234 A MANUAL OF PHYSIOLOGY 



may be extracted by exposure to an atmosphere in which the partial 

 pressure of the gas is made as small as possible. Thus, oxygen can 

 be obtained from liquids in which it is simply dissolved by putting 

 them in an atmosphere of hydrogen or nitrogen, in which the partial 

 pressure of oxygen is zero, or in the vacuum of an air-pump, in 

 which it is extremely small. Heat also aids the expulsion of dis- 

 solved gases. Some gases held in weak chemical union, like the 

 loosely-combined oxygen of oxyhaemoglobin, can be obtained by dis- 

 sociation of their compounds when the partial pressure is reduced. 

 More stable combinations may require to be broken up by chemical 

 agents carbonates, for instance, by acids. 



Extraction of the Blood-gases. This is best accomplished by 

 exposing blood to a nearly perfect vacuum. The gas-pumps which 

 have been most largely used in blood analysis are constructed on the 

 principle of the Torricellian vacuum. A diagram of a simple form of 

 Pfliiger's gas-pump is given in Fig. 83. The gases obtained are 

 ultimately dried and collected in a eudiometer, which is a graduated 

 glass tube with its mouth dipping into mercury. The carbon dioxide 

 is estimated by introducing a little caustic potash to absorb it. The 

 diminution in the volume of the gas contained in the eudiometer 

 gives the volume of the carbon dioxide. The oxygen may be 

 estimated by putting into the eudiometer more than enough hydrogen 

 to unite with all the oxygen so as to form water, and then, after 

 reading off the volume, exploding the mixture by means of an 

 electric spark passed through two platinum wires fused into the glass. 

 One-third of the diminution of volume represents the quantity of 

 oxygen present. It can also be estimated by absorption with a 

 solution of pyrogallic acid and potassium hydrate. The remainder 

 of the original mixture of blood-gases, after deduction of the carbon 

 dioxide and oxygen, is put down as nitrogen (with, no doubt, a small 

 proportion of argon). For the sake of easy comparison, the observed 

 volume of gas is always stated in terms of its equivalent at a standard 

 pressure and temperature (760 mm., or sometimes on the Continent 

 i metre of mercury, and o C.). 



It is also possible in various ways to estimate the amount of 

 oxygen in blood without the use of the pump. Thus, since a definite 

 volume of oxygen (1*338 c.c. at o C. and 760 mm. pressure) 

 combines with a gramme of haemoglobin, we can calculate the total 

 volume of oxygen present if we know how much of the blood-pigment 

 is in the form of oxyhaemoglobin ; and this can be determined by 

 means of the spectrophotometer (Hiifner). Or the blood may be 

 shaken with carbon monoxide (carbonic oxide), which expels the 

 oxygen from its combination with the haemoglobin. The oxygen can 

 then be estimated in the gas collected (Bernard). 



In dog's blood, which has been up to this time chiefly in- 

 vestigated, there are considerable variations in the quantity 

 of oxygen and carbon dioxide which can be extracted ; and 

 this is particularly true of the venous blood, as might 



