238 RESPIRATION 



without some knowledge of the chemical changes in the blood 

 associated with external and internal respiration. We therefore 

 pass to the consideration of 



SECTION III. THE CHEMISTRY OF EXTERNAL RESPIRATION. 



Our knowledge of this subject has been entirely acquired in the 

 last 200 years, and chiefly in the last century. 



Boyle showed by means of the air-pump that animals die in a 

 vacuum, and Bernouilli that fish cannot live in water from which 

 the air has been driven out by boiling. 



Mayow, of Oxford, seems to a considerable extent to have antici- 

 pated Black, who in 1757 demonstrated the presence of carbonic 

 acid (carbon dioxide) in expired air by the turbidity which it causes 

 in lime-water. 



A fundamental step was the discovery of oxygen by Priestley in 

 1771, and his proof that the venous blood could be made crimson, 

 like arterial, by being shaken up with oxygen. 



Lavoisier discovered the composition of carbonic acid, and applied 

 his discovery to the explanation of the respiratory processes in 

 animals, the heat of which he showed to be generated, like that of a 

 candle, by the union of carbon and oxygen. He made many further 

 important experiments on respiration, publishing some of his results 

 in 1789, when the French Revolution, in which he was to be one of 

 the most distinguished victims, was breaking out. He made the 

 mistake, however, of supposing that the oxidation of the carbon 

 takes place in the blood as it passes through the lesser circulation. 



That some carbon dioxide is formed in the lungs there is no reason 

 to doubt, and the quantity may even be considerable. But that 

 they are not the chief seat of oxidation was sufficiently proved as 

 soon as it was known that the blood which comes to them from the 

 right heart is rich in carbon dioxide, while the blood which leaves 

 them through the pulmonary veins is comparatively poor. 



There are two main lines px which research has gone in trying to 

 solve the chemical problems of respiration: (i) The analysis and 

 comparison of the inspired and esepired air, or, in general, the in- 

 vestigation of the-gaseous exchange between the blood and the air 

 in the lungs. (2) The analysis and comparison of the gases of 

 arterial and venous blood, of the other liquids, and of the solid 

 tissues of the body, with a view to the determination of the gaseous 

 exchange between the tissues and the blood. We shall take these 

 up as far as possible in their order. 



The methods which have been used for comparing the composi- 

 tion of inspired and expired air and estimating the respiratory ex- 

 change are very various. 



