22 ProceeiUmjs. 



The second class of the Sanropsitla comprises the birds, 

 and these are so numerous, and the characters by which they 

 are distinguished are so sUght, that my best plan is to make 

 a few general remarks about their characters as a class, and 

 then mention a few of the more striking forms. 



The bird's heart always has four chambers, two auricles 

 and two ventricles, and breathing is effected by two lungs 

 attached to the inner surface of the back. A character 

 peculiar to the birds is the possession of air-sacs. These are 

 a kind of bladder, some of which are situated in the chest, 

 some in the abdomen, and others beneath the skin. They 

 are connected with the lungs, and are capable, through those 

 organs, of being filled with or emptied of air. Their use is 

 very obvious. Hold a common air-ball some distance above 

 the ground, then drop it, and note how slowly it falls ; but 

 let the air out and drop the empty skin, and how quickly it 

 touches the ground. If we then imagine the bird to be partly 

 composed of air-balls (as it literally is), how much greater 

 buoyancy the body of the creature possesses when these are 

 filled than when they are empty. There is another point of 

 structure displayed by the birds, though not entirely confined 

 to them, as they possess it in common with the bird-like 

 Reptiles I mentioned just now, and also with the Crocodile. 

 "This is the possession of pneumatic bones, that is, bones 

 which are filled with air instead of marrow. These bones, 

 among which are the shoulder-bone, the breast-bone or ster- 

 num, the ribs, the blade-bone, the thigh-bone, and even some 

 of the joints of the spinal-column, are the largest and heaviest 

 in the body, and thus acquire greater lightness. Their hollow 

 interiors are in connection with the air-sacs, and through 

 these, of course, with the lungs. The celebrated sm-geon, 

 Hunter, actually made a Pigeon breathe through its shoulder- 

 bone by cutting the bone through, and then tying up its 

 wind-pipe. It is worthy of remark that neither the Gulls, the 

 Penguin, nor the Ostrich possess these pneumatic bones. 



Now, in direct connection with a bird's power of flight we 

 note two things, the peculiar development of the breast-bone, 

 and the large size and flat shape of the shoulder -bone. These 



