144 



THE ESSENTIALS OF HISTOLOGY. 



which are embedded in the fibrous tissue. The muscular tissue, which 

 is of the plain variety, forms a flat band, the fibres of which run trans- 

 versely at the back of the tube. The trachea is lined by a muoc/us 

 membrane (fig. 166, a-c), which has a ciliated epithelium upon its 

 inner surface. The epithelium-cells have been already described 

 (Lesson VII.) ; they rest upon a thick basement membrane. The 

 mucous membrane proper consists of areolar and lymphoid tissue, and 

 contains numerous blood-vessels and lymphatics. In its deepest part 

 is a well-marked layer of longitudinal elastic fibres (d). Many small 

 glands for the secretion of mucus are found in the wall of the trachea. 

 They may lie either within the mucous membrane or in the submucous 



FIG. 166. LONGITUDINAL SECTION OF THE HUMAN TRACHEA, INCLUDING PORTIONS 

 OF TWO CARTILAGINOUS RINGS. (Klein.) (Moderately magnified.) 



a, ciliated epithelium ; 6, basement-membrane : c, superficial part of the mucous membrane, 

 containing the sections of numerous capillary blood-vessels and much lymphoid tissue ; 

 d, deeper part of the mucous membrane, consisting mainly of elastic fibres ; e, submucous 

 areolar tissue, containing the larger blood-vessels, small mucous glands (their ducts and 

 alveoli are seen in section), fat, etc. ; /, fibrous tissue investing and uniting the cartilages ; 

 g, a small mass of adipose tissue in the fibrous layer ; ft, cartilage. 



areolar tissue (e), or, lastly, at the back of the trachea, outside the 

 transverse muscular fibres. 



The two divisions of the trachea, the bronchi, are precisely similar in 

 structure. 



The larynx is also very like the trachea so far as the structure of 



