CHAP, ii.] RESPIRATION. 321 



circle in the vertical plane of the body; as the rib is carried 

 upwards from an oblique to a more horizontal position, the sternal 

 attachment must of necessity be carried farther away in front of 

 the spine. Since all the ribs have a downward slanting direction, 

 they must all tend, when raised towards the horizontal position, to 

 thrust the sternum forward, some more than others according to 

 their slope and length. The elasticity of the sternum and costal 

 cartilages, together with the articulation of the sternum to the 

 clavicle above, permit 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 several ways, then, the elevation 

 of the ribs enlarges the dimensions of the chest. 



The ribs are raised by the contraction of certain muscles. Of 

 these the external intercostals are the most important. Even in the 

 case of two isolated ribs such as the fifth and sixth, the contraction 

 of the external intercostal muscle of the intervening 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 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 begins 

 probably by a contraction of the scaleni. The first two ribs 

 being thus fixed, the contraction of the series of external inter- 

 costal muscles acts to the greatest advantage. 



While the elevating i.e. inspiratory action of the external 

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

 intercostals has been much disputed. Haller may be regarded 

 as the leader of those who regard the internal intercostals as 

 inspiratory, while Hamberger was the first who successfully ad- 

 vocated the perhaps 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 



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