84 THE HUMAN LUNGS. 



vertebra of the back, where it divides into two tubes termed the 

 bronchi. The trachea, the trunk of the windpipe, consists of from 

 fifteen to twenty fibro-cartilaginous rings, which, however, do not 

 form complete circles, being deficient at the back part, where the 

 tube is completed by a strong membrane. These rings, like little 

 ribs, are separated from, or connected with, each other by strong 

 elastic membrane, so that first there is the membrane, then a ring; 

 then again the membrane, then a ring; and so forth. The trachea 

 is lined on the inside by a soft membrane continued from that of 

 the mouth. It is the great stem which bears the ramifications of 

 the lungs. 



The two large branches of the trachea, the first bronchial tubes, 

 run on each side to the lungs. On arriving thither, each divides 

 into two smaller branches, and the subdivision continues, of each 

 little branch into two twigs, and of each twig into lesser twigs, un- 

 til at the last division the air cells terminate the tubes. These air 

 cells are minute hollow chambers or vesicles, which hang like glo- 

 bules or grapes from the ends of the bronchia, and the air passes 

 into them with every breath we draw, and is expelled from them 

 more or less completely during each expiration. They form the 

 characteristic element of the lungs, which are themselves nothing- 

 more than a vast, manifold and corroborated air cell. The amount 

 of surface exposed by the cells is very great. 



The whole of the constituents of the trachea exist virtually, in 

 function and principle, in the smallest elements of the lungs, and 

 the trachea with the lungs is a goodly diagram of the minutest 

 bronchial twig with its delicate air cell. In the grand consistency 

 of nature, the parts belong to the whole, and vice versa, the mass 

 being a spontaneous association of myriads of equally integral and 

 so far independent structures. 



We have now drawn an outline of the pulmonic tree. Its roots 

 are the nose and mouth extending into the atmospheres; its boss is 

 the larynx; its shaft the trachea; its first two branches are the 

 bronchia; its other branches, twigs, and fruits are collectively the 

 lungs; the fruits or air cells, however, are the lungs especially and 

 essentially. 



All the organs of the body are supplied by arteries carrying vivid 



