RESPIRATION 



271 



The results obtained by the microtonometer support the view 

 that the exchange of gases in the lungs is brought about by the process 

 of diffusion, and not by active secretion. The oxygen passes from 

 the alveolar air, where its pressure is higher, into the arterial blood, 

 where its pressure is lower. The carbon dioxide passes from the 

 venous blood, where its pressure is higher, into the alveolar air, where 

 its pressure is lower; and the pressure of carbon dioxide in the arterial 

 blood leaving the alveolus is higher, or at any rate not lower, than that 

 in the alveolar air. 



There is nothing inherently improbable in the conception of the 

 lung as a gas-secreting organ. Gas is secreted by several aquatic organ - 



FIG. 142. FIVE GAS-SECRETING CELLS FROM THE GAS GLAND IN B THE SWIM-BLADDER^ 

 OF THE PARADISE FISH Macropodus V iridi-auratus. (Redrawn after Reis and 

 Nusbaum from Dahlgren and Kepner.) 



b, Thickened distal border of cells on the lumen; vac., gas-vacuoles ; (r., trophospongia, 

 the organs concerned in the elaboration of gas from the materials of the cell ; bl. ca., 

 blood capillary. 



isms for the purpose of notation (Fig. 142). The swim-bladdei^of the 

 fish is an organ developed, like the lung, as an outgrowth from the gut. 

 Gas is secreted in it, so as to render the specific gravity of the fish 

 equal to that of the surrounding water. . In fish at great depths, 

 the gas is compressed by -even hundreds of atmospheres of pressure, 

 due to the superincumbent water. The fish secrete oxygen gas against 

 this enormous pressure, and the swim-bladder is immune to oxygen- 

 poisoning. If the swim-bladder in a codfish is punctured, and the 

 gas drawn off, the bladder fills again; but this does not take place if 

 the vagus nerves be divided. Thus the secretion of the gas is con- 

 trolled by these nerves. Some authorities have sought evidence that 

 the gaseous exchange in the lungs is not only a process of secretion, 

 but one controlled by the vagus nerves. The function of the swim- 

 bladder is manifested by taking two goldfish, and fastening a piece 

 of cork to the dorsal fin of one, and a piece of lead to the ventral fin 

 of the other. Both are returned to a tall jar of water, and the one 



