AVES. 249 



into two secondary bronchi. Other secondary bronchi come off 

 from the vestibule. All these tubes are placed near the ventral 

 surface of the lung, the dorsal part of which organ is mainly 

 made up of their branches, delicate tubes with blind endings. 



Closely connected with the lungs are a number of thin-walled 

 air-sacs into which the secondary bronchi open. 



On each side of the body, in the abdominal region, between the kidneys 

 and the intestines, there is a large posterior air-sac, communicating with 

 the hinder end of the lung. In front of this are two pairs of intermediate 

 air-sacs, situated on the ventral side of the body-cavity. The posterior 

 intermediate air-sacs overlap the hinder part of the lungs, and communicate 

 with them by openings close to those of the posterior sacs, while the 

 anterior intermediate air-sacs are situated ventral to the anterior two-thirds 

 of the lungs, and open into them near the middle of their length. In the 

 region of the clavicle a large, unpaired inter -clavicular air-sac is present 

 (formed by the fusion of two sub-bronchial air-sacs), communicating with 

 either lung by an aperture near the entrance of the bronchus. This air-sac is 

 bilobed, and each lobe passes out of the body-cavity near the origin of the 

 wing to communicate with the hollow shaft of the humerus, while dorsal to 

 it there is a small prebroncJiial air-sac, connected with the anterior end of 

 the lung. 



The most important histological point to be noticed is the finer 

 structure of the delicate tubules making up the greater part of 

 the lungs. The walls of these are raised up internally into 

 intersecting ridges, which form a complicated honeycombing, 

 traversed by networks of capillaries, and covered by simple 

 squamous epithelium. 



The epithelium lining the bronchi and trachea is largely 

 ciliated. 



The lungs are very immobile and take but little part in the 

 respiratory movements. Expiration is effected by a contraction 

 of the body-walls, by which the highly elastic air-sacs are com- 

 pressed. Inspiration is passively effected by the elasticity of the 

 body-walls, which expand and cause air to rush into the air-sacs. 

 Owing to the presence of these, a large amount of air passes 

 through the main passages of the lung. The essential part of 

 respiration takes place in the small tubes making up the bulk of 

 the lung, the walls of which are raised into folds, and the air is 

 kept pure in them by diffusion. 



Respiration is very vigorous, in accordance with the rapid 

 oxidation (metabolism) of the tissues. A great deal of heat 

 results from this latter process, and the body is maintained at 

 a very high temperature (103- 10 4 F.) 



