RESPIRATION. 263 



The elevation of the ribs is accompanied by a slight opening out of 

 the angle which the bony part forms with its cartilage (fig. 211, A); 

 and thus an additional means is provided for increasing the antero- 

 posterior diameter of the chest. 



The muscles by which the ribs are raised, in ordinary quiet inspira- 

 tion, are external inter costals, and that portion of the internal intercostals 

 which is situate between -the costal cartilages; and these are assisted 

 by the scaleni, which fix the first and second ribs, the levatores costarum, 

 and the serratus posticus superior. The action of the levatores and the 

 serratus is very simple. Their fibres, arising from the spine as a fixed 

 point, pass obliquely downward and forward to the ribs, and necessarily 

 raise the latter when they contract. The action of the intercostal mus- 

 cles is not quite so simple, inasmuch as, passing merely from rib to rib, 

 they seem at nrst sight to have no fixed point toward which they can 

 pull the bones to which they are attached. 



Fig. 211. Diagram of movement of a rib in inspiration. 



In tranquil breathing, the expansive movements of the lower part of 

 the chest are greater than those of the upper. In forced inspiration, 

 on the other hand, the greatest extent of movement appears to be in 

 the upper antero-posterior diameter. 



In extraordinary or forced inspiration, as in violent exercise, or in 

 cases in which there is some interference with the due entrance of air 

 into the chest, and in which, therefore, strong efforts are necessary, other 

 muscles than those just enumerated, are pressed into the service. It is 

 very difficult or impossible to separate by a hard and fast line the so- 

 called muscles of ordinary from those of extraordinary inspiration; but 

 there is no doubt that the following are but little used as respiratory 

 agents, except in cases in which unusual efforts are required ihesterno- 

 mastoid, the serratus magnus, the pectorales, and the trapezius. 



