276 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



vesicle now being known as an infundibulum (fig. 299), The result 

 of these many divisions is an enormous amount of internal respiratory 

 surface without great increase in the size of the whole organ. It is 

 to be noticed that in this subdivision the entodermal lining takes the 

 initiative, the outer (serous) surface showing but slight signs of the 

 internal modifications. 



Each infundibulum has its own duct which, when smooth inter- 

 nally, is called a bronchiole, when lined with alveoli, an alveolar duct. 

 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 



J-A^ 



Fig. 298. Fig. 299 



Fig. 298. — Two stages in the development of the lung of the pig, ventral views, after 



Flint. A, pig 5 mm. long; B, 18.5 mm. long, b, gill pouch; d, I, v, dorsal, lateral and 



ventral bronchi; oe, oesophagus; /, trachea. 



Fig. 299. — Scheme of mammalian lung structure, ad, alveolar duct; b, bronchus; 



fl, bronchiole; i, infundibulum lined with alveoli. 



those of the bronchi ciliated columnar. There are no skeletal ele- 

 ments in the bronchioles, but the bronchi have small cartilages in the 

 walls, these exhibiting 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 coelomic epithelium, and are bound to the 

 dorsal wall by a mesenterial-like fold of varying extent. This outer 

 coat of epithelium has received the name of pleura, the term being ex- 

 tended in the case of the mammals to include the whole lining of the 



