VITAL CAPACITY. 141 



called into action to elevate the breast bone and first pair 

 of ribs; and when breathing is difficult, particularly from 

 asthma, additional force is obtained by fixing the arms, as, for 

 example, by laying hold of an arm-chair ; and these muscles 

 extending between the chest and shoulders, being fixed at 

 the latter attachment, exercise their action on the former. 

 It is principally in inspiration that muscular force is called 

 into requisition, expiration being largely aided by the elas- 

 ticity of both ribs and lungs ; for the ribs in inspiration are 

 pulled forcibly out of their natural curve, and the elasticity 

 of the pulmonary texture may be easily demonstrated by 

 noting the rapidity with which the lungs of dead animals 

 collapse after artificial inflation. In certain circumstances, 

 however, a considerable exertion may be required in expira- 

 tion, as, for instance, in playing a wind instrument or in 

 glass-blowing ; and then the abdominal muscles may bo 

 brought strongly into action both to push up the diaphragm 

 and to depress the ribs. 



103. The acts of inspiration take place at a rate usually vary- 

 ing from fifteen to twenty per minute ; but, like the pulsations 

 of the heart, they are much more frequent in childhood, being 

 above forty per minute in the infant. Only a small portion 

 of the air in the lungs is changed in each ordinary respira- 

 tion; the quantity so changed, or the breathing air, as it is 

 termed, being on an average twenty or twenty-five cubic 

 inches, while the vital capacity, or amount of air which can 

 be expelled after a full inspiration, and which therefore 

 includes a complemental supply not usually taken in, and a 

 reserve quantity not iisually parted with, is estimated on an 

 average at 225 cubic inches. Even after the most forcible 

 expiration, a large amount of residual air remains in the 

 chest; and indeed it is impossible, even by direct pressure, 

 completely to expel the air from the healthy lung of an 

 animal which has breathed, so that it shall sink in water, 

 like the lung of an animal born dead, into which the air has 

 never entered. 



104. The vital capacity indicates the mobility of the walls 

 of the chest, but by no means varies according to the dimen- 

 sions of the cavity; for differences in the dimensions occur, 

 irrespective of height, age, or weight, whereas the vital 



