310 PULMONARY MUCOUS TISSUE. 



tion, by ^ of that of the atmosphere. We see that the differ- 

 ence is greater in expiration than in inspiration, in the case 

 of an energetic movement : as we know that more mechani- 

 cal effect is produced, for instance, by blowing through a 

 tube, than by inhaling through it. The reason of this differ- 

 ence is evident, if we remember that the contraction of the 

 inspiratory muscles is impeded by the elasticity of a number 

 of organs which are disturbed by such contraction (the lungs, 

 costal cartilages, abdominal viscera, &<x) ; while the expiratory 

 muscles, which are at least as powerful as their antagonists, 

 have only to add their force to that of these elastic parts 

 acting in the same direction. This power of forced expira- 

 tion, joined to the mechanical conditions produced by the 

 contraction of the trachea and the glottis, serve to promote 

 the expulsion of foreign bodies, or mucosities (coughing). 



We repeat that this difference, on the side of expiration, 

 exists only in the case of forcible respiration : in the normal 

 condition expiration is only a reaction of the elasticity of the 

 organs overcome by inspiration ; and thus one has nearly as 

 much power as the other. But they are not both of the same 

 type, the same form, or the same duration; that is to say, 

 inspiration, produced by muscular contraction, takes place in 

 a manner nearly uniform, and may be represented by a regu- 

 larly ascending line ; while expiration, on account of the way 

 in which it is produced, follows, in its form, the same law as 

 the elastic bodies : for instance, if we compress a gas in a 



Fig. 81. Normal kymographic tracings of the respiratory movements 

 in man (Marey).* 



syringe, by means of a piston, we shall find that the moment 

 we cease to press the latter, it rises .up, suddenly at first, but 



* The descending line is that of inspiration, the ascending that of expira- 

 tion, the pen moving from left to right. 



