872 PHYSIOLOGY OF DIGESTION AND SECRETION. 



Per cent, of iodin. 



Children's thyroid none. 



Maltese kid thyroid none. 



Guinea-pig thyroid 0.05 



Dog thyroid 0.061 



Cat thyroid 0.08 



Sheep thyroid 0.176 



Beef thyroid 0.25 



Hog thyroid 0.33 



Human (Wells) 0.236 



Human (goitre) 0.04 



Opinions in regard to the significance of the iodin have varied 

 from the view, on the one hand, that it is an essential constituent 

 of the physiologically active substance secreted by the gland, to 

 the opposite extreme that it is an injurious substance which is 

 bound and made innocuous by the thyroid cells. The balance 

 of evidence seems to favor the first point of view,* and at present 

 we may conclude that the iodin in some way intensifies the activity 

 of the internal secretion of the thyroid. That it is absolutely 

 necessary to this activity is rendered improbable by the fact that 

 iodin-free thyroids appear still to exercise their normal influence 

 upon metabolism, but administration of iodin in the food not 

 only raises the iodin percentage in the gland, but also increases 

 proportionately the physiological activity of extracts of the tissue. 

 Experiments show also that the known effects of thyroid extracts 

 are greater in the iodin-rich than in the iodin-poor glands. 



Thymus. The physiology of the thymus gland is very obscure, 

 in fact, nothing that is definite can be said about its functions, ex- 

 cept perhaps that the gland is concerned in some way with the 

 processes of growth. It is assumed that it furnishes an internal 

 secretion, and that in early life at least this secretion influences in an 

 important way some phases of the body metabolism. Formerly, 

 it was supposed that the gland reaches its maximum size at birth 

 and afterward undergoes a process of atrophy or involution so that 

 it is entirely absent in adult life. More careful observations indi- 

 cate, on the contrary, that the gland retains its size and presumably 

 its full activity until the period of puberty. Thenceforward it 

 does undergo a gradual atrophy, but apparently throughout life 

 some remnants of the gland tissue persist embedded in fat. It 

 appears also that under pathological conditions there may be a 

 persistence of more of this tissue than is normal, or there may be 

 a real hypertrophy together with an overactivity, a condition 

 which might be designated as hyperthymusism. It is recognized 

 now that this activity may be a complicating factor in exophthal- 



* For discussion and literature, consult Hunt, "Studies on Thyroid," 

 "Hygienic Laboratory Bulletin," 1909, No. 47, Washington, D. C.; and 

 Hunt and Seidell, "Journal of Pharmacology and Experimental Therapeutics," 

 2, 15, 1910. 



