VI FORM AND FUNCTION 93 
But what muscles are brought into play? The 
muscle that connects the wing with the back- 
bone will not help us now. We have to depend on 
the external intercostals, the action of which I have 
already explained (p.g1) for raising the back. Besides 
there are fairly strong muscles running along the 
vertebral column, and these will straighten the back 
at the point just in front of the pelvis, where, as I 
have said, there is a joint which allows a considerable 
rise or fall. 
The machinery of breathing, then, in birds is very 
different from ours. Let us compare the results, 
remembering that the object of breathing is to oxidise 
the blood. In all lungs the blood is separated from 
the air by a very thin membrane which allows the 
passage-out of carbonic acid gas and the passage-in 
of oxygen. An experiment will illustrate this. If 
black venous blood is placed in a bladder and the 
bladder placed in oxygen gas, the oxygen will find 
its way into the bladder, and the blood will be 
arterialised. In fact gases mechanically held in a 
fluid tend to diffuse into any atmosphere to which 
they are exposed—e'g. the carbonic acid gas in soda 
water—and gases separated by a dry porous partition 
diffuse into one another. But the: oxidation of the 
blood is a very complicated process. Here it is 
enough to say that when air as it comes from the 
lungs is:compared with fresh air, it is found to have 
gained about 5 per cent. of carbonic acid and to have 
lost about 5 per cent. of oxygen. This is proof 
positive of the interchange of gases. Moreover it is 
1 See Huxley’s Elementary Phystology, Lesson IV. 
