SECRETION AND ABSORPTION OF GAS, ETC. 239 



Pfliiger for these experiments was his well-known "aero- 

 tonometer ". This instrument consists essentially of a pair 

 of glass tubes, each of which is closed by a tap above and 

 by mercury below, and is so arranged that a stream of blood 

 led from a vessel of the animal can be allowed to trickle 

 slowly down the internal surface of the glass and escape 

 under the mercury. One tube is filled before the experi- 

 ment with a gas mixture containing more, and the other 

 tube with a gas mixture containing less, carbonic acid or 

 oxygen than corresponds to the expected tension of carbonic 

 acid or oxygen in the blood. The tubes are kept in a 

 water bath at the body temperature, and the pressure inside 

 them is kept equal to that of the atmosphere. If the experi- 

 ment is successful, the percentage of carbonic acid or 

 oxygen in the first tube will diminish, and that in the second 

 will increase, so that the percentage found afterwards will 

 be nearly the same in both tubes. Evidently this final per- 

 centage expresses the tension of carbonic acid or oxygen in 

 the blood in terms of the atmospheric pressure at the time 

 of the experiment. 



By means of the aerotonometer the tension of carbonic 

 acid in the arterial blood of the dog was found by Strass- 

 burg to be from 2'i to t>'8 per cent, (mean 2*8 per cent.) of 

 an atmosphere. Unfortunately there is no means of deter- 

 mining directly the tension of carbonic acid in the alveolar 

 air of an animal breathing normally. It is evident, how- 

 ever, that since the expired air consists of a mixture of 

 alveolar air with the pure air contained in the trachea and 

 bronchi at the end of each inspiration, the alveolar air must 

 contain less oxygen and more carbonic acid than the expired 

 air. If the respirations are shallow the difference between 

 expired and alveolar air will be great, whereas with deep 

 respirations there will be little difference. In man the ex- 

 pired air usually contains about 3*5 per cent, of carbonic 

 acid, and according to the experiments of Loewy l the 

 volume of the "dead space" formed by the trachea, etc., is 

 about 140 cc, the corresponding volume of air inspired in 



1 Pfluger's Arckiv, vol. Iviii., p. 416, " Untersuchungen iiber die Re- 

 spiration," etc., p. 26. 



