860 PHYSIOLOGY 



reading of the burette noted as soon as it is constant. The bottle is again shaken, &c., 

 until a constant result is obtained, for which about fifteen minutes from the beginning 

 of the operations are required. The temperature of the water in the jacket of the 

 burette, and the reading of the barometer, are now taken, and the gas evolved is reduced 

 to its dry volume at and 760 mm. To calculate the oxygen evolved from 100 c.c. of 

 blood, allowance must be made for the fact that a 20 c.c. pipette does not deliver 20 c.c. 

 of blood, but only about 19-6 c.c. The actual amount of shortage for a given pipette 

 can easily be determined by weighing the pipette after water, and again after blood, 

 has been delivered from it. A further slight correction is necessary on account of the 

 fact that the air in the bottle at the end of the operation is richer in oxygen than at the 

 beginning, so that, as oxygen is about twice as soluble as nitrogen, slightly more gas will 

 be in solution. With a bottle of 120 c.c. capacity and 20 per cent, of oxygen in the blood, 

 the air in the bottle at the end will evidently contain about 27 per cent, of oxygen, so 

 that, assuming that the coefficients of absorption of oxygen and nitrogen in the 54 c.c. 

 of liquid within the bottle are nearly the same as in water, the correction will amount 

 at 15 C. to -06 c.c. in the reading of the burette, or + 0-30 per cent, in the result." 



THE SPECIFIC GRAVITY OF THE BLOOD 



The specific gravity of the blood may be determined by directly weighing 

 a sample, or more conveniently by collecting blood in a capillary tube and 

 discharging drops of it into a series of vessels containing glycerin and water 

 mixed in varying proportions. When it is found that the drop of blood 

 as it leaves the capillary vessel neither rises nor falls in the glycerin and 

 water mixture, we know that the specific gravity of the blood is identical 

 with that of the mixture. A graduated series of these mixtures is kept 

 in bottles and their specific gravity is generally determined before the 

 experiment. Hammerschlag's method consists in placing a drop of blood 

 in a mixture of chloroform and benzene and then adding chloroform or 

 benzene, as the case may be, until the drop neither rises nor falls. The 

 specific gravity of the mixture is then taken. The specific gravity varies 

 in man between 1057 and 1066, and in woman from 1054 to 1061. It 

 is increased by loss of water, as after profuse perspiration, or by passive 

 congestion of the part from which the sample is taken. It is also increased 

 as a result of any operation upon a serous cavity in consequence of exuda- 

 tion of plasma in the inflamed or irritated part. It is diminished as the 

 result of bleeding. The specific gravity of serum is 1028 to 1032, of cor- 

 puscles about 1090. It is interesting to note that the specific gravity of 

 the blood is highest in the foetus at full term, when it amounts to 1066, 

 contrasting with that of the mother at the same time, the specific gravity 

 of whose blood is only 1050. The specific gravity rapidly falls to the 

 latter figure after birth. 



THE REACTION OF THE BLOOD 



The blood is alkaline to litmus. This fact can be demonstrated by 

 allowing a drop to fall on a piece of glazed litmus paper and then wiping 

 away the blood with a piece of linen moistened with distilled water or neutral 

 saline solution. In order to estimate the alkalinity a small definite quantity 

 of the blood is mixed with sulphate of soda solution containing a definite 



