240 A MANUAL OF PHYSIOLOGY 



a time when no spontaneous respiratory movements are going on. 

 This has been observed, for instance, in cats during resuscitation 

 of the brain after a period of anaemia. In man also, in a case of 

 Cheyne-Stokes respiration accompanied by hiccup, it was seen 

 that the hiccup persisted during the periods of apncea. If the 

 respiratory centre is the centre for the hiccup reflex, it can there- 

 fore be excited by afferent nervous impulses at a time when it is not 

 excited by the normal chemical stimulus (MacKenzie and Cushny). 



Chemistry of 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 

 anticipated 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 most 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 pro- 

 cesses 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, pub- 

 lishing 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 on which research has gone in trying 

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

 and comparison of the inspired and expired air, or, in general, 

 the investigation 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 



