242 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



of the trachea in being incomplete behind. The 

 right bronchus has from six to eight rings, while the 

 left has from nine to twelve. The left bronchus is 

 longer but narrower. 



The mucous membrane is smooth and contains a 

 considerable amount of lymphoid tissue and many 

 mucous glands. The epithelial lining consists of 

 long columnar ciliated cells, often very irregular and 

 even branched at their fixed ends. One or two rows 

 of small irregular cells intervene between the basal 

 ends of the ciliated cells, and this epithelium is 

 therefore stratified although very thin. The cells 

 rest upon a basement membrane through which 

 nerves pass to reach the sensitive epithelium. Ex- 

 ternal to the basement membrane, in what consti- 

 tutes the membrana propria, there is a strong layer 

 of longitudinal elastic fibers. This tissue extends 

 the whole length of the air passages, and gives not 

 only great elasticity but is an obstruction to invading 

 bacteria. A vascular submucosa, not very exten- 

 sive, intervenes between the mucosa and the carti- 

 lage. In this may be found the bodies of small race- 

 mose mucous glands, the largest being in the poste- 

 rior wall. The excretory ducts of these glands open 

 on the inner surface. Lymphoid tissue is present 

 both in the mucosa and the submucosa. 



THE LUNG. 



The lungs occupy the greater part of the chest 

 cavity, of which they form an accurate mould. The 

 right lung is the larger and has three lobes, while the 

 left has two. Each lung is suspended freely in this 



