THE RESPIRATION 



75 



FIG. 70. LOBULE 

 OF LUNG. 



is to draw the ends together at times (for instance, during 

 coughing) and reduce the size of the tube. The function 

 of the hoops of cartilage is to keep the windpipe open at 

 all times. If it should be closed by pressure, life might 

 be lost. These rings of cartilage may be felt in the neck. 

 The lower end of the trachea is just behind the upper 

 end of the breastbone; there it divides into two large 

 tubes. These subdivide into a great 

 number of smaller branches called bron- 

 chial tubes. Cartilage is found in the 

 walls of all but the smallest of the tubes. 

 The subdivision continues, somewhat like 

 the branching of a tree, until the whole 

 lung is penetrated by bronchial tubes. 

 Each tiny tube finally ends in a wider 

 funnel-shaped chamber called a lobule 

 (Fig. 70), into which so many dilated 

 sacs, called air cells, open, that the walls of the terminal 

 chamber or lobule may be said to consist of tiny cups, or 



air cells, placed side by 

 side. The lobules, or 

 clusters of air cells, are 

 chiefly near the surface 

 of the lung. (The word 

 " cell " is here used in 

 its original sense to de- 

 note a cavity or cham- 

 ber, and not in the sense 

 of a protoplasmic cell.) 

 The air cells are elastic 

 FIG. 7 i. CAPILLARIES AROUND AIR SACS and enlarge by stretch- 



OF LUNGS (enlarged 30 diameters). Air ^ thg chegt ex _ 



sacs in white spaces. Dark lines are capil- 

 laries. (Peabody.) pands ; hence, the cells 



