130 RESPIRATION RESPIRATORY MOVEMENTS. 



tion. When either of these acts is the result of irritation from a foreign 

 substance or from secretions, it may be modified or partly smothered by the 

 will, but is not completely under control. The sensibility of the mucous 

 membrane at the summit of the air-passages usually protects them from the 

 entrance of foreign matters, both liquid and solid; for the slightest im- 

 pression received by the membrane gives rise to a violent and involuntary 

 cough, by which the offending substance is removed. The glottis, also, is 

 spasmodically contracted. 



In sighing, a prolonged and deep inspiration is followed by a rapid and 

 generally an audible expiration. This occurs, as a general rule, once in five 

 to eight respiratory acts, for the purpose of changing the air in the lungs 

 more completely, and it 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 efficiently per- 

 formed. Yawning is an analogous process, but it differs from sighing in the 

 fact that it is involuntary and can not 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 deficient oxygenation of the blood 

 and it is followed by a sense of satisfaction, which shows that it meets 

 some decided want on the part of the system. 



Laughing and sobbing, although expressing opposite conditions, are 

 produced by very nearly the same action. The characteristic sounds accom- 

 panying these acts are the result of short, rapid and convulsive movements 

 of the diaphragm, attended with contractions of the muscles of the face, 

 which produce the expressions characteristic of hilarity or grief. Although 

 to a certain extent under the control of the will, these acts are mainly invol- 

 untary. Violent and convulsive laughter may be excited in many individuals 

 by titillation of certain portions of the surface of the body. Laughter and 

 sometimes sobbing, like yawning, may be the result of involuntary imitation. 



Hiccough is a peculiar modification of the act of inspiration, to which it 

 is exclusively confined. It is produced by a sudden, convulsive and entirely 

 involuntary contraction of the diaphragm, accompanied by a spasmodic con- 

 striction of the glottis. The contraction of the diaphragm is more extensive 

 than in laughing and sobbing and occurs only once every four or five respir- 

 atory acts. 



CAPACITY OF THE LUNGS, AND THE QUANTITY OF AIR CHANGED IN 

 THE RESPIRATORY ACTS. 



The volume of air ordinarily contained in the lungs is about two hun- 

 dred cubic inches (3,277 c.c.) ; but it is evident, from the simple experiment of 

 opening the chest, when the elastic lungs collapse and expel a certain quan- 

 tity of air which can not be removed while the lungs are in situ, that a part 

 of the gaseous contents of these organs necessarily remains after the most 

 complete and forcible expiration. After an ordinary act there is a certain 



