THE LUNGS. 



349 



a consideration of the air-sacs of a frog. These consist 

 of membranous sacs, which are filled by the act of deglu- 

 tition. They present upon their inner surfaces numerous 

 reticulations, forming pits, which are the air-cells proper. 

 The capillary vessels are freely distributed to the minute 

 partitions between the pits, and the blood is exposed to 

 the action of the air. In the human subject we have a 

 similar arrangement, but very much dwarfed in size, an 

 arrangement of a microscopic sac, presenting numerous 

 reticulations, forming pits or air-cells, the entire sac of 

 cells being called a cluster. 

 Ten to forty clusters make a 

 lobule, which is surrounded by 

 areolar tissue, containing much 

 yellow elastic and some in- 

 voluntary muscular fibres, 

 which also form the nidus for 

 the ramification of arterioles, 

 venules, nerves, and the lym- 

 phatics. To further elucidate 

 the structure of the lungs, it 

 is necessary to follow the course 

 of a bronchus, which, upon 

 entering the hilum, divides and subdivides repeatedly, 

 until it has attained the diameter of one-fiftieth of an inch, 

 when it loses its encircling rings, which are replaced by 

 delicate cartilaginous plates ; these then become entirely 

 membranous, and terminate in the air-sacs, these air-sacs 

 being simple dilatations, very similar to the large air-sacs 

 of the frog ; they present along their walls a reticulated 

 arrangement of the lining mucous membrane ; these 

 reticulations form numerous pit-like, polyhedral air-cells, 

 separated by very delicate septa, in which the capil- 

 laries ramify; they all open into the common air-sac, and 



FIG. 151. Two CLUSTERS OF AIR- 

 CELLS. 



