250 NATURAL THEOLOGY. 



Were we seeking for the varieties of the organs 

 of voice, we might here start to the conclusion that 

 we had found in the animal body a complex in- 

 strument r?sembling a trombone or Siberian hunt- 

 ing horn ; but by further inquir} we should discover 

 that some of those birds that have the most com- 

 plex and varied windpipe have no cry at all. 



But there is a still more curious provision for 

 the extension and magnitude of the body of a bird, 

 independently of weight. In birds the air does not 

 only pass into their lungs, but through them, so as 

 to fill a series of air-cells, composed of fine mem- 

 branes which are interwoven with all the viscera. 

 The heart is surrounded by such a cell. Two great 

 cells are attached to the liver, and in the same 

 manner are all the viscera of the abdomen inter- 

 spersed with air-cells, and these all communicate. 

 The air thus admitted into the interior of the body, 

 extends even into the bones. Naturalists have mis- 

 taken the end of this structure when they have 

 represented it as a developement of the respira- 

 tory organs. It is not to make the function of res- 

 piration more perfect that the air enters so exten- 

 sively into the body ; as a proof of which, the air 

 in the bones of the head is supplied through the 

 nasal cavities and Eustachian tubes, independently 

 of the lungs altogether.* 



Mr. Hunter has shown us, that in many birds 

 the internal air-cells communicate with the exte- 



* Mr. Hunter's " Animal Economy." 



