564 THE RESPIRATORY EXCHANGE 



dition of expiratory apnoea may be produced, even if hydrogen 

 be used to distend the lungs ; by " negative " ventilation the 

 diaphragm is thrown into a condition of continued contraction. 

 In this way it is maintained that the activity of the respiratory 

 centre under normal conditions is regulated, not by the venosity 

 of the blood, but reflexly by impulses passing up the vagi. 



There is no doubt that the wave of opinion has gone too far ; 

 new work and new evidence often produce a greater effect than 

 is rightly their due, and this at the expense of older work. The 

 balance can only be restored by yet newer work. This has been 

 supplied by the important research of Haldane and Priestley 

 upon the regulation of the ventilation of the lungs in man. Their 

 experiments upon the composition of the alveolar air have already 

 been described, but it may be again mentioned that they found 

 that under ordinary atmospheric pressure the alveolar air of each 

 individual contains a practically constant percentage of carbon 

 dioxide. The smallest increase in the percentage of carbon dioxide 

 in the inspired air is accompanied by an' increase in the ventila- 

 tion of the lung, sufficient to keep constant the percentage of 

 carbon dioxide in the alveoli. During muscular work a similar 

 compensation is present ; the alveolar ventilation is increased in 

 proportion to the extra production of carbon dioxide, and thus 

 the hyperpnoea seen during muscular exercise can be explained 

 as a response of the respiratory centre to the rise in the pressure 

 of carbon dioxide in the arterial blood. Other experiments show 

 that apncsa depends upon a fall in the pressure of carbon dioxide 

 below the amount which will stimulate the respiratory centre ; 

 thus rapid respiration of air containing sufficient carbon dioxide 

 to prevent the pressure of the gas in the alveoli from falling below 

 this threshold value will not produce even a short apnosa. It is 

 unnecessary to assume the existence of a true vagus apnoea, for 

 the apncea produced by distension of the lungs can be explained 

 by the lowering of the pressure of carbon dioxide. 



There is much independent evidence in support of these views. 

 Fredericq proved by his well-known experiment with a crossed 

 circulation that the condition of apncea produced in the one 

 animal by artificial respiration performed on the other is not due 

 to a rise in the pressure of oxygen in the blood, but to a fall in 

 the pressure of carbon dioxide. Mosso has shown that in man 

 a short apnoea can be produced by a deep inspiration of hydrogen, 



