THORACIC CAVITY 



61 



distributed to the middle lobe. It is accompanied by a 

 tributary of the superior pulmonary vein. All the other 

 hyparterial branches are distributed to the lower lobe, and 

 are accompanied by tributaries of the inferior pulmonary vein 

 and branches of the pulmonary artery (Fig. 25). 



Trachea 



Right pulmonary artery 



\ 

 Right bronchus \ 



Left bronchus 



Left pulmonary artery 





Ventral hyparteria 

 bronchi with arteries 

 (blue) and veins (red) 



Dorsal hyp- 



rterial bronchi 

 ' i Left atrium 



Pulmonary artery 

 ' Conus arteriosus 

 ist ventral accessory hyparterial 

 bronchus with blood-vessels 



FIG. 25. Diagram of the Lungs with the Bronchi and Blood- Vessels. 



i. First left ventral hypart 

 First right ventral hypar 



First left ventral hyparterial bronchus. 

 hyarterial bronchus. 



3. Eparterial bronchus. 



All the bronchi, except the smallest terminal branches, 

 are kept permanently open by bars and plates of cartilage, 

 which lie in their walls and enable the bronchi to be 

 so easily distinguished from the blood-vessels. Nevertheless 

 the lumina of all the bronchi can be reduced in size by the 

 contraction of the unstriped muscle fibres in their walls, for 



