94 OK RESPIRATION. 



power of the diaphragm and intercostal muscles is far greater 

 than the elastic power of the lungs, and therefore, when exerted, 

 overcomes it, producing inspiration : but, ceasing to be exerted, 

 the elastic power gains efficiency, and produces expiration. 

 " The contractile power of the diaphragm (and intercostal 

 muscles) in conformity with the laws of muscular motion, is irre- 

 gular, remitting and sometimes, altogether quiescent. The elas- 

 ticity of the lungs, on the other hand, is equal and constant. 

 The superior energy of the former is balanced by the permanency 

 of the latter. By the advantage which the inferior power, from 

 the uniformity of its operations, is enabled to take of the remis- 

 sions of its more powerful antagonist, the ground which had 

 been lost is recovered, and the contest prolonged ; that contest 

 in which victory declaring on one side or the other is the instant 

 death of the fabric." * 



In the common account of respiration, the elasticity of the 

 lungs is unnoticed, and expiration is ascribed to the contractions 

 of the abdominal muscles. Now in the first place, the elasticity 

 of the lungs is of itself sufficient for the purpose ; and in the 

 second, there is no proof of the agency of these muscles in 

 expiration. It proceeds equally well in cases of inanition, when 

 their contraction would rather enlarge than diminish the abdo- 

 minal cavity, and in experiments when they arc entirely removed 

 from animals. 



The beautiful contrivance in the shape of the thorax deserves 

 attention : by its being conical, every degree of motion in the 

 diaphragm produces a greater effect on the capacity of the chest 

 than could occur were it of any other shape. 



The vacuum constantly threatening in the chest, either from 

 tile shrinking of the lungs or the contraction of the inspiratory 

 muscles, and I may add from the expulsion of blood from the 

 ventricles of the heart, will evidently be prevented, not only by 

 the falling of the ribs and the ascent of the diaphragm in the 



* L. c. . 223. 



