

RESPIRATION. 69 



In tranquil breathing the glottis is motionless, but in 

 the more active modes of respiration, the cords diverge in 

 inspiration, resuming their normal position in expiration. 

 In extreme dyspncea, inspiration is accompanied by dila- 

 tation of the nostrils. 



The lungs, mechanically considered, may be regarded as 

 a collection of elastic and very distensible bags, all of which 

 communicate freely with each other as well as with the 

 atmosphere. The volume of each lung, when removed 

 from the chest, is much smaller than that of the cavity in 

 which it is contained, and which, in its normal state of 

 expansion, it completely fills ; consequently, when a pleural 

 cavity is opened, the lung collapses. When the closed 

 pleura is brought into communication with a mercurial 

 manometer, the column in the open limb falls, so that the 

 pressure to which any fluid in this cavity is exposed is less 

 than that of the atmosphere. As measured when the 

 chest is in the condition of equilibrium, the difference can 

 be shown to be about ifo of an atmosphere (7 millimeters 

 mercury). In natural inspiration it increases to 8 or 9 

 millimeters, and in full inspiration it may be increased to 

 30 millimeters. It is increased by any cause which de- 

 stroys the expansibility of any part of the lung. 



As the organs contained in the chest are under the same 

 pressure as the fluid in the pleura, the flow of blood towards 

 the heart is aided by the elasticity of the lungs. The same 

 condition is also favourable to the diastolic filling of the 

 heart, which contracts with more effect after each inspira- 

 tion, so that the arterial pressure rises. 



An adult male inspires from 25 to 30 cubic inches at 

 a time, and about 20 times per minute ; consequently, 

 from 500 to 600 cubic inches per minute. The greatest 

 volume of air which an individual is able to exchange 

 in a breath is called the "vital capacity." The mean 

 "vital capacity" of a man of ordinary height and build 

 (5 feet 7 inches, and 32 inches in girth) is 210 cubic 



