86 STUDIES IN PHYSIOLOGY 



ing from the mouth, the other opening into the gullet), 

 and three for the air (the first two letting in the air from 

 the nose, the third, conducting it through the glottis) to the 

 lungs. But one set of these openings can be used at the 

 same time, for one sees from Fig. 29 that the paths of food 

 and air cross each other in the throat. Hence, if we try to 

 breathe and swallow at the same instant, the food starts 

 " down the wrong way," that is, down the windpipe. 



The Eustachian Tubes. A simple experiment demon- 

 strates the presence of an additional pair of openings, con- 

 necting the throat with the ear. Close the mouth, grasp 

 the nose firmly so as to close its external openings, and then 

 force air upwards several times from the lungs. The crack- 

 ling sound is due to the momentary stretching of the ear 

 drums by the increased pressure of the air on their inner 

 surface. The Eu-sta'chi-an tubes (so named from their dis- 

 coverer, a learned Italian physician), carry this air from the 

 upper part of the throat cavity into the middle region of 

 the ear. The ringing sensation in the ears when one has a 

 cold in the head is due to the temporary closing of these 

 tubes. 



The Process of Swallowing. The food can be kept in the 

 mouth as long as we wish, but when once it has been pushed 

 back into the throat it is beyond our control. The uvula 

 blocks the way toward the nose cavity, the windpipe is 

 closed by the epiglottis, and the muscles that surround the 

 throat cavity quickly close in about the food and force it 

 down the gullet. This rapid clearing of the throat is nec- 

 essary in order that breathing may be resumed. As soon 

 as the food reaches the gullet, the windpipe is opened by the 

 lowering of the larynx and by the elevation of the epiglot- 

 tis, and at the same time the soft palate drops down into a 

 vertical position, thus opening the passages from the nose. 

 Hence, when food particles start down the windpipe and 

 we cough, the food is often forced out through the nose, 

 since this is the only passage way that is clear (see Fig. 29). 



