80 BRITISH BIRDS 



be seen that the chest expands during inspiration, and that its calibre 

 diminishes during expiration. What happens is this. The lungs are 

 contained in a cavity which contains no air. This cavity can be in- 

 creased in size in two directions. When the ribs are moved out 

 which they can be by the movements of the muscles called inter- 

 costal, which lie between them the cavity of the chest from before 

 backwards is evidently enlarged. On the other hand there is the 

 diaphragm, which we have already spoken of as bounding the chest 

 cavity below. Now this diaphragm is muscular, with a tendinous 

 centre. When the muscles contract, like all muscles do, the surface 

 of the diaphragm, which was before rather convex towards the 

 chest cavity, becomes more flat ; hence the cavity lying above it, i.e. 

 the chest cavity, becomes larger in a downward direction also. When 

 it is increased in this way by the action of the two separate sets of 

 muscles, some space more space than before is left between its 

 walls and the lungs which lie within it ; it follows, therefore, that, 

 as there is no air in the cavity, the pressure of air outside the body 

 forces more air into the lungs, because there is no counterbalancing 

 pressure to prevent this. The principle is the same in the bird, 

 but the details are different. If you will turn again to the 

 bird's skeleton, you will see that the backbone and ribs and 

 sternum form a bony box, which is jointed in the middle ; this 

 acts precisely like a pair of bellows : the bones at top and bottom 

 represent the wood, and the soft intervening leather of the bellows 

 is represented by the muscles which lie between, and which connect 

 the sternum with the abdomen and with the ribs. When these 

 muscles contract, the sternum is obviously brought nearer to the 

 backbone, and air is expelled from the inside ; when they are re- 

 laxed, a vacuum is created and air rushes in. The air-spaces, then, 

 are really ramified tags of lung which have no blood-vessels in their 

 walls, and are therefore not meant for respiration, but serve as 

 reservoirs of air, lightening the body of the creature. It is curious 

 that birds are not the only animals which possess expansions of 

 lung that are apparently useless for breathing purposes. The lungs 

 of the Chameleon have quite similar sacs appended to them. There 

 is, it is true, no such complicated a ramification as that which we 

 find hi the bird, but still there is no doubt that the structure is of 

 the same natur 3. It looks almost like a first step in the path towards 

 a bird. Very possibly the extinct Pterodactyles, which flew through 

 the woods of the middle ages of the earth, had bodies lightened in 

 the same or a similar way ; for we know that their bones have thin 



