348 A YEAR IN SCIENCE 



divides into two tubes, the right and the left" bronchus, 

 each of which supplies one lung. 



Nose cavity. The nose cavity is an irregular-shaped 

 passage lined with a mucous membrane. This secretes 

 mucus which aids in catching any dust and germs 

 which escape the many fine hairs at the entrance of 

 the cavity. The air in passing through the nose under- 

 goes several changes; first, it is partly freed of par- 

 ticles of dust and germs; second, it is warmed; and 

 third, it becomes moist. Air may reach the lungs 

 through the mouth, but the mouth is not adapted for 

 the purpose. Mouth breathing is a habit to be 

 avoided. 



Larynx. The structure of the throat has already 

 been studied. The air passes from the throat 

 through the glottis into the voice box, or larynx. The 

 larynx is a cavity on top of the wind pipe. Its walls 

 are stiffened by movable pieces of a substance known 

 as cartilage. The largest of these can be felt on the 

 ventral side of the larynx, commonly called Adam's 

 apple. Within the larynx are folds of mucous mem- 

 brane. These folds form the vocal cords. When these 

 are drawn close together and air passes over them, 

 they vibrate and produce sound. The glottis can be 

 closed by the epiglottis, as we have previously men- 

 tioned. This must be closed during swallowing. If 

 particles of food enter the larynx, choking results. 



Trachea. The trachea, or wind pipe, is immediately 

 below the larynx. It and its branches are kept open 



