behavior is influenced by the hypothalamus. It stimulates the pituitary gland to release the sex hormones. When levels of those hormones falls, so does sexual desire.
The hypothalamus secretes the appropriate releasing factor into the blood, which reaches the pituitary and stimulates it to secrete gonadotrophic hormone.
In the female, the target gland of the gonadotrophic hormone is the ovary. The ovary has two functions, the first one is to produce eggs,