620 Comparative Morphology of Chordates 



the trachea, supported by rings of cartilage. Inside the more elabo- 

 rately subdivided lung are wide and irregularly branching passages 

 which are, in effect, continuations of the main bronchus but not so 

 definitely tubular. In the complex lungs of Chelonia and Crocodilia, 

 a wide central "bronchus" extends through nearly the entire length 

 of the lung (Fig. 473D), opening laterally into several large spaces 

 which, in turn, open into smaller spaces. Small irregular cartilages 

 may occur in the walls of these main internal bronchial passages. 



A study of its embryonic development shows that the adult rep- 

 tilian lung is not to be interpreted as a sac subdivided by inwardly 

 projected folds of its walls. It results, rather, from outgrowth of a 

 cluster of pouches from the distal region of the growing bronchus — as 

 if a cluster of soap bubbles were blown out from a pipe having several 

 holes near its end. These pouches push out into a dense mass of mesen- 

 chyme which is the source of the connective tissue, smooth muscle- 

 fibers, and blood-vessels of the lungs. The "partitions" of the adult 

 lung, which look as if they had been formed by ingrowth of folds from 

 the outer wall, result from the crowding together of walls of adjacent 

 pulmonary "bubbles." The partition consists of pulmonary epithelium 

 on each surface and a more or less thick layer of the mesodermal con- 

 nective tissue between them. The outer wall of the lung as a whole is a 

 mesodermal investment whose outermost layer is the pulmonary 

 peritoneum (pleura). The internal complication of the lung results 

 not from ingrowth of partitions, but from outgrowth of spaces. 



Comparing amniote lungs, the differences seem more striking than 

 the similarities. Even among reptiles the differences seem extreme, 

 but they are differences in degree rather than in kind of complication. 

 If several simple lungs such as occur in Sphenodon and some lizards 

 were assembled around a common bronchial passage, all opening into 

 it, the result would be such a lung as occurs in crocodilians and some 

 turtles. Turning to mammals, it may be suggested that, if a single 

 lobule of a mammalian lung (Fig. 472) were greatly enlarged, the 

 result would be a system of spaces resembling those of the entire lung 

 of a crocodilian, the bronchiole of the lobule corresponding to the 

 central bronchial passage of the reptilian lung. Birds have gone 

 beyond any such comparison, having completely abandoned pouches 

 and saccules in their entirely tubular lungs. 



As for ventilation, the reptilian lung of the simpler type is just 

 one big "dead end." Those of more complex type are no better for 

 having several or many "dead ends." The mammalian lung is a com- 

 plex of millions of microscopic "dead ends." The bird lung, by virtue 

 of its appended air-sacs, has no "dead ends." 



Lungs intermediate in type between those of reptiles and those of 



