Chap xxin.] THE GASES OF THE BLOOD. 295 



globin of the blood. The great difference of pressure, 

 then, between the O in the blood and that in the air 

 cells, while a very important factor in the absorption 

 of that gas by the blood, is not the only one. 

 Similarly, the carbonic acid gas is not in the blood 

 in a simple state of solution. A diminution of 

 pressure will not cause all the C0 2 to be evolved, 

 nor does the evolution follow the law of pressures. 

 It seems to be in loose chemical combination with cer- 

 tain salts of the serum. Here, also, therefore, in 

 addition to the physical explanation offered by the 

 difference in tension between the CO 2 of the blood and 

 that of the air cells, the chemical explanation must 

 be taken into account. 



A sufficiently low pressure, however, will cause to 

 be evolved from the blood the gas it contains in solu- 

 tion as well as the gas held in unstable combination, 

 with the exception of a small percentage (2 to 5) of 

 carbonic acid gas, which requires the addition of some 

 acid to drive it off. The method of obtaining the gases 

 of the blood for analysis proceeds on this principle. A 

 vacuum is created in a receiver, usually by means of a 

 mass of mercury, producing the Torricellian vacuum. 

 The receiver is connected, by means of a short tube and 

 canule, with an artery of the animal whose blood is to 

 be analysed. As soon as a sufficient exhaustion has 

 been obtained, the communication between the artery 

 and receiver is opened and the blood rushes in, the gas 

 being immediately evolved. If the receiver be placed 

 .in an outer vessel containing warm water, the libera- 

 tion of the gas is aided. If, then, a small quantity of 

 carbonic acid solution be permitted to enter the 

 receiver, the " fixed " C0 2 is liberated, and thereafter 

 all the gases may be collected into a graduated tube 

 over mercury, and analysed. 



