330 CHANGES IN THE BLOOD. [BOOK n. 



blood is found to contain more oxygen and less carbonic acid than 

 that obtained from venous blood. This is the real differential 

 character of the two bloods ; all other differences are either, as we 

 shall see to be the case with the colour, dependent on this, or are 

 unimportant and fluctuating. 



If the quantity of gas which can be extracted by the mercurial 

 air-pump from 100 vols. of blood be measured at ()C., and a 

 pressure of 760 mm., it is found to amount, in round numbers, to 

 60 vols. 



The vacuum produced by the ordinary mechanical air-pump is in- 

 sufficient to extract all the gas from blood. Hence it becomes necessary 

 to use either a large Torricellian vacuum or a Sprengel's pump. In the 

 former (Fig. 57) case two large globes of glass, one fixed and the other 

 moveable, are connected by a flexible tube; the fixed globe is made to 

 communicate by means of air-tight stopcocks alternately with a receiver 

 containing the blood, and with a receiver to collect the gas. When the 

 moveable globe filled with mercury is raised above the fixed one, the 

 mercury from the former runs into and completely fills the latter, the 

 air previously present being driven out. After adjusting the cocks, the 

 moveable globe is then depressed thirty inches below the fixed one, 

 in which the consequent fall of the mercury produces an almost 

 complete vacuum. By turning the proper cock this vacuum is put into 

 connection with the receiver containing the blood, which thereupon be- 

 comes proportionately exhausted. By again adjusting the cocks and 

 once more elevating the moveable globe, the gas thus extracted is 

 driven out of the fixed globe into a receiver. The vacuum is then 

 once more established and the operation repeated as long as gas continues 

 to be given off" from the blood. This form of pump, introduced by 

 Ludwig, or a modification of it, with drying apparatus, employed by 

 Pfliiger, or a similar form introduced by French observers, is the one 

 which has been hitherto most extensively used ; but a Sprengel's pump 

 is preferred by some. 



The average composition of this gas in the two kinds of blood 

 is, stated in round numbers, as follows: 



From 100 vols. m &y be obtained 



Of oxygen, of carbonic acid, of nitrogen. 



Of Arterial Blood, 20 vols. 40 vols. 1 to 2 vols. 



Of Venous Blood, 8 to 12 vols. 46 vols. 1 to 2 vols. 

 all measured at 760 mm. and C. 



That is to say, venous blood, as compared with arterial blood, 

 contains 8 to 12 p.c. less oxygen and 6 p.c. more carbonic acid. 

 But the gases of venous blood are much more variable than those of 

 arterial blood. 



It will be convenient to consider the relations of each of these 

 gases separately. 



