180 APPLIED PHYSIOLOGY 



The Trachea and Bronchi. The trachea and large 

 bronchi are wide tubes kept permanently patent by 

 hoops of cartilage. Scattered through their wall are 

 nurnberous bands of elastic fibres, chiefly longitudinal 

 in direction, which permit of a certain amount of stretch- 

 ing as the lungs move up and down in respiration, or 

 when the head is thrown back. The free ends of the 

 hoops of cartilage are connected by unstriped muscle 

 fibres, the function of which it is difficult to understand. 

 Embedded in the wall of these tubes are numerous 

 mucous glands, the secretion of which moistens the 

 surface of the mucous membrane, and entangles such 

 particles of dust as have escaped capture in the nose. 

 Lining the tube is a layer of stratified ciliated epithelium, 

 the cilia of which are constantly engaged in maintaining 

 an upward current in the mucus poured out on the 

 surface, so that particles of foreign matter are floated 

 away from the direction of the lungs. The removal 

 of such particles is also facilitated by the existence in the 

 submucous coat of masses of adenoid tissue, the cells 

 from which wander into the tube and lay hold of 

 intruding bodies, and carry them off to be dealt with in 

 the lymphatic glands. As the bronchi break up after 

 entering the lungs, the hoops of cartilage become re- 

 placed by smaller plates, which allow of partial collapse 

 of the tubes during expiration ; and as the breathing 

 surface is approached these disappear altogether, neither 

 cartilage nor glands being found in the walls of the 

 bronchioles, which measure only half a millimetre in 

 diameter. As the cartilage disappears, however, the 

 muscular layer in the wall becomes more pronounced, 



