448 WILLIAM A. LOCY AND OLOF LARSELL 



It is commonly recognized by morphologists that our knowl- 

 edge of the development of the avian lung including its air-sacs 

 is both inadequate and defective in several important respects. 

 The notable observations of Schulze ('11) and of Juillet ('12) have 

 brought forward a newly recognized structural element, the re- 

 current bronchi, known onlj^ in the lung of birds, and which 

 imparts a renewed interest in the structural peculiarities of the 

 avian lung and the physiology of its air-sacs. It is now more 

 imperative than heretofore that we should have a review of the 

 embryological history of the lung with a more precise study of 

 the development of the bronchial tree, of the air-sacs and their 

 recurrent bronchi. 



The assumption that, except for air-sacs, the lungs of birds 

 and of mammals are essentially similar as to architecture has 

 retarded the recognition of the structural peculiarities of the 

 bird's ung. The beginnings are similar in these two classes of 

 vertebrates but the end-products are very different. There is 

 needed an embryological study to determine the way in which 

 the avian lung departs from the mammalian type and to de- 

 termine the precise nature of the intercommunications between 

 its bronchioles. The development of recurrent bronchi from the 

 air-sacs and the establishment of labyrinthine communications 

 between all parts of the bronchial tree, imparts to the avian lung 

 a unique architecture not found in any other class of vertebrates. 

 There is no ending of the ultimate twigs of the bronchial branches 

 in culs-de-sac, as in the lungs of other vertebrates, so that the 

 facilities for ventilation of the avian lung are very complete. 

 The absence of alveoli in which a portion of the air is retained as 

 residual air, permits the air current to sweep unimpeded through 

 the minutest air passages and affords great opportunities for 

 respiratory exchanges between the blood capillaries and the air 

 capillaries. The air-sacs receive their supply through direct ori- 

 fices, during inspiration, and the air passes from these reservoirs 

 into the lung by way of the recurrent bronchi during expiration. 

 It is essential to understand the intercommunications of the air 

 passages in order to comprehend either the morphology or the 

 physiology of the bird lung. 



