BLOOD SYSTEM. 



359 



Structure. — We have already said (page 204) that 

 two tubes go down from the throat into the body cavity 

 — the posterior one, the gullet, to the stomach, and the 

 anterior one, the windpipe, or trachea, to the lungs. 

 The one is soft and flaccid, but muscular ; the other is 

 a firm, open tube, being kept open by a series of carti- 

 laginous rings. This 

 is necessary in order 

 that the air may come 

 and go with the least 

 resistance possible. 

 The trachea is capped 

 above by the larynx, 

 as already explained 

 (page 204). After a 

 course of about five 

 inches it divides into 

 two great branches, 

 one to each lung, 

 called the bronchi. 

 These are also ringed. 

 The primary bronchi 

 are subdivided into 

 secondary 



and these again into 

 tertiary, and so on 



until the tubes become of capillary fineness and corre- 

 spondingly numerous. This is shown diagrammatically 

 in Fig. 240. They finally terminate in minute cells or 

 cellulated cells (Fig. 241). The ringed structure con- 

 tinues, except in the smallest subdivisions and the ter- 

 minal cells. The terminal cells are y^-j- to -^ of an inch 

 {\ to \ millimetre) in diameter, and their number has 

 been estimated as 600,000,000.' Now conceive this mass 

 of finely divided tubes and terminal cells, lined through- 



bronchi, Fig. 240. — Diagram showing the general 

 structure of the lungs of man : L, larynx ; 

 7V, trachea ; 6r, bronchi. 



