CHANGES IN AIR AND BLOOD IN RESPIRATION. 



671 



to remove the blood from the vessels and bring it into contact 

 with an atmosphere containing a known quantity of O, CO2, or N, 



Fig. 274. — Diagram to show the principle of the aerotonometer; A, The tube containing a 

 known mixture of gases, O, CO2, N ; C, the outside jacket for maintaining a constant body tem- 

 Derature. When stopcock b is open the blood trickles down the sides of A and enters into dif- 

 fusion relations with the contained gases. After equilibrium is reached the stopcock b is closed 

 and a is opened. By means of the mercury bulb the gases can then be forced out of A into a 

 suitable receiver for analysis. 



according to the gas to be measured. By trial an atmosphere can 

 be obtained in which this gas is contained in amounts such that there 

 is no marked increase or decrease in quantity after standing in diffu- 

 sion relations with the blood. The percentage of the gas in the at- 

 mosphere chosen will measure the tension of that gas in the blood. 

 An instrument which has been much used for such determinations is 

 represented diagrammatically in Fig. 274. It is known as a tonom- 

 eter or aerotonometer (Pfluger). It consists of a tube (A) which 

 can be connected through h directly with the blood-vessels. This 

 tube A is surrounded by a jacket (C) containing warm water, so 

 that the blood may be kept at the body temperature during the ex- 

 periment. A is first completely filled with mercury ffom the bulb M 

 to drive out the air. An atmosphere of known composition is 

 then sucked into A by dropping the bulb. Blood is allowed to 



