CHAP, ir.] RESPIRATION. 347 



since by the mere holding of the breath the carbonic acid in the 

 blood and 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 

 disturbance of the general breathing, and therefore without any 

 change in the normal proportions of the gases of the blood, the ex- 

 perimenter 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 its carbonic acid tension. 

 The blood passing through the alveolar capillaries of this limited 

 portion of the lung naturally possesses the same carbonic acid 

 tension as the rest of the venous blood flowing through the 

 pulmonary artery, a tension 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 tension 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 tension 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 tension 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 tension of the venous blood of the pulmonary artery. 

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

 found that the tensions of the carbonic acid of the occluded air and 

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

 equal, that of the occluded air being, if anything, slightly less than 

 that of the venous blood. So that 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 for instance no necessity to suppose that the epithelium 

 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. 



