CAPACITY OF THE LUNGS. 135 



Laughing and sobbing, though expressing opposite conditions, 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 diaphragm, accompanied by con- 

 tractions 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 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 constriction of the glottis. The contrac- 

 tion of the diaphragm is more extensive than in laughing and sobbing and occurs only 

 once every four or five respiratory acts. The causes which give rise to hiccough are nu- 

 merous, 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 effervescing or alcoholic 

 drinks. It occurs frequently in cases of disease. 



Capacity of the Lungs, and the Quantity of Air 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 expel 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 act, there is a certain quantity 

 of air in the lungs which can be expelled by a forced expiration. In ordinary respi- 

 ration, a comparatively small volume of air is introduced with inspiration, which is ex- 

 pelled by the succeeding expiration. 1 By the extreme action of all the inspiratory 

 muscles in a forced inspiration, a supplemental quantity of air may be introduced into the 

 lungs, which then contain much more than they ever do in ordinary respiration. For 

 convenience, many physiologists have adopted the following names, which are applied to 

 these various volumes of air : 



1. Residual Air ; that which is not and cannot 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 by the ordinary acts 

 of inspiration and expiration. 



4. Gomplemental Air ; the excess over the ordinary breathing air, which may be 

 introduced by a forcible inspiration. 



The questions relating to the above divisions of the respired air have been made the 

 subject of numerous investigations; but, although at first it might seem easy to deter- 

 mine all of them by a sufficient number of experiments, the necessary observations are 

 attended with considerable difficulty, and the sources of error are numerous. In measur- 

 ing the air changed in ordinary breathing, it has been found .that the acts of respiration 

 are so easily influenced by the mind and it is so difficult to experiment on any individual 

 without his knowledge, that the results of many good observers are not to be relied 

 upon. This is one of the most important of the questions under consideration. The 

 difficulties in the way of estimating with accuracy the residual, reserve, or complemental 

 volumes, will readily suggest themselves. The observations on these points, which may 

 be taken as the most definite and exact, are those of Herbst, of Gottingen, and Hutchin- 

 son, of England. Those of the last-named observer are exceedingly elaborate and were 



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 T V to ss)- Tm ' s is not taken into account in this connection. 



