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The thymus lies in the upper part of the thoracic cavity (see figure 10.1 and table 10.2). It is important in the function of the immune system. The thymus secretes a hormone called thymosin (thı̄′ mō-sin), which aids the development of white blood cells called T cells. T cells help protect the body against infection by foreign organisms. The thymus is most important early in life; if an infant is born without a thymus, the immune system does not develop normally, and the body is less capable of fighting infec-tions .
The pineal (pin′ ē-ăl; pinecone) gland is a small, pinecone-shaped structure located superior and posterior to the thalamus of the brain . The pineal gland produces a hormone called melatonin (mel-ă-tōn′ in), which is thought to decrease the secre-tion of LH and FSH by decreasing the release of hypothalamic-releasing hormones (see table 10.2). Thus, melatonin inhibits the functions of the reproductive system. Animal studies have demonstrated that the amount of available light controls the rate of melatonin secretion. In many animals, short day length causes an increase in melatonin secretion, whereas longer day length causes a decrease in melatonin secretion. Some evidence suggests that melatonin plays an important role in the onset of puberty in humans. Tumors may develop in the pineal gland, which increase pineal secretions in some cases but decrease them in others.
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