170 RESPIRATION 



Structure of lungs. Besides the capillaries with their 

 connected arteries and veins, the air sacs and their 

 connecting passages, the bronchial tubes and bronchi, 

 the lungs consist of but little more than a strengthening 

 framework of soft elastic connective tissue, which holds 

 all the parts together and forms a strong yet highly 



Fio. 100. Section of lung with distended blopd-veesels, highly magnified, c, c, 

 partitions*between air sacs; 6, small artery giving off capillaries to walls of air 

 sacs. 



elastic spongy mass. The bulk of this mass when empty 

 of air occupies but a small fraction of the space w r hich 

 it fills in the chest cavity during life. In other words, 

 the main bulk of the lungs is due to the air which fills 

 the air sacs. 



Membrane covering lungs. The lungs have for out- 

 side covering an exceedingly thin membrane formed of 

 flattened cells, which also lines the chest cavity within 

 which the lungs lie. This membrane, known as the 



