CHAP. IL] RESPIRATION. 575 



senting the ribs, moving vertically by means of their articulations 

 with an upright representing the spine, and connected by 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 

 their vertebral articulation. The mechanical conditions in fact 

 of these muscles are so complex, that a deduction of their actions 

 from simple mechanical principles, or from the direction of the 

 fibres, must be exceedingly difficult and dangerous. Actual experi- 

 ments on the cat and dog tend to shew that in these animals the 

 contraction of the internal intercostals, along their whole length, 

 takes place, in point of time, alternately with that of the 

 diaphragm, and thus afford an argument in favour of these muscles 

 being expiratory in function. 



Next in importance to the external intercostals "come the 

 levatores costarum, which, though small muscles, are able, from 

 the nearness of their costal insertions to the fulcrum, to produce 

 considerable movement of the sternal ends of the ribs. The 

 external intercostals and the levatores costarum with the scaleni 

 may fairly be said to be the elevators of the ribs, i.e. the chief 

 muscles of costal inspiration in normal breathing. 



It must be added however that some observers deny that either 

 set of intercostal muscles take any important part in raising the 

 ribs. They hold that the chief if not the only use of these muscles 

 is by their contraction to render the intercostal spaces firm and the 

 whole thoracic cage rigid, so that the thorax is moved as a whole 

 by the other muscles mentioned, and the intercostal spaces do not 

 give way during the respiratory movements. 



Additional space in the transverse diameter is afforded probably 

 by the rotation of the ribs on an antero-posterior axis ; but this 

 movement is quite subsidiary and unimportant. When the chest 

 is at rest, the ribs are somewhat inclined with their lower borders 

 directed inwards as well as downwards. When they are drawn up 

 by the action of the intercostal muscles, their lower borders are 

 everted. Thus their flat sides are presented to the thoracic cavity, 

 which is thereby slightly increased in width. 



334. Laboured Inspiration. When respiration becomes 

 laboured, other muscles are brought into play. The scaleni are 

 strongly contracted, so as distinctly to raise or at least give a very 

 fixed support to the first and second ribs. In the same way the 

 serratus posticus superior, which descends from the fixed spine in 



