342 MANUAL OF PHYSIOLOGY. 



established, the tolerably fresh lung of an infant will sink in 

 water. 



In order to have a clear idea of the volumes of air at rest and 

 in motion during pulmonary ventilation, it is convenient to follow 

 the classification from which the nomenclature in common use 

 has been borrowed. 



Tidal air is the current of air which passes into and out of the 

 chest in quiet natural breathing. It amounts to about 500 cc. 

 (30 cubic inches). 



Reserve air is that volume which can be voluntarily emitted 

 after the end of a normal tidal expiration, and which, therefore, 

 during ordinary respiration remains in the lungs ; it is estimated 

 at about 1500 cc. (or nearly 100 cubic inches). 



Complemental air is that which can be voluntarily taken in 

 after an ordinary inspiration by a forced inspiration ; it also 

 amounts to about 1500 cc., but is not used during ordinary 

 breathing. 



Residual air is the air volume which remains in the lungs after 

 a forced expiration, that is to say, which no voluntary effort can 

 remove from the lungs ; it includes the air which leaves the 

 lungs when the pleura is opened after death and the air which 

 persistently remains in the lungs after they have collapsed. This 

 amounts to about 2000 cc. (or about 120 cubic inches). 



Vital capacity is a term given to the greatest amount of air that 

 can be emitted by a forced expiration immediately following a 

 forced inspiration, so that it equals the sum of the tidal, reserve, 

 and complemental air. The vital capacity is estimated by spi- 

 rometers of different kinds, and gives an approximate measure- 

 ment of (1) the capacity of the chest; (2) the power of the respi- 

 ratory muscles; (3) the resistance offered by the elasticity or 

 rigidity of the walls of the thorax ; (4) the working capacity of 

 the lungs, i. e. y their extensibility or freedom from disease. It, 

 therefore, varies greatly according to the age, sex, position of 

 the body, the occupation, weight, height, the fullness of the hol- 

 low viscera of the abdomen, and the pathological condition of 

 the lungs. It can be much increased by practice, and this fact, 

 apart from the injury forced respirations may produce in a mor- 



