THE RESPIRATORY ORGANS 61 



in the wall of all the different orders of 

 bronchi. 



Innumerable openings in the walls of the 

 parabronchia lead into minute canals beset 

 with dilatations corresponding to the air- 

 vesicles of the mammalian lung, and, like them, 

 separated from each other by thin septa con- 

 taining a close-meshed network of capillary 

 blood-vessels connecting the pulmonary artery 

 and pulmonary vein. 



Each tertiary bronchus forms the centre of a 

 lobule of the lung (Fig. 31). Between adjacent 

 lobules are septa of connective tissue in which 

 the branches of the pulmonary artery ramify 

 preparatory to sending twigs into the interior 

 of the lobule. 



Intimately associated with the respiratory 

 organs are thin-walled and membranous sacs 

 filled with air. These air^sacs communicate 

 with the bronchi on the one hand, and, on the 

 other, with the interior of many bones of the 

 limbs and other parts of the skeleton. They 

 are important respiratory organs, and by their 

 presence confer hghtness and buoyancy upon 

 the body, and, since they are mainly dorsal 

 in position, influence the position of the centre 

 of gravity. Structurally, the walls of the sacs 



