THE ANATOMY OF A BIED 27 



of muscles ; these when they contract shorten the syrinx, and 

 of course produce alterations in the note, just as the shortening of 

 the tube in a cornet alters the sound. In many passerine birds, and 

 in most other birds, there is only one pair of these muscles ; but the 

 Parrots agree with the passerines in having several pairs of muscles, 

 and therefore a more easily alterable syrinx. In a good many birds 

 there are no muscles at all in this place ; for example, in the Storks, 

 which have not by any means a flexible voice. The syrinx, in fact, 

 is one of those organs which show a great deal of difference in 

 different kinds of birds. But it is never entirely absent, though 

 rather rudimentary in the Ostrich. The Australian Emu has a 

 curious way of producing its sounds which is not found in any other 

 bird. The cock and hen Emus can only be recognised by their 

 voice, which is duller in the hen and sharper in the cock. When 

 the bird is uttering its note, it seems almost to come from some- 

 where else, and not from the throat of the bird ; the bird is some- 

 thing of a ventriloquist. The sound, which is a low bellow, is pro- 

 duced by a bag of skin opening into the windpipe some way up 

 the neck ; a current of air passing down the tube is believed to set 

 the air in this bag in vibration, just as the air in a key may be 

 caused to vibrate by blowing over its edge. Generally speaking, 

 the windpipes of birds are straight tubes running to the lungs by 

 the shortest route ; but in the Cranes, and in a few other birds, the 

 pipe is coiled upon itself once or twice, and the coils are even hidden 

 in an excavation of the breast-bone. The increased length of tube 

 gives a louder and more resonant note, such as we know character- 

 ises the Crane. 



Lungs and Air-sacs. 



Ifc is not only by virtue of their powerful muscles and stiffened 

 fore limbs that birds can fly. The body is rendered lighter in pro- 

 portion to its bulk by air-cavities, which permeate everywhere, even 

 into the substance of the bones. So thorough is this aeration in the 

 Screamer of South America, that when the skin of the recently dead 

 bird is roughly pressed it crackles. Curiously enough, there seems 

 to be no very definite relation between the degree of thoroughness 

 to which the aeration of the body is carried out and the capacity 

 for flight. The Screamer, that has just been mentioned, is fuller of 

 air-cavities than the Frigate-bird, in which the art of flying is carried 

 to the highest extreme the ' triumph of the wing,' as Michelet says 



