THE LUNGS AND RESPIRATION. 



123 



The Trachea. The trachea is a membranous tube ex- 

 tending down from the larynx for about four and a 

 half inches to the fourth or fifth dorsal vertebra, where 

 it divides into the right and left bronchi. It is formed 

 of sixteen to twenty imperfect cartilaginous rings, 

 open behind, enclosed in a double elastic fibrous 



FIG. 47. The larynx, trachea and bronchi. (After Sobotta.) 



membrane, and is lined with ciliated mucous mem- 

 brane. The rings are for strength and in the interval 

 at the back where they are wanting there is one layer 

 of longitudinal and another of transverse unstriped 

 muscle fibers. The passage is kept clear by the action 

 of the cilia, which sweep up and out any particles of 

 dust that become entangled in the mucus. Trache- 

 otomy is generally done about one inch below the cri- 



