THE EMBRYOLOCxY OF THE BIRD's LUNG 35 



with the heart is exhibited. The puhnonary arteries are now 

 hxrge and the ducti BotalU are atrophied. 



The effect of the obhteration of the ducti BotaUi is that the 

 blood from the right ventricle can no longer pass through the 

 sixth arch to the aorta but is sent entirely through the pulmonary 

 arteries to the lungs. 



COMMENTS 



The morphology of the avian lung can be made clear only by 

 observations of its development. It is through this channel alone 

 that one becomes acquainted with the nature of the modifica- 

 tions of the bird's lung that place it in a class by itself. It can be 

 safely said that the facts of morphology separate the avian lung 

 from the lungs of all vertebrates, with the possible exception of 

 the reptiles. The excellent papers of Milani ('94) and of Hesser 

 ('05) on the embryology of the reptilian lung are classics, but the 

 embryology of the air-sacs is insufficiently known. On account 

 of the development of air-sacs, of recurrent bronchi and laby- 

 rinthine connections between all air passages, the avian lung ex- 

 hibits a special architecture and upon our understanding of this 

 architecture will depend our conception of its physiology. While 

 we have described what we conceive to be its physiological anat- 

 omy, we have not engaged in experiments that entitle us to make 

 any special comment on its physiology. 



The steps in arriving at a conception of the bird's lung have 

 been gradual and dependent upon increasing definiteness in the 

 knowledge of its internal architecture. Since the observations of 

 the early investigators have been a factor in molding this concep- 

 tion, and since ideas of lung anatomy have been so dominated by 

 their results, they may be briefly summarized. Moreover, the 

 sketches of Selenka ('66) are still used in text-books to illustrate 

 the development of air-sacs (vide Lillie, Development of the 

 Chick, '08) Hertwig-Kingsley, Manual of Zoology, Revised Edi- 

 tion, '12), accordingly a summary will not deal with obsolete 

 matters. 



