CAPACITY OF THE LUNGS. 397 



face, which produce the expressions characteristic of hilarity 

 or grief. Though to a certain extent under the control of 

 the will, they are mostly involuntary. Violent and convul- 

 sive laughter may be excited in many individuals by titilla- 

 tion of certain portions of the surface of the body. Laugh- 

 ter and sometimes sobbing, like yawning, may be the result 

 of involuntary imitation. 



Hiccough is a peculiar modification of the act of inspira- 

 tion, to which it is exclusively confined. It is produced by 

 a sudden, convulsive, and entirely involuntary contraction of 

 the diaphragm, accompanied by a spasmodic constriction of 

 the glottis. The contraction of the diaphragm is more exten- 

 sive than in laughing and sobbing, and occurs only once in 

 four or five respiratory acts. The causes which give rise tb 

 hiccough are numerous, and many of them are referable to 

 the digestive system. Among these may be mentioned the 

 rapid ingestion of a quantity of dry food, or of efferves'cing or 

 alcoholic drinks. It occurs frequently in cases of disease. 



Capacity of the Lungs, and the Quantity of A.ir changed in 

 the Respiratory Acts. 



Several points of considerable physiological interest arise 

 in this connection. It is evident from the simple experiment 

 of opening the chest, when the elastic lungs collapse and ex- 

 pel a certain quantity of air which cannot 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 expiration, 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 is introduced with inspiration, which is 

 expelled by the succeeding expiration. l By the extreme action 



1 Experiments have shown that a certain volume of air is lost in the lungs, 

 the expired air being a little less in volume than the quantity inspired (from ^o 

 to gV). This is not taken into account in this connection. 



