however: another portion of the hypothalamus, when stimulated, actively inhibits eating by promoting a feeling of satiety. In experimental animals, damage to this portion of the brain is associated with continued excessive eating, eventually leading to obesity.

In addition to these numerous functions, there is evidence that the hypothalamus plays a role in the induction of sleep. For one thing, it forms part of the reticular activating system, the physical basis for that hard-to-define state known as consciousness (about which more later); for another, electrical stimulation of a portion of the hypothalamus has been shown to induce sleep in experimental animals, although the mechanism by which this works is not yet known. In all, the hypothalamus is a richly complex cubic centimeter of vital connections, which will continue to reward close study for some time to come. Because of its unique position as a midpost between thought and feeling and between conscious act and autonomic function, a thorough understanding of its workings should tell us much about the earliest history and development of the human animal.

The pituitary and the pineal glands function in close association with the hypothalamus. The pituitary responds to signals from the hypothalamus by producing an array of hormones, many of which regulate the activities of other glands: thyroid-stimulating hormone, adrenocorticotropic hormone (which stimulates an outpouring of epinephrine in response to stress), prolactin (involved in the production of milk), and the sex hormones follicle-stimulating hormone and luteinizing hormone, which promote the development of eggs and sperm and regulate the timing of ovulation. The pituitary gland also produces several hormones with more general effects: human growth hormone, melanocyte-stimulating hormone (which plays a role in the pigmentation of skin), and dopamine, which inhibits the release of prolactin but is better known as a neurotransmitter (see Chapter 5 ).

The pineal gland produces melatonin, the hormone associated with skin pigmentation. The secretion of melatonin var-