vi FORM AND FUNCTION 93 



Hut what muscles arc 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. 91) 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. 1 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 Physiology, Lesson IV. 



