THE CHEMISTRY OF RESPIRATION 



1069 



the other tube R with a mixture containing 5 per cent. CO 2 . a is now connected with 

 the distal end of the jugular vein, or with the central end of the carotid artery, and 

 blood is allowed to flow in a thin stream down the walls of the tubes R and R, thus 

 presenting a large surface to the contained gases. The blood collects in the lower 

 narrower portions of the tubes, and runs out into the vessels b, b, whence after defibrina- 

 tion it is returned at intervals into the veins of the animal. 



Bohr's aerotonometer was built on the plan of the Stromaiche devised by Ludwig, 

 and could be inserted in the course either of an artery or of a vein. In using this instru- 

 ment it is advisable to inject some sub- 

 stance like peptone or, better, hirudin, 

 in order to prevent coagulation of the 

 blood. 



In all these instruments the 

 main difficulty is in obtaining 

 a sufficient surface of the blood 

 exposed to the gaseous mixture. 

 The interchange of- gases is thus 

 very slow, and it is difficult to be 

 certain at any time that the blood 

 and the gas with which it is in con- 

 tact are really in equilibrium. Krogh 

 therefore adopted an ingenious de- 

 vice of limiting the volume of air to 

 a small bubble, the superficial area 

 of which is large in proportion to 

 its bulk. This bubble, after it has 

 been in a stream of blood for some 

 minutes, is transferred to a special 

 capillary tube in which its analysis can be carried out with a fair degree of 

 accuracy. 



The performance of a tonometer may be expressed by the ratio of the surface of 

 blood exposed to the volume of the air used. The ' specific surf ace ' of an aerotonometer 



is represented by ^ -. The specific surface of Pfliiger's instrument is only 



volume in c.c. 



3-3 and of Bohr's only 5-2. In Krogh's microtonometer the absolute volume of air 

 employed is reduced to a bubble of about 2 mm. in diameter, having a volume of -004 c.c. 

 and a surface of 0-125 sq. cm., so that its specific surface is 30. In such a bubble the 

 equalisation of the tensions takes place with extreme rapidity and only a minute 

 quantity of fluid is necessary. The microtonometer consists of the tonometer proper 

 and the apparatus for the microanalysis of the gas bubble. In the latter the measure- 

 ment of the gas bubble is carried out in a capillary tube, the absorption of carbon dioxide 

 and of oxygen being carried out in the usual way with pptash and with pyrogallic acid. 

 The tonometer is represented in Fig. 502. It is kept in a small water-bath at the tem- 

 perature of the blood to be examined. The tonometer is filled with saline solution and 

 contains the gas bubble 2, which can be drawn up by means of the screw 4 into the 

 narrow graduated tube 3, where its volume is measured. The blood from the artery or 

 vein, in which we wish to examine the tension of the gases, passes by a cannula through 

 the tube 1, and enters the tonometer as a fine jet. It forces its way up above the gas 

 bubble, which is pressed a little down by the current, and kept oscillating with great 

 rapidity. From the tonometer the blood flows back through the tube 7 and is collected 

 in a vessel where it can be measured and afterwards drawn off and reinjected into the 



FIG. 501. Pfliiger's aerotonometer. 



