RESPIRATION. THE MECHANISM OF BREATHING 389 



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. 

 Here the wall consists of smooth muscle. Against this the gul- 

 let lies, the absence of cartilage facilitating swallowing. The 

 bronchi are similar in structure. 



The Structure of the Lungs. These consist of the bronchial 

 tubes and their terminal dilations; numerous blood-vessels, nerves, 

 and lymphatics; and an abundance of connective tissue, rich in 

 elastic fibers, binding all together. The bronchial tubes ramify 

 in a tree-like manner (Fig. 112). The larger ones resemble the 

 trachea, except that the cartilage rings do not have their open 

 parts all turned one way, and the smooth muscle encircles the 

 tube completely. As the tubes become smaller their constituents 

 thin away; the cartilages become less frequent and finally dis- 

 appear; the epithelium is reduced to a single layer of cells which, 

 though still ciliated, are much shorter than the columnar super- 

 ficial cell-layer of the larger tubes. The terminal alveoli (a, a, Fig. 

 114) 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-ves- 

 sels. 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 fitted for gaseous 

 diffusion. 



The Pleura. Each lung is covered, 

 except at one point, by an elastic se- 

 rous membrane which adheres tightly 

 to it and is called the pleura; that 



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

 n j . i . ,. , , ! j . ~ bronchial tube. 



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 

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



