MECHANICAL PHENOMENA OF RESPIRATION. 299 



the abdomen be opened and emptied, and the lower surface of 

 the diaphragm examined, it will be found to have ascended 

 automatically, as it were: this is because the lung has a 

 tendency to ascend quite high, and draws the diaphragm 

 forcibly with it, by means of the pleural vacuum, which 

 obliges the diaphragm to follow the lung as we saw that the 

 lung followed the diaphragm. Thus, in a corpse, the 

 diaphragm is found greatly arched at the top, and 

 very tense. Anatomists know well how favorable this 

 circumstance is to the dissection of this muscle, but they also 

 know that the slightest stroke of the scalpel, by which it is 

 divided and the air allowed to enter between the two folds 

 of the pleura, immediately causes the muscle to descend, 

 when it becomes flabby, loose, and no longer capable of being 

 handsomely dissected. 



In the normal condition, therefore, the mechanism of 

 inspiration and expiration is entirely different ; the former is 

 active, and is produced by muscular contraction ; the latter is 

 passive, and is dependent on phenomena of elasticity on the 

 part of those organs which have been opposed by inspiration ; 

 for it is not the elasticity of the lung alone which produces 

 this reaction, that of the walls of the thoracic cage, which 

 have been equally opposed, must also be taken into account; 

 the costal cartilages, for instance, which during inspiration 

 are twisted around their axis in a rather remarkable manner. 

 Finally, the viscera and the walls of the abdomen, having 

 been displaced during inspiration, return to their original 

 position, while the stomach and intestine, which contain elas- 

 tic gases, by this means force the diaphragm upwards. 



Expiration may, however, be active in certain cases. As 

 we have seen that there is a natural, and a forced, inspiration, 

 so we find that there is a natural, and a forced, expiration ; 

 in the latter only do the muscles come into play, those, 

 namely, of the abdomen, the serrati postici, and, in. general, 

 all those by which the ribs can be depressed. This active 

 expiration takes place especially in coughing : the walls of 

 the thorax then no longer simply follow the lung as it draws 

 back, but compress it, thus increasing the rapidity and energy 

 of the expiratory current of air. 



We cannot lay too much stress upon that special function 

 of the pleural cavity by which, while it permits the lungs 

 to move along the inner surface of the thoracic wall, binds 

 these two surfaces together, in such a manner that if the 

 thorax expands, the lung expands also ; and if the latter con- 



