THE RESPIRATORY SYSTEM. 249 



and they do not collapse when the chest is opened. In the new-born 

 infant, the lungs, even in their collapsed condition, fill the chest. As 

 the child grows, the capacity of the chest increases more rapidly than 

 does the size of the lungs ; and the lungs of the adult are considerably 

 expanded even at the end of expiration. 



Since with each expansion of the chest the lungs increase in size 

 so as to fill completely the extra space thus provided, the amount of 

 air entering the lungs at each breath varies with the extent of the 

 respiratory movement. In quiet respiration it amounts to 350-500 c.c., 

 and is spoken of as tidal air. The additional volume of air, which can 

 be taken into the lungs by forced inspiration, amounts on an average to 

 about 1500 c.c. and is called complemented air. The largest amount of 

 air which can be expelled from the lungs by the most violent expiration, 

 made at the end of an ordinary breath, is termed supplemental air ; it 

 varies in different individuals from 1000 to 1500 c.c. Even after the most 

 forcible expiration, a considerable amount of air usually about 1000 c.c. 

 still remains in the lungs, and is spoken of as residual air. The 

 total volume of air which can be taken into and expelled from the 

 lungs by the most forcible inspiration and expiration, namely, the sum 

 of the tidal, complemental, and supplemental air, is termed the vital 

 capacity of the chest, and is from 3000 to 3500 c.c. These figures are 

 obtained by allowing the individual to breathe into a spirometer, which 

 is a small gasometer provided with a graduated scale. When the 

 subject breathes into or out of the spirometer, the air chamber rises 

 or falls, the increase or decrease of its contents, thus produced, being 

 read off on the scale. 



Tidal air 500 c.c. . . . j 



Complemental air 1500 c.c. . > Vital capacity 3500 c.c. 



Supplemental air 1500 c.c. . j 



Residual air 1000 c.c. 



The normal rate of respiration in adults is 15 to 18 a minute. 

 Expiration follows inspiration immediately, and is succeeded by a 

 slight pause before the next inspiration begins. Children breathe more 

 rapidly, the rate in the infant being about 40 a minute. 



Ordinary quiet breathing is usually called eupnoea, and an increase 

 in the depth of the respiratory movements is called hyperpn&a ; if these 

 movements are not only deeper, but also laboured, the term dyspnoea is 

 applied to them. A temporary cessation of breathing is known as 

 apnoea. 



If the ear is placed in contact with the chest wall, a faint sound 

 the vesicular murmur is heard during inspiration ; it^ is believed to be 



