196 HUMAN PHYSIOLOGY 



giving it up at a moderate temperature and under a low 

 pressure of oxygen is the great function of the blood pigment 

 in the body. The haemoglobin plays the part of a middle- 

 man between the air and the tissues, taking oxygen from the 

 one and handing it on to the others, just as it may be made 

 to act when in solution in a test tube containing sulphide of 

 ammonia or a ferrous salt. (Chemical Physiology, p. 14.) 



Haemoglobin constitutes about 13 or 14 per cent, of the 

 blood, but in various diseases its amount is decreased. The 

 best method of estimating its amount is by Haldane's Hsemo- 

 globino meter. This consists of two tubes of uniform calibre, 

 one tilled with a 1 per cent, solution of normal blood 

 saturated with CO, and another in which 20 cmm. of blood 

 to be examined, measured in a pipette, is placed in water, 

 mixed with coal gas to saturate with CO, and then diluted 

 till it has the same tint as the standard tube. The per- 

 centage of haemoglobin in terms of the normal is indicated by 

 the mark on the tube at which the fluid stands. (Chemical 

 Physiology, p. 15.) 



Methaemoglobin. Haemoglobin forms another compound 

 with oxygen methaemoglobin. The amount of oxygen is 

 the same, but methaemoglobin must be acted on by the 

 strongest reducing agents before it will part with its oxygen. 

 When therefore this pigment is formed in the body, the 

 tissues die for want of oxygen. It may be produced by the 

 action of various substances on oxyhsemoglobin. Among 

 these are ferricyanides, nitrites, and permanganates. It 

 crystallises in the same form as oxyhaemoglobin, but has a 

 chocolate brown colour. Its spectrum is also different from 

 haemoglobin or oxyhaemoglobin, showing a narrow sharp band 

 in the red part of the spectrum, with two or more bands in 

 other parts according to the reaction of the solution in which 

 it is dissolved (Fig. 101). It is of importance since it occurs in 

 the urine in some pathological conditions. In all probability 



the molecule of oxyhaemoglobin has the formula Hb/ 



X) 



while in methaemoglobin the atoms are arranged 



