RESPIRATION. THE MECHANISM OF BREATHING 351 



each somewhat the shape of a horseshoe, and the deficient part 

 of each ring being turned backwards, it comes to pass that 

 the deeper or dorsal side of the windpipe has no hard parts in 

 it. Against this side the gullet lies, and the absence there of the 

 cartilages no doubt facilitates swallowing. The bronchi resem- 

 ble the windpipe in structure. 



The Structure of the Lungs. These consist of the bronchial 

 tubes and their terminal dilatations; numerous blood-vessels, 

 nerves, arid lymphatics; and an abundance of connective tissue, 

 rich in elastic fibers, binding all together. The bronchial tubes 

 ramify in a tree-like manner (Fig. 113). In structure the larger 

 ones resemble the trachea, except that the cartilage rings are not 

 regularly arranged so as to have their open parts all turned one 

 way. As the tubes become smaller their constituents thin away; 

 the cartilages become less frequent and finally disappear; the 

 epithelium is reduced to a single layer of cells which, though 

 still ciliated, are much shorter than the columnar superficial cell- 

 layer of the larger tubes. The terminal alveoli (a, a, P"ig. 115) 

 have walls composed mainly of elastic 

 tissue and lined by a single layer of 

 flat, non-ciliated epithelium, immedi- 

 ately beneath which is a very close 

 network of capillary blood-vessels. 

 The air entering by the bronchial tube 

 is thus only separated from the blood 

 by the thin capillary wall and the thin 

 epithelium, both of which are moist, 

 and well adapted to permit gaseous 

 diffusion. 



The Pleura. Each lung is covered, 

 except at one point, by an elastic se- 

 rous membrane which adheres tightly ^S^^ag^^SS^. k, b, 

 to it and is called the pleura; that | lollow protrusions of the aiveo- 



r , . jus, opening into its central cav- 



point at which the pleura is Wanting ity; c, terminal branches of a 

 11 i .1 < ,i i i bronchial tul i-. 



is called the root of the lung and is on 



its median side; it is there that its bronchus, blood-vessels and 

 nerves enter it. At the root of the lung the pleura turns back 

 and lines the inside of the chest cavity, as represented by the 

 dotted line in the diagram Fig. 3. The part of the pleura at- 



