880 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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.* The active 

 substance in the thyroid secretion is an iodin compound, the 

 efficacy of which is dependent not upon the iodin constituent 

 alone, since other organic iodin compounds, iodin-protein com- 

 plexes, for example, are inactive, but upon the special form in 

 which it is combined. 



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- 

 mic goiter (Graves' disease). The anatomical facts in regard to 

 the involution of the gland after puberty justify the suggestion 

 that the function it exercises is of especial importance in the period 

 preceding the maturation of the sexual glands, but that in post- 

 pubertal life it continues to play some r61e, although of subordinate 

 importance. Very many experiments have been made to deter- 

 mine the nature of the function of this tissue, but at present it is 

 not possible to interpret the results in a satisfactory manner, f 

 Removal of the gland in young dogs (Basch) is said to cause a 

 retarded growth of the bony tissues and to induce a condition re- 

 sembling rickets. At the same time the peripheral nervous sys- 



1910, and Kendall, 1916, loc. cit. 



t References: Friedleben, "Die Physiologic der Thymusdriise," 1858: 

 Verdun "Derives branchiaux chez les vertebres," 1898; Henderson, "Journal 

 of Physiology " 1904, xxxi., 222; Basch, "Jahrbuch f. Kinderheilkunde," 64, 

 1906, and 68/1908; Klose and Vogt, "Klinik u. Biologie d Thymusdriise " 

 Tubingen 1910; Halsted, "Johns Hopkins Hospital Bulletin," August, 1914; 

 Hart, "Virchow's Archiv," 214, 1, 1913. 



