REGULATION OF ENVIRONMENT 63 



their specific gravity equal to that of the water at 

 whatever depth they may be, or even to counterbal- 

 ance the effects of a float or weight attached to them. 

 I have in my library Ludwig's copies of Moreau's 

 papers. They are an interesting clue to what was 

 passing through his mind in suggestions he made as 

 to the possibility of oxygen secretion in the lungs. It 

 was discovered by Bohr that the oxygen secretion in 

 the swim bladder is, like salivary secretion, under 

 nervous control ; and Dreser found that oxygen secre- 

 tion can be excited by pilocarpin, a drug which also 

 excites secretion in other glands. 



The cells in the wall of the swim bladder which 

 secrete the oxygen are columnar, and arranged like 

 the cells of many other secreting glands, whereas the 

 lung epithelium is extremely thin. Nevertheless the 

 elementary structure of the lung is glandular, just as 

 in the case of the swim bladder; and both lung and 

 swim bladder are developed as outgrowths of about 

 the same part of the alimentary canal. Before the 

 lungs expand at birth the lung epithelial cells are cubi- 

 cal, and similar to those of other secreting glands. 

 That the secreting cells should be thicker in the swim 

 bladder is natural considering the enormously greater 

 pressure against which the cells have to secrete. 



The pressure difference against which oxygen can 

 be secreted in the lungs is evidently quite limited. This 

 is shown by measurements of the oxygen pressures in 

 the blood in CO poisoning, wnen the stimulus to secre- 

 tion is pushed up to what is presumably a maximum. 

 If there were no limit the secretion of oxygen would 



