Chap. XIII.] 



KESPIRATION. 



153 



elastic cords into vibrations. These impart their vibrations to 

 the column of air above them, and so give rise to the sound 

 which we call the voice. 



The larynx is placed in communication with the external air 

 by two channels : the one, supplied by the nasal passages, is 

 always open ; the other, furnished by the mouth, can be opened 

 and closed at will. 

 One advantage of this 

 arrangement is, that 

 when exposed to a 

 very cold temperature, 

 we can close our 

 mouths and breathe 

 through the nasal pas- 

 sages, which, being nar- 

 row, thickly lined, and 

 freely supplied with 

 blood - vessels, warm 

 the air before it reaches 

 the lungs. 



The trachea. — The 

 trachea or windpipe is 

 a fibrous and muscu- 

 lar tube, the walls of 

 which are strengthened 

 and rendered more 

 rigid by hoops of car- Fig. lUU. — The Larynx as seen by Means of 

 tilace embedded in the ^^^ Laryngoscope in Different Conditions 

 * . of the Glottis. A, while singing a high note ; B, 



llbrOUS tissue. ihese in qniet breathing; C, during a deep inspiration. 

 hoODS are C-shaned ^' ^^^^ "^ tongue ; e, upper free edge of epiglottis ; 

 ■'■, . ^'> cushion of the epiglottis; p/i, part of anterior 



and incomplete behind, wall of pharynx; cv, the true vocal cords; cvs, the 

 the cartilaginous rings ^^^^^^^cal cords; rr, the trachea with its rings; b, 

 o o the two bronchi at their commencement. 



being completed by 



bands of plain muscular tissue where the trachea comes in con- 

 tact with the oesophagus. Like the larynx it is lined by mucous 

 membrane, and has a ciliated epithelium upon its inner surface. 

 The mucous membrane, which also extends into the bronchial 

 tubes, keeps the internal surface of the air passages free from 

 impurities ; the sticky mucus entangles particles of dust and 

 other matters breathed in with the air, and the incessant 



