THE BESFIBATOBT APPARATUS IN BIRDS. 483 



been able to prove that they act as antagonists to the first, by contracting during inspira- 

 tion and expanding in expiration. No doubt, at the time of the contraction of tlie middle 

 reservoirs, a small quantity of the air they contain is driven back into the anterior and 

 posterior sacs in passing across the lung ; and without dovxbt, also, these latter give a part 

 of their contents to the diaphragmatic sacs at the moment of the expansion which draws 

 the air into these reservoirs. M. Sappey has also noted that these contents are always 

 iormed of entirely vitiated air, while the air of the middle reservoirs has only been 

 partially respired. 



It is necessary to add that the functions of the air-sacs do not cease here ; for it has 

 been demonstrated that they exercise a very marked influence : 1, On locoinotion, by 

 diminishing the weight of the body, and, by theii- position, rendering ec[uilibrium more 

 stable ; 2, On the voice, the extent and power of which they augment. 



