THE EMBRYOLOGY OF THE BIRD's LUNG 495 



This dorsal plexus, or network, is an important one and in one 

 respect it is unique. Instead of uniting two different groups of 

 parabronchi (the usual situation) this network is composed only 

 of dorsobronchi. It is central in position and at its periphery it 

 connects with the parabronchi of the different systems with which 

 it is surrounded. In this manner there is established b}^ the in- 

 termediation of the reseau anastomotique a general communica- 

 tion between the bronchial circuits. In addition to this it forms 

 connections with recurrent bronchi from the abdominal air-sac. 



The parabronchi. The parabronchi, frequently designated lung 

 pipes (Bronchuli respiratorii of Schulze) belong to the third order 

 of bronchial tubes. In order to make this classification clear we 

 must bear in mind that the central lung tube is primary ; and that 

 all air-tubes opening directly on the primary are secondar}^ 

 Branches of small and nearly uniform calibre, arising from subdi- 

 visions of the secondaries, are tertiar3^ For a specific illustra- 

 tion take the ento- and ectobronchi: The subdivisions of the 

 ento- and ectobronchi form fan-like and feather-like groups, 

 the branches of which diminish in diameter until they reach a 

 certain size, which thereafter they maintain, and continue as 

 cylindrical pipes; these tubes of uniform diameter are the ter- 

 tiaries, or parabronchi. By their anastomosis they form the net- 

 work of passages so characteristic of the avian lung. Since 

 parabronchi are incomparably more numerous than the second- 

 aries, the great mass of the lungs is composed of tertiary bronchi 

 and the air-capillaries that spring from them. 



With the exception of the plexus of dorsobronchi, mentioned 

 above, the parabronchi unite two opposite systems of secondary 

 bronchi. Owing to this union, as C'ampana pointed out, there 

 is no bronchial tree, but instead bronchial circuits, the middle 

 part of which is multiplied into small tubes (tertiaries) while the 

 extremes (secondaries) open on the primary bronchus. The pri- 

 mary bronchus is tracheal, its branches only are pulmonary. 



The network of parabronchi is formed relatively late and, 

 chronolog cally, as w^ll as w4th respect to their anatomical rela- 

 tions, the account of the parabronchi might well follow that of 

 the air-sacs and their recurrent bronchi but unity of treatment 

 makes it ad\antageous to consider them at this jioint. 



