56 



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 total mass of blood in a living man has been estimated by caus- 

 ing the person to inhale a mixture of carbon monoxide with oxygen 

 or air. The amount of carbon monoxide taken up is determined and 

 also, 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. Notwith- 

 standing the elegance of this method in principle, it is by no means 

 easy to obtain accurate results with it in practice. It unques- 

 tionably gives values for the total quantity of blood which are too 

 low. A better method is to inject into a vein a measured quantity 

 of a solution of a pigment (" vital red "), which is only slowly elimin- 

 ated and is not taken up to a sensible degree by the erythrocytes. 

 Samples of blood are drawn before and a short time after the injec- 

 tion and the plasma separated from each by the centrifuge. The 

 amount of pigment is then determined which must be added to the 

 first sample of plasma to make its tint the same as that of the 

 second. The total quantity of plasma can thus be calculated and 

 from it, by determining the relative proportion of corpuscles and 

 plasma in the blood, the total quantity of blood (Rowntree, etc.). 



The quantity of blood in the body was greatly over-estimated 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. By Welcker's method the proportion of 

 blood to body-weight has been found to be in the dog i : 13, cat 1 : 14, 

 horse i : 15, frog i : 17, rabbit 1 : 19, fowl i : 20. In new-born 

 children the proportion was i : 19, in adult human beings (executed 

 criminals) i : 13. By the ' vital red ' method, the amount of 

 plasma was found to be one-twentieth and that of blood one- 

 eleventh to one-twelfth of the body-weight. 



According to Dreyer, the blood volume is a function of the surface 

 of the body, so that the smaller and lighter animals in any given species 

 have a relatively greater blood volume than the larger and heavier 

 individuals. Accordingly, he considers that the practice of expressing 

 the volume of blood as a percentage of the body-weight should be 

 abandoned. 



Fig. 16 (p. 54) illustrates the distribution of the blood in the 

 various organs of a rabbit. The liver and skeletal muscles each con- 

 tain rather more than one-fourth; the heart, lungs, and great vessels 



