. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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 ccelomic regi
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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 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). \J A B DIPNOI.—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. /. FIG. 266.—Different types of am- phibian lungs. A, Necturus, without alveoli; B, alveoli in the proximal por- tion; C, frog, alveoli throughout. AMPHIBIA.—In
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