THE RESPIRATORY MECHANISM. 383 



Adam's apple, as a rigid cylindrical mass. It consists funda- 

 mentally of a fibrous tube in which cartilages are imbedded, 

 so as to keep it from collapsing; and is lined internally by a 

 mucous membrane covered by several layers of epithelium 

 cells, of which the superficial is ciliated. The elastic car- 

 tilages imbedded in its walls are imperfect rings, each some- 

 what the shape of a horse shoe, 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 re- 

 semble the windpipe in structure. 



The Structure of the Lungs. These consist of the bron- 

 chial tubes and their terminal dilatations; numerous blood- 

 vessels, nerves and lymphatics; and an abundance of connec- 

 tive tissue, rich in elastic fibres, binding all together. The 

 bronchial tubes ramify in a tree-like manner (Fig. 122). 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 re- 

 duced 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, Fig. 124), and the 

 air-cells, ft, which open into them, 

 have walls composed mainly of 

 elastic tissue and lined by a 

 single layer of flat, non-ciliated 

 epithelium, immediately 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 wail 

 and the thin epithelium, both of 

 which are moist, and well adapted 



,.~ . FIG. 124. Two infuudibula of the 



to permit gaSeOUS diffusion. lung: much magnified 6, 6. the air- 



m , -TV, - -i i cells, or hollow protrusions of the 



The Pleura. Each lung IS alveolus, opening into its central 



covered, except at one point, by ' 



an elastic serous membrane which adheres tightly to it and 



