208 THE BLOOD. 



until the column of mercury in the descending limb of the 

 Sprengel tube is broken into fragments by intervening spaces 

 containing air. This happens whenever the quantity of mer- 

 cury which reaches the bend by the ascending limb in any given 

 time, is less than that which leaves it by the descending limb. 

 In a time which varies according to the capacity of the cavity 

 to be exhausted, vacuum is attained. No more bubbles are 

 discharged at the lower end of the Sprengel. Each drop of 

 mercury as it falls produces a peculiar click, and if the current 

 is stopped, it is seen that the height of the column in the de- 

 scending limb is less than that of the barometer at the time, 

 by a number of millimetres which is equal to the tension of 

 aqueous vapor at the temperature. The apparatus is so 

 arranged that the bend of the first tube is supported at a level 

 several inches higher than that of the second. Consequently, 

 as the process of exhaustion approaches, the bulb with which 

 it communicates becomes emptied of mercury, the vacuous 

 space thus formed gradually extending till the level of the mer- 

 cury in the descending limb coincides with that of the bend of 

 the second tube. 



We next pass to the description of the method of obtaining 

 blood from an artery or vein, and of transferring it to the 

 vacuum. Although it is not possible to produce a vacuum 

 with the Sprengel pump above described, as rapidly as with 

 the ordinary mercurial pump, 1 its action in other respects is 

 very satisfactory. It completely fulfils the conditions enume- 

 rated by Ludwig as essential to an efficient blood-pump. The 

 vacuum produced is perfect; it is bounded by mercury which, 

 having previously passed through a vacuum (in the first tube), 

 is completely deprived of air; and it can be renewed any num- 

 ber of times after the blood is introduced. 



29. Method of Transferring the Blood to be Ex- 

 hausted from the Artery or Vein to the Vacuum. It 

 is essential that the transference should be effected without 

 contact with air; the blood must therefore either flow as directly 

 as possible from the artery or vein into the vacuum tube: or, 

 if it is intended to defibrinate it, it must be received in a space 

 previously occupied by mercury. Before describing the mode 

 of transferring, an account must be given of the chamber or re- 

 cipient in which the blood is exhausted, and of the mode in 

 which it communicates with the pump. The exhaustion tube 

 (see Fig. 199, 11) is connected by a vulcanite union, inclosed in 

 an external tube containing glycerin, with a long nearly capil-. 

 lary tube, of such form and length as to reach the table by the 

 side of which the pump stands. Near its lower end it is bent 



1 The instrument probably admits of considerable improvement in this 

 respect. 



