Gases of the Blood 597 



ments of the experiment and to be held completely motionless. The 

 front feet are fastened to the bars of this cage, and for the hind 

 feet, two bars mounted in grooves in an arc can be separated more 

 or less according to the size of the animal. 



In this position one can draw blood either from one of the 

 carotid arteries or from one of the femorals. The carotids are more 

 convenient on account of both their size and their nearness to the 

 wall of the cylinder, and I almost always used them. 



This wall, opposite the place where the artery has been exposed, 

 is pierced by several holes, like the head of a watering-pot; through 

 one of these holes is passed the cannula used to extract blood; the 

 other holes are closed by a handful of modeling wax firmly pressed 

 over them. 



And now how to extract the blood? In the artery it is subjected 

 to a pressure equivalent to about 15 to 18 centimeters of mercury, 

 which makes extraction very easy when the operation takes place 

 in normal pressure. But the animal is placed in an apparatus in 

 which the pressure is to be diminished and we are to expose the 

 artery to the outside air. It is quite evident that when the pressure 

 is lowered 15 to 18 centimeters, the blood will no longer have any 

 tendency to flow from the vessel, and that when the pressure is 

 carried still lower, the outside air will tend to rush into the ani- 

 mal's artery and from there to spread through the whole circulatory 

 system. 



There is the danger and there lies the difficulty. To avert the 

 one and solve the other, I used (Fig. 29) first a cannula A forked 

 at its free extremity, into which slipped a stylet ending in an olive. 

 The latter was arranged so that it could exactly cover the orifice of 

 the cannula when it was placed in the animal's artery. When I 

 wished to extract blood, I pulled the stylet until the olive reached 

 the fork. All this time the cavity of the cannula was completely 

 closed, the stylet sliding tightly in a pierced rubber stopper which 

 a head a fastened closely. Then, by fitting the syringe in Figure 23 

 to the orifice a' by means of a rubber tube with thick walls, and by 

 opening the cock, blood could be extracted without danger. 



But in spite of all the precautions taken, I have had difficulties 

 resulting from the entrance of a certain quantity of air. Indeed, 

 a microscopic orifice is enough to let bubbles enter, and these, 

 reaching the left heart and being pumped thence into the arteries, 

 can, as you will see, cause very serious troubles. Sometimes the 

 quantity of air admitted thus was even enough to bring on imme- 

 diate death. 



