AIR CHANGED IN THE RESPIRATORY ACTS ill 



Laughing and sobbing, although expressing opposite conditions, are 

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

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

 ments 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 involuntary. 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 re- 

 sult 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 with 

 spasmodic constriction of the glottis. The contraction of the diaphragm 

 is more extensive than in laughing and sobbing and occurs only once in 

 every four or five respiratory 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 

 hundred cubic inches (3277 cubic centimeters); but it is evident, from 

 the simple experiment of opening the chest, when the elastic lungs col- 

 lapse and expel a certain quantity 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 forcible expiration. After an ordinary 

 act there is a certain quantity of air in the lungs, which can be expelled 

 by a forced expiration. In ordinary respiration a comparatively small 

 volume of air enters with inspiration, and a nearly equal quantity is ex- 

 pelled by the succeeding expiration. By the extreme action of all the 

 inspiratory muscles in a forced inspiration, a supplemental quantity of 

 air may be taken into the lungs, which then contain much more than 

 they ever do in ordinary respiration. For convenience of description, 

 physiologists have adopted the following names, which are applied to 

 these various volumes of air : 



1. Residual Air; that which is not and can not be expelled by a 

 forced expiration. 



2. Reserve Air ; that which remains after an ordinary expiration, 

 deducting the residual air. 



3. Tidal, or Ordinary Breathing Air ; that which is changed in the 

 ordinary acts of inspiration and expiration. 



