THE RESPIRATORY APPARATUS IN BIRDS. 567 



dilatation of that viscus during inspiration. And the entrance of air into the 

 pulmonary tissue is not due to this dilatation ; it is due to the dilatation of the 

 diaphragmatic reservoirs ; the position of these effectively admits of their expan- 

 sion, by the play of the inferior on the superior ribs. The air is then drawn 

 into their cavity after traversing the larger bronchial tubes which open into 

 them, and also after passing across a certain region of the capillary network 

 formed by the canaliculi, where it comes into mediate contact with the blood, 

 and is submitted to the necessary transformations. The atmosphere, therefore, 

 arrives in the diaphragmatic sacs partly pure and partly altered by its contact 

 with the blood. During expiration, it again resumes the course it followed on 

 its introduction, traverses a second time the lung, and is thus respired once 

 more before being expelled from the body. It is, therefore, obvious that the 

 hasmatosic transformations accomplished in the lung take place during the two 

 acts of respiration — inspiration and expiration. 



In studying the part that the other reservoirs play m this function, 

 Sappey has been able to prove that they act as antagonists to the first, by 

 contracting during inspiration and expanding in expiration. No doubt, at the 

 time of the contraction of the 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 doubt, 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. Sappey has also noted that these contents are always 

 formed 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 

 locomotion, by diminishing the weight of the body, and, by their position, 

 rendering equiUbrium more stable. 2. On the voice, thfi range and power of 

 which they augment. 



