48 A MANUAL OF PHYSIOLOGY 



cleared away, and extracting it with water. The extract and 

 washings are mixed and weighed ; a given quantity of the mixture 

 is placed in a haematinometer (a glass trough with parallel 

 sides, e.g.), and a weighed quantity of the unmixed blood diluted 

 in a similar vessel till the tint is the same in both. From the 

 amount of dilution required, the quantity of blood in the watery 

 solution can be calculated. This is added to the amount of 

 unmixed blood directly determined. Since haemorrhage is imme- 

 diately followed by the entrance of liquid into the bloodvessels 

 from the lymph and tissue fluids, somewhat too high a result 

 will be obtained if the bleeding is at all prolonged. It is well, 

 therefore, to take only a moderate amount of blood for direct 

 estimation, and to compute the balance by the colorimetric 

 method. 



Many other methods have been devised on the principle of 

 injecting a known quantity of some substance into the circulating 

 blood, and then, after an interval has been allowed for mixture, 

 determining the change produced in a sample. Thus, the specific 

 gravity of a drop of blood having been measured, a certain 

 quantity of a solution of sodium chloride isotonic with the 

 plasma may be injected into a vein, and the specific gravity 

 again determined. Or the electrical resistance of a small sample 

 of blood may be measured before and after injection of a given 

 quantity of isotonic salt solution. 



The quantity of blood in the body was greatly overestimated 

 by the ancient physicians. Avicenna put it at 25 lb., and many 

 loose statements are on record of as much as 20 lb. being lost 

 by a patient without causing death. The proportion of blood 

 to body-weight has been found to be in the dog i : 13, new-born 

 child i : 19, cat i : 14, horse i : 15, frog I : 17, rabbit i : 19, 

 fowl i : 20. The total mass of the blood in a living man has been 

 estimated by causing the person to inhale a known volume of 

 carbon monoxide mixed with oxygen or air, and then determining 

 in a sample of blood taken from the finger the percentage amount 

 to which the haemoglobin has become saturated with carbon 

 monoxide. All that remains is to estimate the volume of carbon 

 monoxide (or, what is precisely the same thing, the volume of 

 oxygen) which 100 c.c. of blood will take up. This latter 

 quantity is called the percentage oxygen capacity. From these 

 data the total volume of the blood can be calculated. If the 

 volume is multiplied by the specific gravity the mass is obtained. 



Thus, if the haemoglobin was found to be 25 per cent, saturated 

 with carbon monoxide after the person had absorbed 150 c.c. of that 

 gas, the whole of the blood would require 600 c.c. of carbon monoxide 

 to saturate it completely. If the percentage oxygen capacity was 

 20, 20 c.c. of oxygen or carbon monoxide would be needed to saturate 



