RESPIRATION 



8l 



must be kept open and free from obstructions. They are 

 kept open by special contrivances found in their walls 

 which, by supplying a degree of stiffness, cause the tubes 

 to keep their form. In the trachea, bronchi, and larger 

 bronchial tubes, the stiffness is supplied by rings of carti- 

 lage, while in the smaller tubes this is replaced by connect- 

 ive and muscular tissue. The walls of the larynx contain 

 strips and plates of cartilage ; while the nostrils and the 

 pharynx are kept open by their bony surroundings. 



The air passages are kept clean by cells especially 

 adapted to this purpose, known as the ciliated epithelial 

 cells. These are slender, wedge-shaped cells which have 

 projecting from a free end many small, hair-like bodies, 

 called cilia (Fig. 35). They line the mucous membrane 



in most of the air 



j 

 passages, and are so 



placed that the cilia 

 project into the tubes. 

 Here they keep up 

 an inward and out- 

 ward wave-like move- 

 ment, which is 

 quicker and has 

 greater force in the 

 outward direction. By this means the cilia are able to 

 move small pieces of foreign matter, such as dust particles 

 and bits of partly dried mucus, called phlegm, to places 

 where they can be easily expelled from the lungs. 1 



sages by pressure, or by the presence of foreign substances, would keep the air from 

 reaching some part of the lungs. 



1 Coughing, which is a forceful expulsion of air, has for its purpose the ejection 

 of foreign substances from the throat and lungs. Sneezing, on the other hand, has 

 for its purpose the cleansing of the nostrils. In coughing, the air is expelled 

 through the mouth, while in sneezing it is expelled through the nostrils. 



B 



FIG. 35. Ciliated epithelial 

 A cells. A. Two cells highly mag- 

 nified, c. Cilia. . Nucleus. B. 

 Diagram of a small air tube 

 showing the lining of cilia. 



