xvi. 24 RESPIRATION 473 



The lungs are rather small spongy organs, with little elasticity. The 

 air passes backwards in a large main bronchus running through the 

 lungs and giving off branches to the lung substance, but continuing 

 beyond to the inspiratory air-sacs (Figs. 283 and 284). These are thin- 

 walled chambers, divided into two sets, the posterior inspiratory and 

 anterior expiratory. The posterior, inspiratory, air-sacs are the ab- 

 dominal and posterior thoracic and they 

 are filled by the air rushing into them 

 through the main bronchus. The anterior 

 or expiratory air-sacs include an anterior 

 thoracic, median interclavicular, and cer- 

 vical, these often communicating with 

 spaces in the bones. At expiration the 

 air passes from the more posterior sacs 

 through the lungs by special recurrent 

 bronchi into the anterior sacs. From 

 these the air may be expelled to the 

 exterior, return to the lungs being pre- 

 vented by closure of sphincters. In some 

 conditions, however, especially in diving 

 birds, the air may be passed backwards 

 and forwards through the lungs several 

 times, until all its oxygen has been used. 

 The branches of the bronchi in the lungs 

 do not end blindly in alveoli, but make an 

 elaborate system of lung capillaries. Air 

 sweeps through the larger channels at 

 inspiration and expiration, but probably 

 reaches the finer capillaries by diffusion. 



The mechanism by which the ventilation is produced is complicated 

 and depends largely on the movements produced during locomotion. 

 The upper surface of the lung adheres to the ribs, its lower surface is 

 covered by a special membrane derived from the peritoneum and 

 known as the pulmonary aponeurosis (Fig. 285). This is connected 

 with the ribs by costopulmonary muscles. The floor of the thoracic 

 air-sacs, which lie below the lungs, is also covered by a fibrous mem- 

 brane, the oblique septum, but the walls of the remaining air-sacs are 

 very thin. Quiet respiratory movements are produced by the inter- 

 costal (inspiratory) and abdominal (expiratory) muscles, acting upon 

 the thoracic and abdominal cavities so as to enlarge and contract the 

 thorax, drawing air in and out of the air-sacs, through the lungs. During 



Fig. 284. The air-sacs of a bird. 



1.. right lung; c. cervical air-sac; ICL. 

 interclavicular; A.S. outgrowth into 

 humerus (h.); A.th. anterior thoracic 

 air-sac; p.TH. posterior thoracic; 

 AnD. abdominal air-sac; tr. trachea. 

 (From Thompson, Biology of Birds, 

 Sidgwick & Jackson, Ltd.) 



