CHAP, ii.] RESPIRATION. 545 



side, in proportion as the fifth arch is wider than the fourth. 

 Thus the elevation of the rib increases not only the antero-posterior 

 but also the transverse diameter of the chest. Further, on account 

 of the resistance of the sternum, the angles between the ribs and 

 their cartilages are, in the elevation of the ribs, somewhat opened 

 out, and thus also the transverse as well as the antero-posterior 

 diameter, somewhat increased. In more than one way, then, the 

 elevation of the ribs enlarges the dimensions of the chest. 



333. The ribs are raised by the contraction of certain 

 muscles. Of these the external intercostals are perhaps the most 

 important. Even in the case where two ribs, such as the fifth and 

 sixth, are isolated from the rest of the thoracic cage, by section of 

 the structures occupying the intercostal spaces above and below, 

 the contraction of the external intercostal muscle of the inter- 

 vening space raises the two ribs, thus bringing them towards the 

 position in which the fibres of the muscle have the shortest 

 length, viz. the horizontal one. This elevating action is, in the 

 entire chest, further favoured by the fact that the first rib is 

 less moveable than the second, and so affords a comparatively fixed 

 base for the action of the muscles between the two, the second in 

 turn supporting the third, and so on, while the scaleni muscles in 

 addition serve to render fixed, or to raise, the first two ribs. So 

 that in normal respiration, the act may probably be -described as 

 beginning by a contraction of the scaleni. The first two fibs 

 being thus raised or at least fixed, the contraction of the series of 

 external intercostal muscles acts at a great advantage. 



While the ele.vating, i.e. inspiratory action of the external 

 intercostals is admitted by nearly all authors, the function of the 

 internal intercostals has been much disputed. Some regard their 

 action as wholly inspiratory ; others maintain, what is perhaps the 

 more commonly adopted view, that while those parts of them 

 which lie between the sternal cartilages act like the external 

 intercostals as elevators, i.e. as inspiratory in function, those parts 

 which lie between the osseous ribs act as depressors, i.e. as ex- 

 piratory in function. 



In the well-known model consisting of two rigid bars, repre- 

 senting the ribs, moving vertically by means of their articulations 

 with an upright representing the spine, and connected at their free 

 ends by a piece representing the sternum, it is undoubtedly true 

 that stretched elastic bands attached to the bars in such a way as 

 to represent respectively the external and internal intercostals, viz. 

 sloping in the one case downwards and forwards and in the other 

 downwards and backwards, do, on being left free to contract, in the 

 former case elevate and in the latter depress the ribs. Such a 

 model however does not fairly represent the natural conditions of 

 the ribs, which are not straight and rigid, but peculiarly curved 

 and of varying elasticity, capable moreover of rotation on their own 

 axes, and having their movements determined by the characters of 



F. 35 



