CH. XXIX.] THE BLOOD 445 



of blood is fcakeu from an animal by venesection ; it is defibrinated 

 and measured, and used to make standard solutions of blood. The 

 animal is then rapidly bled to death, and the blood which escapes is 

 collected. The blood-vessels are next washed out with saline solu- 

 tion until the washings are no longer coloured, and these are added 

 to the previously withdrawn blood ; lastly, the whole animal is finely 

 minced with saline solution. The fluid obtained from the mincings 

 is carefully filtered and added to the diluted blood previously obtained, 

 and the whole is measured. The next step in the process is the com- 

 parison of the colour of the diluted blood with that of standard solu- 

 tions of blood and water of a known strength, until it is discovered 

 to what standard solution the diluted blood corresponds. As the 

 amount of blood in the corresponding standard solution is known, as 

 well as the total quantity of diluted blood obtained from the animal, 

 it is easy to calculate the absolute amount of blood which the latter 

 contained, and to this is added the small amount which was with- 

 drawn to make the standard solutions. This gives the total amount 

 of blood which the animal contained. It is contrasted with the 

 weight of the animal, previously known. The result of experiments 

 performed in this way showed that the quantity of blood in various 

 animals differs a good deal, but in the dog averages T V to ^ of 

 the total body-weight. In smaller animals the proportionate blood 

 volume is greater. 



Haldane and Lorrain Smith have invented another method which 

 has the advantage of being applicable to man. The data required 

 are (1) the percentage of haemoglobin in the blood, and (2) the extent 

 to which the haemoglobin is saturated by a measured amount of 

 carbonic oxide absorbed into the blood. 



The percentage of haemoglobin is determined colorimetrically by 

 the Gowers or Gowers-Haldane haemoglobinometer (see p. 470). In 

 the latter instrument the standard 100 per cent, of colour corresponds 

 to a capacity of 18'5 c.c. of oxygen or carbonic oxide per 100 c.c. of 

 blood. The subject whose blood is to be measured breathes a known 

 volume of carbonic oxide, and a few drops of the blood are taken and 

 the saturation of his haemoglobin is determined colorimetrically. 

 From this result the total capacity of the blood for carbonic oxide is 

 calculated. The "carbonic oxide capacity" is the same as the 

 " oxygen capacity." The volume of the blood is then calculated from 

 the total " oxygen capacity," and the percentage capacity as deter- 

 mined by the haemoglobinometer. The following is an example : The 

 subject's blood in a given case has, let us say, the colour of the 100 

 per cent, standard, and therefore has a capacity of 18'5 c.c. per 

 100 c.c. blood. He is allowed to breathe 75 c.c. of carbonic oxide, 

 and it is then found that his blood is 15 per cent, saturated with 

 that gas. That is to say, instead of there being 18-5 c.c. of oxygen 



