THE MECHANICS OF PULMONARY RESPIRATION. 335 



as yet come into existence ; the walls of the bronchioles are similarly in a 

 collapsed condition, with their walls touching ; the more rigid bronchia, like 

 the trachea, possess some amount of lumen, which, however, is occupied by 

 fluid. When the chest expands with the first breath taken, the pressure of 

 the inspired air has to overcome the " adhesion " obtaining between the walls 

 of the alveoli, thus in contact with each other and also those of the bronchi- 

 oles. The force spent in thus opening out and unfolding, so to speak, the 

 alveoli and bronchioles is considerable, and in the expiration succeeding the 

 first inspiration most of the air thus introduced remains, the force exerted 

 by the chest in returning to its previous dimensions after the breathing in 

 and the elastic action of the alveoli being insufficient to bring the walls of 

 the alveoli again into contact. Succeeding breaths unfold the lungs more 

 and more, until all the alveoli and bronchioles are opened up, and then the 

 whole force of the expiratory act is directed to driving out the previously 

 inspired air. 



It is not, however, until some time after birth that the lungs pass into 

 that further distended state of which we spoke above. In a newly-born 

 animal there is no negative pressure obtaining in the pleural cavities; the 

 lungs, when at rest, are not on the stretch, and opening the thorax does 

 not lead to collapse of the lungs. The state of things obtaining later on 

 is established, not at once, but gradually, and is apparently brought about 

 by the thorax growing more rapidly, and so becoming relatively more 

 capacious than the lungs. The distention of the lungs in the adult may 

 be familiarly described as being due to the chest being too large for the 

 lungs. 



270. In man the pressure exerted by the elasticity of the lungs alone 

 amounts to about 5 or 7 mm. of mercury. This is estimated by tying 

 a manometer into the windpipe of a dead subject and observing the rise 

 of mercury which takes place when the chest-walls are punctured. If we 

 took 7.6 mm. as the pressure this would be just T ^ of the pressure of 

 the atmosphere. If the chest be forcibly distended beforehand, a much 

 larger rise of the mercury is observed, amounting, in the case of a dis- 

 tention corresponding to a very forcible inspiration, to 30 mm. In the 

 living body this mechanical elastic force of the lungs may be assisted by 

 the contraction of the plain muscular fibres of the bronchi ; the pres- 

 sure, however, which can be exerted by these probably does not exceed 

 1 or 2 mm. 



When a manometer is introduced into a lateral opening of the windpipe 

 of an animal, the mercury will fall, indicating a negative pressure, as it is 

 called, during inspiration, and rise, indicating a positive pressure, during 

 expiration, both fall and rise being slight and varying according to the 

 freedom with which the air passes in and out of the chest. When a man- 

 ometer is fitted with air-tight closure into the mouth, or better, in order to 

 avoid the suction-action of the mouth, into one nostril, the other nostril and 

 the mouth being closed, and efforts of inspiration and expiration are made, 

 the mercury falls or undergoes negative pressure with inspiration, and rises 

 or undergoes positive pressure during expiration. It has been found in this 

 way that the negative pressure of a strong inspiratory effort may vary from 

 30 to 74 mm., and the positive pressure of a strong expiration from 62 to 

 100 mm. 



The total amount of air which can be given out by the most forcible 

 expiration following upon the most forcible inspiration, that is, the sum of 

 the complemental, tidal, and reserve airs, has been called the " vital capa- 

 city ;" " extreme differential capacity " is a better phrase. It may be meas- 

 ured by a modification of a gas-meter called a spirometer ; and though it 



