SECRETION OF THE DUCTLESS GLANDS. 869 



that in a case of exophthalmic goiter the heat loss per square 

 meter of skin was 75 calories, as compared with the normal of 

 39.7 calories. The internal secretion or hormone produced by the 

 thyroid acts as a stimulant to the general metabohc processes of 

 the body. Efforts to discover the active principle or hormone of 

 the thyroid secretion have been successful. Baumann (1896) 

 isolated a substance which he designated as iodothyrin that shows 

 in large measure the therapeutic value of thyroid tissue, and is 

 characterized chemically by its large content of iodin. More 

 recently Kendall* has succeeded in obtaining a crystalline body of 

 known composition which seems to possess the full physiological 

 activity of the thyroid tissue. He gives to this substance the name 

 of thyroxin. Chemically, it is described as a tri-hydro tri-iodo 

 oxyindol propionic acid, CuHioOsNIa, or 



HI 

 C 

 /\ 

 HIC C = C-CH>CH2C00H 



HIC C C = 



This substance is very active physiologically. As little as 1 milli- 

 gram given to an adult will cause an increase of 2 per cent, in the 

 body metabolism. Kendall believes that the thyroxin is con- 

 stantly secreted and that it serves as a regulator of the body 

 metabolism. The presence of iodin in the thyroid gland has long 

 been known, and it will be noted that it forms a part of the iodo- 

 thyrin of Baumann and the thyroxin of Kendall. There has been 

 much discussion as to whether the physiological action of the 

 thyroid secretion is dependent upon the iodin, that is, whether 

 the iodin is an essential constituent of the hormone. Since other 

 organic iodin compoimds have no such physiological effect, and 

 since in some entirely healthy animals iodin may be lacking in 

 the gland or be present only in traces, it would seem that the 

 action of the hormone is not an iodin effect. The presence of the 

 iodin probably increases the reactivity of the thyroxin, but is not 

 absolutely essential (Kendall). When solutions of thyroxin are 

 injected into human beings it has been shown (Plummer) that the 

 physiological effect develops slowly during a number of days and 

 persists for a long time. Evidently, therefore, the specific hor- 

 mone of the thyroid tissues, if it is thyroxin or resembles thyroxin, 

 is a stable body which is destroyed or removed very slowly. 



Thymus. ^The physiology of the thymus gland is very ob- 

 scure, in fact, nothing that is definite can be said about its func- 



* Kendall, "Endocrinology," 2, 81, 1918, and 3, 156, 1919. Kendall and 

 Osterberg, "Journal of Biological Chemistry," 40, 265, 1919. 



