136 



THE CIRCULATION OF THE BLOOD 



To appreciate the significance of the results, it is important to bear in 

 mind that the total volume of the blood constitutes from 5 to 7 per cent 

 of the weight of the animal. This fact has been determined partly by 

 postmortem, and partly by antemortem measurements. In the post- 

 mortem method, the total amount of blood is determined by collecting the 

 blood while bleeding the animal to death and then washing out the 

 vessels with saline solution until the escaping fluid is no longer tinged 

 with red. The amount of blood contained in the saline solution is estimated 

 by colorimetric methods (see page 92), and is added to that of the blood 



Fig. 28. Effect of stimulating the peripheral end of the 



pressure. 



right vagus on the arterial blood 



directly collected. In the antemortem method some substance that does not 

 diffuse through vessel walls or become quickly destroyed is added to the 

 blood. By determining the concentration of this substance in a speci- 

 men of blood, the volume with which it has become mixed can readily be 

 calculated. Acacia has recently been found suitable for this purpose 

 (Meek), but the best known work (of Haldane) was done by causing the 

 animal to inspire a known amount of carbon monoxide. This combines 

 with the hemoglobin of the blood (see page 401) to displace an equal 

 quantity of oxygen. By determining the difference between the volume 



