18 PSYCHOTHERAPY : SCIENTIFIC AND RELIGIOUS chap.
and carried to all parts of the body. Glands that act in this way are termed endocrine glands. The name ' ductless glands ' was formerly applied to them, since several of the more important endocrines have no outlets, discharging only into the blood, but it is now recognized that some glands with ducts have endocrine functions also. The glands definitely known to secrete hormones are the pituitary, the thyroid, the parathyroids, the adrenals, the pancreas and the gonads or sex glands. It is possible that the pineal and thymus bodies are also endocrines, and that the liver, the heart and the spleen also secrete hormones.
In earlier times many incorrect and often absurd conjectures were made as to their role in the body, the most frequent guess being that they were vestigial remnants of primitive organs, and no longer of any importance. The first experimental work with the endocrines was performed by Berthold in 1849 on the sex glands of chickens. Since then research has progressed rapidly, especially in the present century, but much is still undiscovered.
More is known about the functions and disorders of the thyroid gland than about the other endocrines. The thyroid consists of two lobes which straddle the trachea in the base of the neck. The thyroid hormone, thyroxin, has been isolated and even prepared synthetically. It is of very complex structure and contains about 60 per cent, of iodine. Disorders of the thyroid are especially prevalent in certain regions in which the natural waters are lacking in iodine content (such as the Great Lakes region of the United States, Tibet, and some parts of Switzerland). Secretions of endocrine glands are produced in minute amounts, and very small concentrations are effective in performing the necessary functions. The total amount of thyroxin present in the blood at any one