Up till the year 1883 the function of the thyroid gland was unknown, and considered to be of slight importance, at least to the adult animal. The theories concerning its function were— (1.) The one propounded by Mr. Simon, “ Phil. Trans.,” 1844, &c., viz., that the thyroid body acted as a regulator of the circulation in the brain, and possibly manufactured some substance which was of primary importance for the nutrition of the central nervous system.