282 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



mesobronchus and usually four of the secondary bronchi do not stop 

 at the lung wall, but are continued as thin- walled sacs which occur in 

 all living species of birds. Each of these sacs has received several 

 names; for details of these reference should be had to special works. 

 The interclavicular sac lies anterior to the furcula. It arises from 

 two parts on either side of the body (fig. 306), but these usually 

 unite so that the result is an unpaired structure. The cervical sac 

 of either side lies at the base of the neck and gives off a branch which 

 extends to an axillary sac in the region of the axilla. The other sacs 

 lie in the trunk, lateral to the viscera, and are called the anterior 

 intermediate, posterior intermediate, and abdominal, the relations of 

 which are shown in figures 306 and 307. These sacs receive their air 

 from the ends or from branches of the bronchi. Recently it has 

 been found that the air does not return by the same passages, but 

 that there are recurrent bronchi leading from the air sacs back to the 

 lungs (fig. 303). From the air sacs slender tubes extend among the 

 viscera and into certain of the bones. The pelvis, humerus, ster- 

 num, coracoid, and ribs most frequently contain prolongations of the 

 air sacs — are pneumatic — less frequently the femur, furcula and 

 scapula. 



Usually there are eight entobronchi (fig. 303), these arising from the medial 

 side of the mesobronchus, but bending to the ventral side of the lung. The 

 ectobronchi vary in number from six to ten. They arise on the dorso-median 

 side of the mesobronchus and pass to the dorsal side of the lung. The latero- 

 bronchi are comparatively few. They supply the lateral part of the lung. The 

 dorsibronchi are smaller and more numerous than the others, some arising from 

 the ectobronchus and some from the mesobronchus. In the chick there are 

 about twenty-five of these. 



The cervical air sac is connected with the second entobronchus (fig. 303). 

 The duct leading to the anterior intermediate sac arises from the middle part of 

 the mesobronchus, that for the posterior intermediate from one of the latero- 

 bronchi, while the abdominal sac is a direct continuation of the meso- 

 bronchus. Of the recent reptiles only the chameleons have anything like air 

 sacs. The bones of the fossil bird Archoeopteryx were not pneumatic, but those 

 of some of the extinct dinosaurian reptiles were. 



There is considerable uncertainty as to the functions of the air 

 sacs. The fact that the walls are supplied with arterial blood, to- 

 gether with the absence of any respiratory capillary hetwork, nega- 

 tives the idea that they are to any great extent respiratory. It has 

 been suggested that they are concerned with the maintenance of 

 equilibrium during flight and that they also lessen the specific gravity 



