Internal secretions, using the expression in its general sense, are necessary for the co-ordination of the work of the various parts of the animal mechanism. Colloid is the internal secretion of the thyroid gland and of the parathyroids. Unlike most other internal secretions, it is a substance easily analysed, and startlingly definite in its chemical characteristics. It is composed mainly of a protein which contains iodine. From this protein a substance termed “iodothyrin” may be obtained, of which no less than 10 per cent. is iodine; but it is uncertain whether iodothyrin is preformed in the gland. The exact nature of the active substance of the internal secretion of the thyroid gland matters little. Whether it be iodothyrin or a protein, its activity depends upon the fact that it contains iodine in large quantity. Iodine amounts to from 0·3 per cent. to 0·9 per cent. of the weight of the whole thyroid gland in Man. Iodine is very widely distributed in Nature. It is present in the air, in rain-water, in herbage. It is also present in all parts of the body, although in quantities which are infinitely minute. It is found in sea-water, and is relatively abundant in marine plants. There is no reason for supposing that it is deficient in districts in which _goitre_ is common. It would appear more likely that the soil has properties which result in the fixing of iodine in a form in which it is not available for plant-food, and that in consequence animals are unable to obtain a sufficient supply. Careful analyses have shown that the thyroid glands of sheep bred in mountainous districts where goitre is common contain but one-thirtieth part as much iodine as the thyroid glands of sheep bred in places where goitre is rare. In ancient times burnt sponge and seaweed were esteemed useful in the treatment of goitre. Later, iodide of potassium given internally, and tincture of iodine as an outward application, were the approved remedies. It is now known that myxœdema and certain forms of goitre may be checked, and even cured, by administering uncooked thyroid gland or even tabloids of dried extract. Fortunately, it is not necessary to inject it subcutaneously; the iodine-containing compound is so stable as to resist the action of gastric juice. Iodine stored in the thyroid and parathyroid glands is distributed to all the tissues. The remarkable symptoms which indicate that the tissues are not receiving an adequate supply may occur under either of two conditions. Iodine may be deficient in the food, or the thyroid gland may be incompetent; the former is the commoner cause. And here we see the explanation of the formation of a goitre. By increasing the size of the organ which selects iodine, Nature attempts to obtain and store an adequate supply for distribution to the tissues.