306 THE ZOOLOGIST. 
too long your respiratory movements will cease. If, on the 
other hand, you diminish the supply of oxygen, you will become 
“asphyxiated.” What you want, then, is oxygen, but you want 
this diluted. Pi - 
The first point, then, to which you must hold is this; you 
cannot continue to live without fresh supplies of oxygen. What 
is true of you is true of every living thing, whether animal or 
plant, that is made up of protoplasm. Supposing these supplies 
to be present, how are they used? They are used up, we know, 
because after a time the air of a space, into which fresh oxygen 
cannot enter, becomes unfit for respiratory purposes. They are, 
so far as we can judge from the condition of the air, used up in 
making carbonic acid; in other words, inspired air contains 
more oxygen and less carbonic acid than does expired air. The 
proportions per hundred parts being— 
Oxygen. Nitrogen. Carbonic Acid. 
In inspired air ...... 20°81 79°15 “04 
In expired air......... 16°083 79°557 4-380 
So that about 4 per cent. of oxygen is lost from the air in the 
process of breathing. 
But now arises this question: If every living thing requires 
oxygen, your great toe no less than your brain requires at times 
a fresh supply—how does it get it? It gets it from that medium 
which is the source of communication between the various parts 
of the body—it gets it from the blood. If you were to take 
some blood which had just been pumped out of the heart into 
the great blood-vessels, and compare it with an equal quantity 
of blood just returned from the body to the heart, you would 
see that the former would be bright scarlet, the latter of a 
purplish hue; and if, with a suitable air-pump, you removed 
the gases from the blood, you would find that the amount of 
oxygen in the purple blood was only one-half of that in the 
scarlet fluid, and that the carbonic acid had increased by about 
eight parts in a hundred; and further, were you to shake up the 
purple blood with air or with oxygen you would find it become 
again of a bright scarlet colour. 
The second point, then, is this: on its way round the body 
the blood loses its oxygen and gains carbonic acid. Though we 
call this blood a fluid, yet it is, we know, “‘ thicker than water ” ; 
