25 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



surface of this bronchus. On the left side the corre- 

 sponding artery turns backward above the level of 

 the first bronchial branch. The bronchus to the 

 upper lobe of the right lung is therefore called an 

 eparterial bronchus. All the other bronchi are be- 

 low their respective arteries and are called hyparterial 

 bronchi. Because of these relations it is believed 

 that the upper lobe of the right lung has no homo- 

 logue on the left side, and that the middle lobe on 

 the right side is homologous to the upper lobe on the 



left. The pulmonary ar- 

 tery divides with the 

 bronchi and closely ac- 

 companies them along 

 their posterior or superior 

 walls. The correspond- 

 ing veins pass along the 

 anterior or inferior walls. 

 These blood-vessels are 

 very large, often as large 

 as the bronchial tubes, 

 but in no case do they 

 supply blood to the walls 

 apex of the pulmonary 

 lobule, the pulmonary artery breaks up into several 

 small twigs, one for each antrum, supplying blood 

 to an extensive capillary plexus that spreads over 

 the surface of atria and air sacs. The capillary 

 meshes are very dense, and the capillary tubes very 

 large, so that the intervening spaces are barely wider 

 than the capillaries themselves. Because of thelarge 

 size of the lung capillaries it is possible for fine 



Fig. 193. Section from in- 

 jected lung showing capillaries of 

 an air sac. 



of the bronchi. At the 



