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A TEXTBOOK OF PHYSIOLOGY 



is drawn by the cork to the surface, and the other by the lead to the 

 bottom. Next day they have adjusted their specific gravity by means 

 of their swim-bladders, and are swimming about easily. On removing 

 the lead, that fish irresistibly floats to the surface; on removing the 

 cork, the other one sinks to the bottom. They have again to adjust 

 their swim-bladders. If goldfish in water are placed in a pressure 

 chamber, and suddenly compressed, they sink to the bottom, owing 

 to the shrinkage of the gas in the bladder. If a fish is hooked in deep 



Fid. 143. FlSH BROUGHT UP FROM A CONSIDERABLE DEPTH WITH SWOLLEN SwiM- 



BLADDER PROJECTING FROM MOUTH. (After Regnard.) 



water, and started on the way up, the gas in the bladder expands, 

 and the fish floats to the surface, and the bladder often bursts 

 (Fig. 143). There is a glandular mechanism in the swim-bladder for 

 secreting gas, and another mechanism for absorbing it. 



Against the theory of pulmonary secretion is the fact that the 

 pulmonary endothelial cell is a flattened structure entirely unlike the 

 granular secreting cells typical of glands. 



Evidence in favour of pulmonary secretion has been sought by 

 a method quite different to that of the aerotonometer. If blood is 

 shaken with air containing, say, 0-05 per cent, of carbon monoxide, 

 the haemoglobin is shared between the oxygen and carbon monoxide 



