THE LUNGS AND RESPIRATION. 131 



respiratory center, thus adjusting the effort to get oxy- 

 gen to the need of it. Respiration may be stopped 

 by stimulating the mucous membrane of the nose, as 

 with strong ammonia. 



Variations. Certain variations from the ordinary 

 respiration might be mentioned here. A deep inspira- 

 tion followed by a long expiration is known as a sigh 

 and a very deep inspiration through the mouth only as 

 a yawn. Hiccough results from a sudden inspiratory con- 

 traction of the diaphragm during which the glottis is 

 suddenly closed. In sobbing the inspirations are short 

 and rapid with a prompt closing of the glottis be- 

 tween. Both coughing and sneezing consist of a deep 

 inspiration followed by complete closure of the glot- 

 tis and then its sudden opening and the forcible ex- 

 pulsion of air. Coughing, however, is generally caused 

 by an irritation or obstruction of the larynx or trachea 

 and the air is expelled through the mouth, while sneez- 

 ing is caused by irritation of the nasal passages and the 

 air is driven out through the nose. Laughing and 

 crying also resemble one another in that each is an in- 

 spiration followed by a series of short, spasmodic ex- 

 pirations, during which the glottis is open and the 

 vocal cords in characteristic vibration. They differ, 

 however, in rhythm and in the facial expression that 

 accompanies them. 



