CHAP, ii.] RESPIRATION. 427 



speak of them as being in a state of 4 adhesion,' such as obtains 

 between two wet membranes superimposed. And it has been 

 suggested 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 shewn that this is really of importance 

 in the matter. 



257. 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, con- 

 sisting 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 " adhesion," 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 unfold- 

 ing, 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 bron- 

 chioles 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 sometime after birth that the lungs 



?ass into that further distended state of which we spoke above. 

 n. 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. 



258. 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 



