RESPIRATORY MOVEMENTS OF THE CHEST. 



237 



Fig. 71. 



scaleni muscles, and the intercostal muscles then contracting simul- 

 taneously, the ribs are drawn upward. In this movement, as each 

 rib rotates upon its articulation with the 

 spinal column at one extremity, and with 

 the sternum at the other, its convexity is 

 necessarily carried outward at the same 

 time that it is drawn upward, and the pa- 

 rietes of the chest are, therefore, expanded 

 laterally. The sternum itself rises slightly 

 with the same movement, and enlarges to 

 some extent the antero-posterior diameter 

 of the thorax. By the simultaneous action, 

 therefore, of the diaphragm which descends, 

 and of the intercostal muscles which lift 

 the ribs and the sternum, the cavity of the 

 chest is expanded in every direction, and 

 the air passes inward, through the trachea 

 and bronchial tubes, by the simple force of 

 aspiration. 



After the movement of inspiration is ac- 

 complished, and the lungs are filled with 

 air, the diaphragm and intercostal muscles 

 relax, and a movement of expiration takes 

 place, by which the chest is partially col- 

 lapsed, and a portion of the air contained 

 in the pulmonary cavity expelled. The 

 movement of expiration is entirely a passive 

 one, and is accomplished by the action of 

 three different forces. First, the abdominal 

 organs, which have been pushed out of their 

 usual position by the descent of the diaphragm, fall backward by 

 their own weight and carry upward the relaxed diaphragm before 

 them. Secondly, the costal cartilages, which are slightly twisted 

 out of shape when the ribs are drawn upward, resume their natural 

 position as soon as the muscles are relaxed, and, drawing the ribs 

 down again, compress the sides of the chest. Thirdly, the pul- 

 monary tissue, as we have already remarked, is abundantly sup- 

 plied with yellow elastic fibres, which retract by virtue of their 

 own elasticity, in every part of the lungs, after they have been 

 forcibly distended, and, compressing the pulmonary vesicles, drive 

 out a portion of the air which they contained. By the constant 



DIAGRAM ILLUSTRATIKO 

 THE RESPIRATORY MOVE- 

 MEXTS. a. Cavity of tbe chest. 

 b. Diaphragm. The dark out- 

 lines show the figure of the chest 

 when collapsed ; the dotted lines 

 show the same when expauded. 



