396 RESPIRATION. 



membrane, the accumulated mucus is carried by the act 

 of coughing either to the mouth, or well into the larynx, 

 whence it is expelled by the act of expectoration. When 

 either of these acts is the result of irritation, either from a 

 foreign substance or secretions, it may be modified or partly 

 smothered by the will, but is not completely under control. 

 The exquisite sensibility of the mucous membrane at 

 the summit of the air-passages, under most circumstances, 

 protects them from the entrance of foreign matter, both 

 liquid and solid ; for the slightest impression received by the 

 membrane gives rise to a' violent and involuntary cough, by 

 which the offending matter is removed. The glottis is also 

 spasmodically closed. 



In sighing, a prolonged and deep inspiration is followed 

 by a rapid and generally audible expiration. This occurs, as 

 a general rule, once in every five to eight respiratory acts, 

 for the purpose of changing the air in the lungs more com- 

 pletely, and is due to an exaggeration of the cause which 

 gives rise to the ordinary acts of respiration. When due 

 to depressing emotions, it has the same cause ; for at such 

 times, respiration is less effectually performed. Yawning 

 is an analogous process, but differs from sighing in the 

 fact that it is involuntary, and cannot be produced by an 

 effort of the will. It is characterized by a wide opening of 

 .the mouth, and a very profound inspiration. Yawning is 

 generally assumed to be an evidence of fatigue, but it often 

 occurs from a sort of contagion. When not the result of 

 imitation, it has the same exciting cause as sighing, viz., defi- 

 cient oxygenation of the blood, and is followed by a sense of 

 satisfaction, which shows that it meets some decided want on 

 the part of the system. 



Laughing and sobbing, though expressing opposite condi- 

 tions, are produced by very much the same mechanism. 

 The characteristic sounds accompanying these acts are the 

 result of short, rapid, and convulsive movements of the dia- 

 phragm, accompanied by contractions of the muscles of the 



