54 



VERTEBRATE RESPIRATION 



Coracoid 



External 

 intercostal 



Backbone 



Vertebral 

 rib 



Sternal 

 rib 



Sternum 



Fig. 17. 



Diagram to show the ventilation movements of the ribs and 

 sternum of a bird. Their position during expiration are shown by full 

 lines. Following activity of the external intercostal muscles the 

 positions shown by dotted lines are taken up during inspiration. 



direct the flow along the various respiratory tubes as had been 

 suggested by several authors. Nevertheless, the parabronchial 

 walls contain muscles which by contracting can vary the cross 

 sectional area of the tube and hence vary their resistance. It is 

 almost certain that air passes along the parabronchi during both 

 the inspiratory and expiratory movements. There will therefore 

 be a continuous flow of air past the areas from which the 

 air capillaries take their origin. The absence of dead spaces near 

 the sites of gaseous exchange is mainly the result of the bellows- 

 like action of the air sacs. 



The two most probable theories of ventilation diff'er in their 

 views concerning the direction of air flow during the two phases 



