REGULATION OF BREATHING 7 



changes colour from a dark purple to bright scarlet 

 when it takes up oxygen. The oxygen is taken up in 

 the form of a weak chemical combination, the com- 

 pound having the property of being stable only in 

 presence of a certain concentration of free oxygen, 

 and dissociating rapidly as the concentration of oxygen 

 falls. The function fulfilled by haemoglobin as a car- 

 rier of oxygen from the lungs to the tissues is thus 

 readily intelligible, as well as the difference in colour 

 between arterial and venous blood. Substances in the 

 blood combine to form similar readily dissociable com- 

 pounds with carbon dioxide, but no change in colour 

 is associated with this process. 



Both deficiency of oxygen and excess of carbon 

 dioxide in the air were found to produce increase in 

 the breathing, and till recently the respective parts 

 played by oxygen and carbon dioxide in regulating 

 the breathing were by no means clear, and opinions 

 on the subject were divided. I was myself led to 

 investigate the whole subject through observations on 

 the effects of air vitiated by respiration or by the gases 

 met with in coal-mines and other confined spaces. 



When air highly vitiated by respiration or combus- 

 tion of carbonaceous material is breathed the amount 

 of air inspired or expired is increased. The increase 

 is due to the carbon dioxide in the air ; for when this 

 is removed there is no increase unless the deficiency of 

 oxygen is extreme. The effect produced on the breath- 

 ing by carbon dioxide in the inspired air increases out 

 of proportion to increase in the percentage of the car- 

 bon dioxide. This fact suggested that in ordinary 



