1110 



THE ORGANS OF VOICE AND RESPIRATION. 



ward, constantly diminishing in calibre until it ends, as such, in the lower and pos- 

 terior part of the inferior lobe of the corresponding lung. In its course each bronchus 

 gives off four ventral and four dorsal branches, the right bronchus also giving off 

 an additional or accessory bronchus, the so-called "heart-bronchus" which passes 

 mesially and dorsally into the inferior lobe. Its name comes from the fact that it 

 is the homologue of a bronchus which, in certain animals, runs to the infracardiac 



Bight bronchus 

 Eparterial branch 



End of bronchus 



End of bronchus 



FIG. 706. Diagram (after Aeby) of a cast of the "bronchial-tree:" v 1 , v 2 , v 3 , v 4 , the ventral branches; 

 d 1 , d 2 , d 3 , d 4 , the dorsal branches ; C, the " heart-bronchus." 



lobe. Of the right bronchus, the first ventral branch goes to the middle lobe ; the 

 other ventral and all the dorsal passing to the inferior lobe. Of the left bronchus, 

 the first ventral branch passes to the superior lobe, all the others, ventral and 

 dorsal, going to the inferior lobe. All these branches, on both sides, are hyp- 

 arterial as well as the " heart-bronchus." The characteristic general course of each 

 bronchus is outlined in the Diagram. 



Structure. The trachea is composed of imperfect cartilaginous rings, fibrous 

 membrane, muscular fibres, mucous membrane, and glands. 



The cartilages vary from sixteen to twenty in number ; each forms an imper- 

 fect ring which surrounds about two-thirds of the cylinder of the trachea, which is 

 completed behind by fibrous membrane. The cartilages are placed horizontally 

 above each other, separated by narrow membranous intervals. They measure 

 about two lines in depth and half a line in thickness. Their outer surfaces are 

 flattened, but internally they are convex from being thicker in the middle than at 

 the margins. Two or more of the cartilages often unite partially or completely, 

 and are sometimes bifurcated at their extremities. They are highly elastic, but 

 sometimes become calcified in advanced life. In the right bronchus the cartilages 

 vary in number from six to eight ; in the left, from nine to twelve. They are 

 shorter and narrower than those of the trachea. 



