336 



MANUAL OF PHYSIOLOGY. 



View from behind of four dorsal verte- 

 brae and three attached ribs, showing the 

 attachment of the elevator muscles of the 

 ribs and the intercostals. (Allen Thom- 

 son.)!. Long and short elevators. 2. Ex- 

 ternal intercostal. 3. Internal intercostal. 



FIG. 152. and lateral downward slopes ; 



by so doing they increase the 

 intervals between the ribs and 

 enlarge the lateral and the 

 antero-posterior diameters of 

 the chest. Thus they are purely 

 muscles of inspiration, and 

 probably, acting with the dia- 

 phragm and the scaleni, are 

 the chief workers in ordinary 

 breathing. 



The Intercostals produce dif- 

 ferent effects on the ribs accord- 

 ing to the different sets of 

 muscles with which they act in 

 association. They never act 

 alone, and it is therefore idle 



to try to ascribe to them any constant specific inspiratory or 

 expiratory action. Generally speaking, the intercostals approxi- 

 mate the ribs, and by this action they stiffen the thoracic wall 

 and help to elevate the thorax when its upper part is fixed, or, 

 when its lower part is fixed, to depress it. 



Now, if both the upper and lower margins of the thorax be held 

 firmly by strong muscles, as really occurs in inspiration from 

 the action of the quadratus and scaleni the intercostals cannot 

 approximate the ribs. Under these circumstances the results 

 which follow their contraction will be twofold, viz. : (1) the 

 sternum will be pushed forward, and the antero-posterior diam- 

 eter of the thorax thus increased ; and (2) the spaces between 

 the ribs, which are widened by the other muscles, are kept rigid 

 and prevented from sinking inward when the intrathoracic pres- 

 sure falls. When acting with the elevators of the ribs, both 

 intercostal layers of muscle have an inspiratory effect. But when 

 the elevators of the ribs are passive, the intercostals, acting with 

 the anterior abdominal muscles, draw down the ribs, and act as 

 muscles of expiration. 



For forced breathing an enormous number of muscles may 



