MECHANISM OF EXPIRATION. 811 



especially the subpleural elastic tissues, aid powerfully in compressing 

 every portion of the lung- 



The importance of the elasticity of the component structures of the 

 lungs, as an expiratory force, is shown by an experiment, which also 

 illustrates the mode in which a lung expands by the removal of atmo- 

 spheric pressure from its outer surface, and by the concurrent entrance 

 of air, under its ordinary pressure, into the bronchial tubes. A bell- 

 shaped glass jar, having a wide mouth below, and a strong open neck 

 at its upper end, has the latter opening fitted with a perforated cork, 

 tightly cemented in. The lower end of a glass tube, about J inch 

 wide and 1 foot long, is closely secured into the bronchus of a sheep's 

 lung; the upper end of this tube is then passed up into the bell- shaped 

 glass jar, and pushed through the hole in the cork, until the lung is 

 suspended high up in the jar; the tube is then hermetically cemented 

 to the orifice in the cork, its upper end being left free and open. A 

 piece of moistened bladder, in the centre of which a stop-cock is closely 

 tied, so as to project outwards, is now placed loosely over the mouth 

 of the jar, and is tightly secured to its rim by cord. When the appa- 

 ratus is held upright, the glass-tube represents the trachea, and the 

 bell-shaped jar may be compared to the thorax, with this difference, 

 however, that its sides are not movable, and are not in contact with 

 the lungs; lastly, the loosely extended moist sheet of bladder occupies 

 the position of the diaphragm, the upward vaulted form of which may 

 now be imitated by opening the stop-cock in the centre of the bladder, 

 thrusting this latter up into the bell-jar, and then closing the stop- 

 cock. In this position the bladder is supported by the atmospheric 

 pressure, and the suspended lung is quiescent. On now pulling the 

 stop-cock downwards, the bladder descends, imitating the descent of 

 the diaphragm, the atmospheric pressure on the surface of the sus- 

 pended lung is removed, and, in anticipation of the threatened vacuum, 

 air enters, through the glass-tube, into the interior of the lung, which, 

 as may be seen through the jar, immediately becomes inflated. In 

 this condition the elastic tissues of the lung are put upon the stretch. 

 But if the stop-cock be let go, the elastic resilience of this organ 

 causes the lung once more to contract, and the artificial diaphragm of 

 moist bladder again ascends into the jar, until the atmospheric equi- 

 librium, inside and outside the lung, is re-established. The experiment 

 may be repeated, again and again, the accidental entrance of an excess 

 of air between the sides of the jar and the surface of the lung, being 

 remedied by opening the stop-cock, thrusting the bladder well up into 

 the jar, and then closing it again. If desired, a manometer may be 

 adapted by a separate opening to the top of the jar, so as to measure 

 the expanding force used in distending tl?e lung, as the bladder is 

 drawn down. The two lungs of a dog, connected with 'the trachea, 

 answer for this experiment as well as the sheep's lung; but the elas- 

 ticity of the lung of the seal or the lion, is much greater. The elastic 

 force of the human lung has been calculated at 8 oz. per square inch 

 of surface, being equal to about 150' Ibs. in the male, and about 124 

 Ibs. in the female. (Hutchinson..) 



It has been shown experimentally, that the contractility of the un- 



