344 THE AMERICAN NATURALIST [Vol. XLIX 



the liver in the form of bile pigments, bnt the iron is con- 

 served for use in the formation of new hemoglobin. But 

 little iron is eliminated by the liver, although much iron 

 may be stored there. The milk of higher animals con- 

 tains but little iron, and the young mammal needs much 

 iron for the formation of new blood cells during the rapid 

 growth of early life. Provision is made for this by the 

 storage of iron in the liver of the fetus. As postpartum 

 growth proceeds, the amount of iron in the liver is re- 

 duced and the amount occurring in hemoglobin is in- 

 creased. 



8. The Internal Secretions 

 The internal secretions form another group of chemical 

 conditions which vary within narrow limits in a state of 

 health. Many years ago Caspar Friederich Wolff ex- 

 pressed the idea that each organ of the body stood in the 

 relation of an organ of internal secretion to some other 

 organ in the body. The liver liberates sugar which is 

 necessary for the action of the muscles, and it also lib- 

 erates urea which has a definite relation to the action of 

 the kidneys. The muscles set free carbon dioxide which 

 acts upon a particular group of cells in the nervous sys- 

 tem, and nitrogenous waste products, some of Which are 

 transformed to urea in the liver. But apart from these 

 more general relationships, a system of ductless glands 

 sets free a variety of chemical substances which bear a 

 rather different relation to the development and activity 

 of many other organs and tissues of the body. 35 The 

 relation of the adrenal gland to the myo-neural junction 

 of sympathetic and smooth muscle fiber 36 is well known. 

 The dependence of the development of the secondary 

 sexual characters upon the internal secretion of the sexual 

 glands is also familiar to biologists. The internal secre- 

 tions in general have become a part of the internal con- 

 ditions of the organism and of the chemical environment 

 of the cells of the organism. 



35 Mathews, Science, 1897, N. S., V, pp. 683-685. 



s« Elliott, Journal of /'/,//*"<'"<///. 1 !"'.">, XXXTT. p. 401. 



