CHAP. IL] RESPIRATION. 537 



that this adhesion, having to be overcome before the two surfaces 

 can separate, assists in preventing the entrance of air into the 

 pleural cavity after puncture of the thorax ; but it has not been 

 clearly shown that this is really of importance in the matter. 



326. Before birth the lungs contain no air ; they are in the 

 condition called atelectatic. The walls of the alveoli, the epithelial 

 lining of which is at that time well developed, consisting of 

 distinctly nucleated cells with granular cell-substance, are in 

 contact, the cavity of the alveolus not having 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 " ad- 

 hesion," obtaining between the walls of the alveoli thus in contact 

 with each other and also those of the bronchioles. 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 in- 

 sufficient 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 in- 

 spired air. 



It is not, however, until sometime 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 distension of the lungs in the 

 adult may be familiarly described as being due to the chest being 

 too large for the lungs. 



327. 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 1/100 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 distension 

 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 



