52 



VERTEBRATE RESPIRATION 



mesobronchus although they are frequently referred to as ventro- 

 bronchi. This group of four anterior secondary bronchi are 

 amongst the largest tubes of the respiratory system expanding 

 rapidly from narrower origins on the mesobronchus. The third 

 of them communicates with both the anterior thoracic and inter- 

 clavicular sacs whereas the first sends a branch forwards to the 

 cervical air sac. The posterior group of secondary bronchi 

 (sixteen) arise both dorsally and ventrally from the meso- 

 bronchus. The posterior thoracic sacs expand from one of the 



Anterior 



secondary 



bronchi 



(c) Air 



capillaries 



Posterior group 



of 

 secondary bronchi 



Fig. 16. 



Diagrams of (a) the main respiratory passages and air sacs of a 

 bird viewed from the left medial side, (based on Akester, 1960.) 

 (b) Cross-section of many parabronchi. (c) A single parabronchus 

 with the air capillaries radiating from its central canal, (after 

 Hazelhoff, E. H. Poultry Sc, Vol. 30. 1951.) 



posterior bronchi or perhaps directly from the mesobronchus 

 itself. The secondary bronchi of the anterior and posterior 

 groups are joined by a very large number of tertiary bronchi 

 (parabronchi) which number over a thousand and form the main 

 bulk of the lung. Their path between the anterior and posterior 

 groups varies in relation to the position of their origin. The para- 



