MECHANICS OF RESPIRATORY MOVEMENTS 1047 



into the spirometer. The air left in the lungs after the most vigorous 

 expiration is known as the residual air. 



The residual air may be determined by letting a person expire to the utmost extent 

 and then connecting with his mouth or nose a bag of known capacity filled with hydrogen. 

 The subject of the experiment then inspires and expires into the bag two or three times, 

 ending in the same state of forced expiration as he began. Any diminution of the total 

 volume of gas in the bag will represent the gas lost during the experiment by diffusion 

 into the blood-vessels. By analysis of the gaseous mixture in the bag, it is possible to 

 determine the amount of air in the lungs at the beginning of the experiment. Supposing, 

 for example, the bag held 4000 c.c. hydrogen, after two respirations the total volume is 

 unaltered, but the gas is found to consist of 3000 c.c. hydrogen and 1000 c.c. oxygen, 

 nitrogen, and CO 2 , i.e. pulmonary gases. Since the gas in the lungs must have the same 

 composition and 1000 c.c. hydrogen have disappeared from the bag, it is evident that the 

 lungs will contain 1000 c.c. hydrogen and ^- , i.e. 330 c.c. pulmonary gases. Thus the 

 total volume of gas left in the lungs at the end of the forced expiration was 1330 c.c., 

 which is the residual volume for the individual. 



The above example is purely imaginary. As a result of actual deter- 

 minations carried out we may assume the residual air in the lungs as some- 

 thing between 600 and 1200 c.c. 



Of the 500 c.c. of tidal air taken in at each inspiration, only a certain 

 part reaches the alveoli, part being required to fill the air-tubes, trachea, 

 bronchi, and bronchioles which lead to the air-cells. The volume of the 

 aii-tubes has been reckoned to amount to 140 c.c., so that of the 500 c.c. about 

 360 c.c. reach the alveoli. For the same reason the expired air represents 

 the air from the alveoli (360 c.c.) diluted with 140 c.c. of air which has 

 remained in the air-tubes and undergone very little change, other than the 

 elevation of temperature and saturation with aqueous vapour. We have 

 therefore to allow for this air contained in the so-called ' dead space ' of the 

 lungs when we seek to arrive at the composition of alveolar air from an 

 analysis of expired air. 



THE BRONCHIAL MUSCULATURE 



Both the large and smaller air-tubes have a coating consisting of un- 

 striated muscle-fibres which in the bronchioles is complete. Contraction 

 of these fibres must have the following effects : (1) a constriction of the 

 bronchi and bronchioles ; (2) a diminution of the air space of the lungs and 

 therefore of the volume of the lung ; (3) an increased resistance to the 

 passage of the air into and out of the alveoli. Changes in the condition of 

 contraction of these muscle-fibres may be studied in two ways. In the first 

 method artificial respiration is carried out, a constant volume of air being 

 blown in and sucked out at each respiration. Any diminution in the 

 calibre of the bronchioles must increase the resistance to the incoming 

 current of air and so cause a rise of pressure in the tracheal tube. Einthoven 

 in investigating this subject, has made use of an arrangement by means of 

 which a mercurial manometer is connected with the trachea for a brief 

 space of time during one part of the inspiratory phase. Any resistance 

 to the current of air raises the pressure during the whole inspiration and 



