192 BIOLOGICAL CHEMISTRY 



and overgrowth of mucoid tissue under the skin were compared 

 with similar symptoms resulting from removal of the thyroid 

 gland. Experiments showed that removal of the thyroid 

 glands causes mental dullness and a tendency to deposit 

 mucin in the subcutaneous tissues,* that these symptoms 

 can be prevented by grafting the gland into other portions of 

 the body or by feeding with the fresh or dried gland. f 



Various conditions, such as cretinism (congenital) or 

 myxcedema (acquired), are associated with deficient activity 

 of the thyroid gland, whilst exophthalmic goitre, which is 

 characterised by excessive nervous activity, is associated with 

 increased activity of the thyroid. The former conditions are 

 relieved by feeding with thyroid glands, and the symptoms 

 of the latter can be produced by excessive doses of the 

 same. 



The effect of removal of the thyroid varies with the species J 

 and age of the animal. In some cases death occurs, whilst 

 in others the results are very slight. 



Associated with the thyroid are structures termed para- 

 thyroids. It is claimed that removal of the thyroid causes 

 death when the parathyroids are also removed, but not when 

 the parathyroids are uninjured. 



The thyroid glands contain spaces filled with a material 

 which stains uniformly with acid dyes, and it has been given 

 the non-committal name of " colloid." The thyroid gland, 

 unlike other tissues, contains appreciable amounts of iodine 

 in organic combination, and this iodine is said to be connected 

 with the active substance of the gland. 



Thyreoglobulin is a protein which can be isolated from 

 the thyroid gland : it contains iodine and has the curative 

 effect of thyroid tissue. || 



The thyroid gland is thus seen to contain a hormone which 

 stimulates metabolism, causing increased mental and physical 

 activity. The active substance is Associated with the presence 

 of a compound containing iodine. 



Quite recently Kendall has isolated a crystalline iodine 

 containing a compound which he calls thyroxin,^ and to which 

 he ascribes the tautomeric formulae : 



* Schiff, Arch. f. exper. Path. u. Pharm., 1884, vol. 18, p. 25. 

 f H. W. Mackenzie, Brit. Med. Journ., 1892, vol. ii, p. 940, and 

 E. F. Fox, ibid., p. 941. 



| V. Horsley, Brit. Med. Journ., 1892, vol. i, pp. 215 and 265. 



V. Horsley, Proc. Roy. Soc., 1886, vol. 40, p. 6. 



I! A. Oswald, Zeit. physiol. Chem., 1899, vol. 27, p. 14. 



T| E. C. Kendall, Journ. Biol. Chem., 1919, vol. 39, p. 125. 



