206 RESPIRATION. 



as bounded above by the surface of the lungs, and always in the 

 sound state possessing the same dimensions, the expansion of 

 the lungs being commensurate with the descent of the diaphragm 

 and ascent of the ribs, and the descent of the diaphragm and 

 ascent of the ribs commensurate with the shrinking of the lungs. 

 The fact that air does not stream into the wounded pleura in 

 expiration, but even streams from it, while the ribs are moveable 

 and the abdominal muscles active, proves, I think, that the descent 

 of the ribs and ascent of the diaphragm, one or both, in ordinary 

 expiration, do partly occasion, by compression*, the diminution of 

 the lungs, or, at least, are not its passive effect, but coincide with 

 it by independent powers, which are, the elasticity of the elevated 

 ribs (and displaced abdominal organs ?), if not the contraction of 

 the extended abdominal muscles. We shall presently see another 

 reason for believing that the organs of the chest are really 

 compressed during expiration. Haller refers expiration to the 

 pressure of the lungs by the elastic ribs and the abdominal and 

 other muscles, and to the elastic and muscular contraction of the 

 lungs themselves, which he considers more forcible than the 

 compression. It appears to me that he is right ; but that, never- 

 theless, either the lungs alone, or the walls of the chest alone, are 

 able, when unassisted by the other, to produce expiration. The 

 change in the situation of the ribs is, moreover, trifling compared 

 to that of the diaphragm, and respiration often proceeds very 

 well by the diaphragm alone. Animals which are remarkable for 

 swiftness and perseverance in the race scarcely employ the inter- 

 costal muscles, using the diaphragm almost solely. f 



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 passage of the air into and from the cells may be distinctly 

 heard on applying the ear to the corresponding part of the chest, 

 and is called by Laennec the respiratory murmur. It is much 

 louder in children, and in them the cells are far more numerous 

 and small. Whence an equal portion of lung from an infant a few 

 days old weighs fourteen times more than from a man of seventy.? 

 When the air tubes are constricted or supplied with too copious 



e 1. G. lib. viii. sect. iv. p. 275. sq. 



f Dr. Carson. 1. c. p. 226. In disease I have seen the diaphragm regularly 

 relax when the intercostals contracted, and contract when they relaxed. Any one 

 may readily make them act oppositely. 



K Dr. Magendie, Journal de Physiologic, t. i. p. 81. 



