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 
