RESPIRATORY MUSCLES. 



331 



FIG. 152. 



this action it may be regarded as the antagonist of the next 

 group. 



The Scaleni Muscles, which pass down from the lateral aspects 

 of the cervical vertebrae to the first two ribs, which they raise so 

 as to draw up the upper outlet of the thorax. The quadratus 

 and scaleui muscles thus act upon the thorax in the same way as 

 the hands when extending a concertina. 



The Levatores Costarum are small muscles, but on account of 

 their number, their aggregate force is much greater than is com- 

 monly thought. They are short, thick muscles, which pass ob- 

 liquely downwards and out- 

 wards from the transverse pro- 

 cesses of the dorsal vertebrae to 

 the angle of the ribs. Their 

 only action is to raise the angle 

 of the ribs, and thus remove 

 their anterior and lateral down- 

 ward slopes ; by so doing they 

 increase the intervals between 

 the ribs and enlarge the lateral 

 and the antero-posterior diame- 

 ters of the chest. Thus they are 

 purely muscles of inspiration, 

 and probably, acting with the 

 diaphragm and the scaleui, are 

 the chief workers in ordinary 

 breathing. 



View from behind of four dorsal 

 vertebrae and three attached ribs, 

 showing the attachment of the eleva- 

 tor muscles of the ribs and the inter- 

 costals. (Allen Thomson.) 1. Long 

 and short elevators. 2. External in- 

 tercostal. 3. Internal intercostal. 



The Inter costals produce dif- 

 ferent effects on the ribs accord- 

 ing to the different sets of mus- 

 cles 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 iutercostals approximate 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. 



