Chap, ii.] RESPIRATION. 435 



forward, some more than others according to their slope and 

 length. The elasticity of the sternum and costal cartilages, 

 assisted by the articulation of the sternum to the clavicle 

 above, permits the front surface of the chest to be thus thrust 

 forwards as well as upwards, when the ribs are raised. By this 

 action, the antero-posterior diameter of the chest is enlarged. 



Since the ribs form arches which increase in their sweep as 

 one proceeds from the first downwards as far at least as the 

 seventh, it is evident that when a lower rib such as the fifth is 

 elevated so as to occupy or to approach towards the position of 

 the one above it, the chest at that level will become wider from 

 side to 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 dimen- 

 sions of the chest. 



§ 264. 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 intervening space raises the two ribs, thus bring- 

 ing them towards the position in which the fibres of the muscle 

 have the shortest length, viz. the horizontal one. This elevat- 

 ing 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 ribs being thus raised or at least fixed, 

 the contraction of the series of external intercostal muscles acts 

 at a great advantage. 



While the elevating, 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 func- 

 tion, those parts which lie between the osseous ribs act as depres- 

 sors, i.e. as expiratory in function. 



