RESPIRATORY ORGANS. 



257 



The alveoli of infundibulum and duct are lined with squamous 

 epithelium, and in the walls is an extensive network of capillary blood- 

 vessels. The lining cells of the bronchioles are cubical and those of the 

 bronchi ciliated columnar. There are no skeletal elements in the bron- 

 chioles, but the bronchi have small cartilages in the walls, these ex- 

 hibiting a tendency in the larger tubes to approximate the rings or 

 semi-rings of the trachea. 



In their backward growth into the coelomic region the lungs either 

 insinuate themselves dorsal to the lining of the dorsal side of the 

 body cavity (dipnoi and a few scattered forms) so that only their ventral 

 surface has a serous coat; or they grow out as free structures, covered 

 on all sides by the ccelomic epithelium, and are bound to the dorsal wall 

 by a mesenterial-like fold of varying extent. This outer coat of epithe- 

 lium has received the name of pleura, the term being extended in the 

 case of the mammals to include the 

 whole lining of the pleural cavity, 

 separated from the rest of the ccelom 

 by the diaphragm (p. 135). 



DIPXOI, In Ceratodus there is a single 

 lung sac; Protopterus and Lepodosiren have 

 paired lungs, the two being united in front at 

 the entrance of the air-duct. In all three the 

 inner surface is divided more or less regu- 

 larly into groups of alveoli, separated by 

 more prominent partitions. The pulmonary 

 arteries arise from the last efferent branchial 

 artery of either side, and hence the blood 

 supply, under normal conditions, is arterial 

 and the lungs cannot act as respiratory 

 organs. In times of drought (Protopterus) 

 or of foul water (Ceratodus) the gills no longer function and the pulmonary arteries 

 bring venous blood to the lungs. 



AMPHIBIA. In the lower urodeles the two lungs are elongate (the 

 left the longer) and are united at their bases, true bronchi being absent. 

 Internally they may be entirely smooth as in Necturus, or there may be 

 alveoli in the basal portion (fig. 266), the whole representing a terminal 

 vesicle either connected directly with the trachea (^4) or by the interven- 

 tion of an alveolar duct (B) . In the caecilians the left lung is very short; 

 the other elongates, with alveoli developed throughout. In the frogs 

 (fig. 266, C) the two lungs are distinct, and their walls are divided into 

 17 



FIG. 266. Different types of am- 

 phibian lungs. A, Necturus, without 

 alveoli ; B, alveoli in the proximal por- 

 tion; C, frog, alveoli throughout. 



