EXPIRATION. 



337 



FIG. 153. 



be called into play during the inspiratory effort, as may be seen 

 during occlusion of the air passages, where all the thoracic, cer- 

 vical, facial, abdominal muscles, and even the muscles of the 

 extremities, one after another, are thrown into a recurring spasm 

 before suffocation ends the patient's life. 



Among the muscles which lend their aid when more energetic 

 inspiratory movements are required, may be mentioned the sterno- 

 mastoid, which helps the scaleni to elevate the front of the tho- 

 racic wall ; the pectoral muscles and the great serratus, which assist 

 when the arms are fixed ; and also the deep muscles of the back, 

 which straighten the spine and act upon the vertebral attach- 

 ments of the ribs so as to elevate them and widen the intervals 

 between them. Owing to the ribs being fixed to the sternum 

 in front, they can only separate 

 laterally when the dorsal curve is 

 lessened, and this tends to approxi- 

 mate the sternum and the verte- 

 brae, thus narrowing the antero- 

 posterior diameter of the thorax. 

 It is in preventing this flattening 

 of the chest that the intercostals 

 are particularly useful ; by holding 

 the ribs together they push forward 

 the sternum, when the dorsal curve 

 is extended. 



During quiet breathing expira- 

 tion requires no muscular effort, 

 the expulsion of the air from the 

 chest being accomplished by the 

 elasticity of the parts. 



The most powerful force is the 

 elasticity of the lungs, which are 

 on the stretch even after a forced 

 expiration, and when distended 

 by inspiration are capable of ex- shows the position of the Ribs and 

 citing considerable traction on the th * ^ inal Colu 7 iu nor '" al forra of 



the thorax, . e., that assumed in expira- 



thoracic wall. 

 29 



tion. 



