CARBON MONOXIDE IN BLOOD 537 



mention that the nitrogen in solution in the blood becomes of 

 the utmost importance in cases of caisson disease ; the gas, which 

 is set free in the blood-vessels and tissues of a man who too 

 quickly passes from the high pressure of the caisson to the ordi- 

 nary pressure of the atmosphere, is almost entirely composed of 

 nitrogen ; the gas cannot be utilised by the tissues, and thus will 

 cause gas-embolism, and, it may be, paralysis and death. This 

 interesting subject is fully discussed in another part of this work. 1 



Carbon Monoxide in Blood. Traces of carbon monoxide 

 have been found by various observers in the blood of healthy 

 animals ; this was probably due to the absorption of the gas 

 during the breathing of the air of large cities where small quantities 

 of carbon monoxide are present, or it might have been due to 

 the leakage of small quantities of coal-gas into the rooms in which 

 the animals lived. Although the gas is not to be regarded as a 

 normal constituent of blood, it is of the greatest interest to the 

 physiologist and the physician ; its property of displacing oxygen 

 and forming a combination with haemoglobin throws great light 

 upon the processes of respiration, and is the cause of its very 

 poisonous nature. Carbon monoxide is formed by the incomplete 

 combustion of coal-dust, and is thus responsible for numerous 

 deaths among miners after an explosion in a mine ; ( 19 ) it is present 

 as an impurity in coal-gas, and in this form is the cause of many 

 deaths, some accidental, others suicidal. 



Carbon monoxide combines with blood in the same proportion 

 as oxygen, but the compound, carboxy -haemoglobin, cannot be 

 decomposed by ordinary reducing agents. The gas can be ex- 

 tracted by the vacuum of a gas-pump, or by the passage of a 

 stream of oxygen or an indifferent gas, for the combination depends 

 upon the partial pressure of the gas. It is attached to the iron 

 portion of the molecule of haemoglobin, and thus is explained 

 its poisonous property, for it displaces an equal volume of oxygen 

 and causes death from lack of oxygen ; the red corpuscles are 

 put out of action as oxygen-carriers. Haldane ( 20 ) has shown that 

 the poisonous action of carbon monoxide diminishes as the pressure 

 of oxygen increases, and is abolished in the case of mice, when 

 the pressure of oxygen is raised to two atmospheres ; the mice 

 can then dispense with the oxygen- carry ing function of the haemo- 



1 See article by Leonard Hill, p. 233. 



