CHAP. IL] RESPIRATION. 607 



but we cannot take this as a measure of the normal percentage 

 of carbonic acid in the pulmonary alveoli, since by the mere 

 holding of the breath the carbonic acid in the blood and hence in 

 the pulmonary alveoli is increased beyond the normal. 



The difficulties of the problem seem however to have been 

 overcome by an ingenious experiment in which there is introduced 

 into the bronchus of the lung of a dog a catheter, round which is 

 arranged a small bag; by the inflation of this bag the bronchus, 

 whenever desired, can be completely blocked up. Thus, without 

 any marked disturbance of the general breathing, and therefore 

 without any marked change in the normal proportions of the 

 gases of the blood, the experimenter is able to stop the ingress 

 of fresh air into a limited portion of the lung. At the same 

 time he is enabled by means of the catheter to withdraw a 

 sample of the air of the same limited portion, and by analysis 

 to determine the amount of carbonic acid which it contains, 

 or in other words, the partial pressure of the carbonic acid. 

 The blood passing through the alveolar capillaries of this limited 

 portion of the lung naturally possesses the same carbonic acid 

 pressure as the rest of the venous blood flowing through the 

 pulmonary artery, a pressure which, though varying slightly 

 from moment to moment, will maintain a normal average. On 

 the supposition that carbonic acid passes simply by diffusion 

 from the pulmonary blood into the air of the alveoli, because the 

 carbonic acid pressure of the latter is normally lower than that of 

 the former, one would expect to find that the air in the occluded 

 portion of the lung would continue to take up carbonic acid 

 until an equilibrium was established between it and the carbonic 

 acid pressure of the venous blood. Consequently, if after an 

 occlusion, say of some minutes (by which time the equilibrium 

 might fairly be assumed to have been established), the carbonic 

 acid pressure of the air of the occluded portion were determined, it 

 ought to be found to be equal to, and not more than equal to, the 

 carbonic acid pressure of the venous blood of the pulmonary artery. 

 And this is the result which has been arrived at; it has been 

 found that the pressures of the carbonic acid of the occluded air 

 and of the venous blood of the right side of the heart are just 

 about equal. Hence the evidence so far as it goes is distinctly in 

 favour of the view that the escape of carbonic acid from the blood 

 into the pulmonary alveoli is simply due to diffusion, and that 

 there is no need to seek for any further explanation. There is, 

 so far as we can see at present at all events, no necessity, any 

 more than in the case of oxygen, to suppose and no adequate 

 evidence to shew that the wall of the pulmonary alveoli has any 

 specific secretory power of discharging carbonic acid from the 

 blood independently of or in antagonism to the influence of 

 pressures, or that it exerts any special influence at all as a 

 diffusion septum. 



