MAMMALIA— PHYSIOLOGY 439 



lodin. — It has long been known that iodin is extremely important 

 in connection with the normal functioning of the thyroid gland 

 and that the latter is a powerful regulator of metabolism. It in- 

 creases the rate of oxidation. Minute quantities of iodin are neces- 

 sary, the human thyroid containing about one-fifteenth of a grain 

 of iodin, but lack of the essential amount may cause a disease known 

 as cretinism in which bodily and mental development are both 

 subnormal. Iodin is an important constituent of vegetable foods 

 and is found in large quantities in the plants and the animals of the 

 ocean. In all probability it is in part responsible for the preserva- 

 tion of the carnivorous Eskimos of Greenland. It has been suc- 

 cessfully used in the treatment of pernicious anemia, given as 

 thyroid extract. 



Recent discoveries would seem to indicate that the iodin-fat 

 balance is important in several of the so-called vitamins, and that 

 the effectiveness of cod liver oil in certain diseases is really due to 

 the very small quantities of iodin present. The author of this text 

 would also like to link the iodin-fat balance with his own theory 

 of Fish Migration, since it is possible that the eel, fattened in fresh 

 water, may need iodin to induce development of its gonads, while 

 the anadramous fishes, like the salmon, possibly need to seek water 

 of lower iodin content and finally to go into fresh water before their 

 iodin-fat balance is reached, and eggs mature. (See page 263.) 



R. McCarrison (1919) and later the Mellanbys (1921) have 

 emphasized the importance of iodin-fat balance in goiter, but until 

 certain studies on the Vitamins were made (Chidester, Eaton, 

 Thompson, Speicher, and others, 1928-) the significance of their 

 finding was not appreciated. 



Arsenic^ found in the earth as a sulphide, occurs in minute 

 amounts in animals and vegetables. Gautier and more recently 

 Bertrand have emphasized the importance oi arsenic as a constituent 

 of the living cell. The yolk of the hen's egg usually contains twice 

 as much arsenic as the white. Arsenic enters into the composition 

 of the epidermis and its appendages, the thyroid and the mammary 

 glands, and the central nervous system. Arsenic resembles phos- 

 phorus in its chemical affinities. It is reported that the Tyrolean 

 mountain climbers habituate themselves to quite considerable doses 

 of arsenic, for its energizing effects. 



Iron is an important constituent of both plants and animals. It 

 Is necessary for the formation of chlorophyll In plants. Without 



