NORMAL HISTOLOGY. 127 



CHAPTEE XVIII. 



THE RESPIRATORY SYSTEM. 



The respiratory system includes the air passages of the nose, 

 mouth and pharynx, and the epiglottis, larynx, trachea, bronchi, 

 lungs, and pleura. Its structures are derived wholly from the epi- 

 derm and mesoderm. Its nerve supply is from the cerebro-spinal 

 and sympathetic systems. 



Epiglottis. This structure consists chiefly of yellow elastic car- 

 tilage. Its mucosa possesses many taste-buds and leucocytes. 



Larynx. The thyroid, cricoid, and arytenoid cartilages make up 

 the bulk of the larynx. These are united by fibrous tissue and liga- 

 mentous membranes and are covered internally with a mucosa, and 

 externally with fibrous tissue. 



THE TRACHEA. 



The trachea comprises a mucosa, a sub-mucosa, and fibrous coat. 

 It is largely a mesodermic structure. 



OUTLINE OF THE TRACHEA. 



f Ciliated epithelium. 

 1 M 



Mucosa < Membrana propria. f Inner layer. 



^ Tunica propria { 



[ Outer layer. 

 f Connective tissue. 



Sub-mucosa 1 g lood vessels? i ymph atics, and nerves. 



Smooth muscle. 



f Perichondrium. 



Hyaline cartilage < Matrix. 



Fibrous coat <{ [ Cells. 



Connective tissue. 



The mucosa presents a superficial layer of stratified ciliated epi- 

 thelium supported by a basement membrane and a tunica propria 

 which possesses a large amount of elastic tissue and is disposed in 

 two layers : An inner layer of loose fibrous tissue containing elastic 

 fibres, nerve fibres, etc., and an outer layer, consisting of a dense 

 network of longitudinal elastic fibres. Of the epithelium, only the 



