THE RESPIRATION 



157 



their walls, and the walls being made partly of elastic tis- 

 sue, the cells contract again when the muscles of inspiration 

 cease to act. When the ribs are lifted up during inspira- 



tion, the costal cartilages 

 that connect them with 

 the sternum are slightly 

 bent, and the elasticity 

 of these cartilages, as 

 well as the weight of the 

 chest wall, causes the 

 ribs to become lower 

 when the muscles of 

 inspiration cease to act. 



Many DUDils T6t the QT- 

 " 



TOnCOUS idea that the 



Diagram to show action of intercostal muscles. At 

 A two ribs are represented in passive position. If 

 line ac shortens, the ribs will go down as in B. 



If line bd shortens, the ribs will rise as in C. One . . , . 



line represents an inner, and the other an outer, diaphragm IS alSO 

 intercostal muscle. A frame made with jointed cor- 

 ners (with strings to represent the muscles) will 



4.* J 



tlC aild 



j 

 Upward 



readily demonstrate the action of the two layers of W hen it relaXCS, thus aid- 

 intercostal muscles. 



ing expiration. When 



relaxed, it has no more elasticity than a piece of cloth, 

 and no more power to push itself upward. However, 

 the abdomen has been somewhat compressed, and its walls 

 somewhat stretched during the inspiration. When the 

 diaphragm relaxes, the elasticity of the muscular walls of 

 the abdomen presses the organs against the under side 

 of the diaphragm, pressing that in turn against the base 

 of the lungs, and aiding expiration. 



262. Quiet or Passive Expiration is, therefore, a rebound 

 brought about by the elasticity of the air cells, costal 

 cartilages, and abdomen, and by the weight of the chest 

 wall. Active expiration adds muscular contraction to the 

 above forces. The two layers of intercostal muscles are 

 among those used in inspiration and active expiration 

 (Figs. 136, 140). The outer muscles diminish the spaces 

 between the ribs when they contract, and thus move the 



