THE RESPIRATORY CHANGES IN THE BLOOD. 349 



for the collection of the gases, can be inverted. A tube g fused on laterally to 

 one opening of the three-way tap d places the latter in connection with a thick- 

 walled Woulff's bottle G containing a layer of strong sulphuric acid. The second 

 tubulure of this bottle is similarly connected by an elastic tube with the vessel 

 I), into which blood or other fluid may be introduced by means of the tap h. 

 All the movable joints of the apparatus are protected by India-rubber tubes 

 into which water can be poured, and a metal casing around the tap d, which 

 may also be filled with water, similarly prevents the possibility of any leakage 

 here. 



The pump is used as follows : By placing the tap d in the position shown in the 

 figure and raising B, the bulb A may be filled with mercury up to the top, the con- 

 tained air being expelled through the upper end of l>. By a slight turn of the tap 

 all connection between A and either the tube g or the upper part of b may be cut 

 off, and on lowering B a vacuum is established in the bulb A and part of the tube 

 (t. A may now be connected by the tap d with the tube g, and hence with C and 

 Z>, and, h being closed, a partial vacuum is established in fand D. By means of 

 the tap d the air in A may be cut off from g, and on raising B and placing the plug 

 of d as shown in the figure this air may be expelled through the upper end of b. 

 By slightly turning d and lowering B a vacuum is again established in A. and, as 

 before, a further portion of air in C and D may be allowed to pass over into A and 

 the vacuum in D and C increased. In this way all the air in I) can be extracted, 

 the final stages being facilitated by the admission of a little water into Z>, the last 

 traces of air being driven over into A by the rush of vapor from the water. A 

 known volume of blood having been collected over mercury in a small tube is now 

 allowed to enter D through the tap h and yields up its gases to the vacuum. A 

 repetition of the processes by which the air in D was originally extracted will now 

 remove the gases which have been given off from the known volume of blood, the 

 only difference being that now the tube / filled with mercury is inverted in the 

 trough e over the upper end of the tube b. In this way the gases originally in D 

 are not allowed to escape into the air, as was the case when the apparatus was 

 being originally made vacuous, but are collected in / for subsequent analysis. 

 During the extraction of the gases from the blood the bulb D is immersed in a 

 vessel of warm water, to facilitate the exit of the gases and, by causing the for- 

 mation of large quantities of aqueous vapor, to sweep the gases rapidly over into 

 A. The sulphuric acid chamber C dries the vacuum before the admission of the 

 blood into />, and hence makes it more perfect and causes the most complete and 

 rapid evolution of gases from the blood. 



The average composition of the gas thus obtained from each of the two 

 kinds of blood (the arterial blood being taken from a large artery, and the 

 venous blood from the right side of the heart) is, stated in round numbers, 

 as follows: 



From 100 vols. may 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 per cent, less oxygen and 6 per cent, more carbonic acid. It must be 

 remembered, however, that while arterial blood from whatever artery taken 

 has always nearly the same proportion of gases, or at all events the same 

 amount of oxygen, the amount of oxygen in venous blood, even when taken 

 from the same vein, may vary a good, deal, still more so when it is taken 

 from different veins. The reason of this we shall see hereafter. 



It will be convenient to consider the relations of each of these gases sep- 

 arately. 



