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 _vings or 
semi-rings of the trachea. 
In their backward growth into the ccelomic 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). 
DIPNOI.—In Ceratodus there is a single 
lung sac; Protopterus and Lepodosiren have Cc 
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- é; 
larly into groups of alveoli, separated by A B 
more prominent partitions. The pulmonary 
arteries arise from the last efferent branchial 
artery of either side, and hence the blood Hid, 266 —Ditiereut aypes of ane 
supply, under normal conditions, is arterial phibian lungs. A, Necturus, without 
and the lungs cannot act as respiratory @lveoli; B, alveoli in the proximal por- 
organs. In times of drought (Protopterus) Benge frog, atyeolt roughstt: 
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 (A) or by the interven- 
tion of an alveolar duct (B). In the cecilians 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 
