400 PEACTICAL PHYSIOLOGY 



determined the tensions of C0 2 and 2 to be as follows in per cent, 

 atmosphere : 



Tissues. Venous blood. Alveolar air. External air. 



C0 2 5-9 > 3-81-5-4 > 2-8 > 0'03 



Arterial blood. 



2 < 14 < 18 < 20-94 



Bohr, on the other hand, with another form of aerotonometer, found 

 that the tension of 2 in the blood is higher than in the bronchial air, 

 and that the tension of C0 2 may be higher in the bronchial air than 

 in the blood. ' Haldane confirms Bohr as regards 2 . 



The exchange of gases, if these observations are correct, must be due 

 'to secretory activity of the lungs or more probably of the blood. In 

 the swim-bladder of the fishes taken from great depths (4000 feet) 

 gas has been found containing about 90 per cent, of oxygen. The 

 oxygen in the bladder at this depth must be at a pressure of over 

 100 atmospheres, while in the sea water the partial pressure of 2 is 

 only about 0*2 atmosphere. The swim-bladder and the pulmonary 

 epithelium are both developed from the alimentary canal. 



CHAPTER XLIII. 

 THE EFFECTS OF CHANGES IN ATMOSPHERIC PRESSURE. 



Decreased Atmospheric Pressure. A mouse and a frog are placed 

 under the bell glass of the air pump. A side tube is connected with 

 a mercury manometer. The latter must be long enough to indicate 

 the pressure of the atmosphere. On lowering the pressure J-J- of the 

 atmospheric pressure the mouse is asphyxiated, while the frog is un- 

 affected. The effect of lessening the atmospheric pressure depends entirely 

 on the partial pressure of oxygen. The normal pressure of 2 is 20-94 

 per cent, of an atmosphere. At 10 per cent, of an atmosphere there 

 arises restlessness and dyspnoea, and at about 7 per cent., death. A 

 partial pressure of 2 = 7 per cent, of an atmosphere corresponds to an 

 altitude of 30,000 feet. Death from want of oxygen is common in foul 

 wells, mines, etc., where c choke-damp ' collects. 



Increased Atmospheric Pressure. A curarised frog, with the brain 

 pithed, is placed in the high-pressure chamber, the web of one foot is 

 spread out on a wire ring beneath one of the glass observation discs. 

 The apparatus is screwed up and connected with an oxygen cylinder. 

 The circulation in the web is observed with a microscope using an inch 



