a diet to fit any particular individual can best be made under competent medical advice, since personal peculiarities have to be taken into account in selecting among the various foods those best adapted for the purpose. The variations in resting metabolism that fall under the head of disease are, first, the increase of metabolism in fever, about which we shall speak in detail in the next chapter, and, secondly, variations in metabolism that result from variations in the activity of the thyroid gland. The thyroid gland is an organ at the front of the neck; when it is enlarged, as it is in some people, we have the condition known as goiter. This gland is now known to manufacture and pour out into the blood a hormone which is a regulator of metabolism. When it is produced in normal amounts, the metabolism goes on at the rate that we find in healthy people. If the gland is inactive and does not secrete enough of the hormone, there is a reduction in the metabolism. Since this implies a lowering of the vigor of the life processes, we might expect it to have important effects. The most marked of these are in the nervous system. Persons whose thyroid glands are relatively inactive are mentally sluggish; the less active the gland is, the more marked is this sluggishness, and in case of practically complete absence of the hormone the condition amounts to idiocy. Occasionally a child is born without an active thyroid gland; this child is doomed for life, unless artificial aid can be procured, not only to complete idiocy but to all the other results of lowered metabolism; these show themselves in dwarfishness and a misshapen body. One of the conspicuous and beneficent discoveries of medicine in comparatively recent years has been that extracts of the thyroid glands of meat animals, when eaten by persons whose own thyroids are not sufficiently active, supply the lack, and so they may be restored to the normal condition. There are many people alive to-day who are in all respects normal, but who, if they were to discontinue taking thyroid extract, would relapse rapidly into a condition of idiocy.