148 PRACTICAL PHYSIOLOGY 



fluid a mixture can be obtained in which the drop of blood remains 

 suspended. The specific gravity of the mixture at this stage is 

 determined by a small hydrometer. 



Hrematocrite. The relative volumes of corpuscles and plasma 

 may be determined by centrifugalising a small quantity of blood 

 in, a graduated glass tube of narrow bore (haematocrite). The 

 rotation must be at a high speed (10,000 revolutions per minute) 

 in order to bring about the separation of the corpuscles from the 

 plasma before the blood clots. The usual proportions are plasma 

 two-thirds, corpuscles one-third. 



CHAPTER XXVII 

 THE INFLUENCE OF CARBON MONOXIDE 



Carbon monoxide is a poisonous gas in virtue of its great affinity 

 for haemoglobin ; oxygen is displaced and carboxyhsemoglobin is 

 formed. Unconsciousness, convulsions, and death are produced 

 by the lack of oxygen which arises when a large portion of the 

 haemoglobin is combined with carbon monoxide and thus deprived 

 of its power of carrying oxygen. 



Carbon monoxide is present as an impurity in coal-gas, and in 

 water-gas, which is often used in the adulteration of coal-gas, the 

 percentage is a very high one. It is due to this gas that death so 

 often results from coal-gas poisoning. In the air of mines after an 

 explosion there is present a large quantity of carbon monoxide, 

 due to the incomplete combustion of coal-dust ; miners overtaken 

 by such a disaster generally die from poisoning by this gas. 



DEMONSTRATION. A white rat or mouse is selected for the 

 experiment, for it is easier in such animals to see in the snout 

 and feet the change of colour due to the formation of the carb- 

 oxyhaemoglobin. The animal is placed under a glass bell jar and 

 coal-gas is admitted ; it becomes restless, unconscious, convulsed, 

 and dies within a few seconds. This is one of the quickest methods 

 of killing an animal, and has the advantage that it rapidly produces 

 unconsciousness. 



If the animal be removed to free air at the beginning of the stage 

 of unconsciousness it may recover. The carboxyhaemoglobin is 

 gradually dissociated and oxyhaemoglobin is formed in its place. 

 In rabbits this occurs very rapidly ; the animal quickly recovers, 

 passing through a stage of incoordination. 



Haldane has shown that the best indicator of the presence of 

 poisonous doses of carbon monoxide is a small warm-blooded 

 animal, such as a mouse or bird, which is affected, owing to its 

 rapid respiratory exchange, much sooner than a man. This method 

 has been employed with success by rescue parties entering a coal 

 mine after an explosion. 



The colour of the snout and feet of the white mouse or rat killed 



