THE TRACHEA. 



635 



and elastic tissue, mucous membrane and glands, with 

 muscular ^fibres. 



The cartilage, FIG. 165. 



thin, flexible, read- 

 ily compressed, but 

 very elastic, as- 

 sumes the form of 

 flattened rings. 

 These rings, how- 

 ever, are not com- 

 plete, being defi- 

 cient in their poste- 

 rior part, and form- 

 ing only about 

 three-fourths of a 

 circle. They resem- 

 ble in structure that 

 of the nose and 

 external ear more 

 than those of the la- 

 rynx. The rings run 

 transversely, being 

 placed one above the other, and averaging in number about 

 eighteen. Each ring is convex externally and concave 

 internally, enclosed within the fibrous, and lined by the 

 mucous membrane. These cartilages preserve the trachea 

 as a permanently open tube for the free ingress and egress 

 of the air. Each ring is about two lines wide, an inch and 

 a half in length, and a line in depth ; their upper and 

 lower edges are thin, and their extremities blunted. Their 

 size is irregular, being sometimes larger in one part than 

 another, and not always parallel. The lower cartilages 

 are occasionally bifid, resembling those of the bronchi. 



The fibrous tissue is regarded as the fundamental part, 

 forming the continued tube of the trachea, being at- 



FIG. 165 represents the'Trachea and Bronchi. 1 Thyroid cartilage. 2 

 Cricoid cartilage. 3 3 Trachea. 4 Left bronchus. 5 Right bronchus. 



