248 A. T. CAMEKON 



average amount present is of a lower order than that of iodin, and its 

 presence in the gland is probably without significance. Numerous experi- 

 ments have been carried out on the effect of bromid medication (cf. Wolff 

 and Opp, who give a complete account of the literature) and the general 

 consensus of opinion is that the bromid ion acts passively in replacing the 

 chlorid ion. 



Our knowledge of fluorin distribution is based on the work of Gautier 

 and Clausmann. Their method is probably the only accurate one, but it is 

 difficult and tedious and has not been tested by other workers. They con- 

 clude from their results that fluorin is localized in a specific fashion, 

 accompanies phosphorus, and is concerned with phosphorus metabolism. 

 The amount in thyroid (0.002 per cent, dried tissue) is of the same order 

 as that in most of the body tissues. 



lodin, therefore, is the only halogen present in marked amount in thy- 

 roid tissue, and in thyroid tissue only, in vertebrates. 



The lodin Content of the Normal Thyroid Gland. lodin has been 

 found in the thyroids of five species of elasmobranch, and one of the holo- 

 cephaloid fishes, of one amphibian, two reptilians, six species of birds, and 

 fourteen species of mammals. Wherever accurate methods have been 

 employed the minimal figure found for dried tissue was at least 0.01 per 

 cent. The following maxima have been observed (figures in percentages, 

 dry tissue) : fish (Scyllium), 1.15 (Cameron (a), 1913); birds (scoter), 

 1.14 (Cameron, 1915) ; sheep, 1.05 (Hunter and Simpson) ; dogs, 0.69 

 (Marine and Lenhart, 1909); man, 0.59 (Seidell) ; stags, 0.54 (Blum, 

 1899) ; pigs, 0.53 (Seidell and Fenger, 1913) ; cattle, 0.48 (Marine and 

 Lenhart (&) (c)). 



Most of the investigations for human thyroids have been obtained for 

 tissue from individuals dead through various causes, which conceivably 

 may have affected the gland though it appeared normal. Recently Zunz 

 (1919) has determined the content in glands from normal men killed in 

 war. His extremes are 0.044 to 0.426 per cent, dried tissue (0.011 to 

 0.121 fresh tissue). The whole thyroid contained from 0.00315 to 0.0449 

 gram of iodin, the mean being 0.01553 gram. His figures indicate that 

 the thyroids of normal individuals have a much higher iodin content than 

 those of individuals dead from disease. 



The following table illustrates some further points in connection with 

 the distribution of iodin in the thyroids of different species. The figures 

 have been selected as far as possible from data for large numbers of 

 normal individuals, whose thyroids were collected under similar conditions. 



These figures for iodin content are derived from the following 

 sources: for fish, birds, guinea pigs and clogs, Cameron, (1914, 1915) ; for 

 American hogs, sheep (the percentage of iodin is much lower than the 

 yearly average), and cattle, Fenger (1912, 1914); for English sheep, 

 Martin (a) (b) ; for man, Baumann (6) (1896) ; for normal man, Zunz. 



