6 ORGANISM AND ENVIRONMENT 



of the centre. A further very significant fact, ob- 

 served originally by Hook in the seventeenth century, 

 but forgotten and rediscovered by Rosenthal in 1875, 

 is that if the blood in the lungs is over-aerated by 

 artificial ventilation, the breathing stops for a time, the 

 condition known as apnoea being established. It 

 seemed, therefore, that just as increased breathing, or 

 hyperpnoea, is due to defective aeration of the blood, 

 so apnoea is due to excessive aeration. This interpre- 

 tation of apnoea was soon challenged, as we shall see, 

 but was firmly established by an ingenious experiment 

 of Fredericq. He crossed the circulation of two 

 animals, so that the blood coming from the lungs of the 

 first animal passed to the respiratory centre of the 

 second, and vice versa. It was then found that when 

 excessive artificial ventilation was applied to the lungs 

 of the first animal the second became apnoeic, or vice 

 versa ; while great hyperpnoea in the first animal was 

 produced by the stoppage of the breathing in the 

 second. 



When aeration of the blood is defective in the 

 lungs two changes in the arterial blood occur. On 

 the one hand its content in oxygen becomes less, and 

 on the other hand it becomes more highly charged 

 with carbon dioxide. Blood which is not aerated 

 with oxygen has a dark purple tint, contrasting with 

 the bright scarlet of fully aerated blood. This dif- 

 ference in colour is due to the fact that haemoglobin, 

 the substance which gives blood its colour and is 

 contained in the red blood corpuscles, is the substance 

 which carries nearly the whole of the oxygen, and 



