436 MOVEMENTS OF THE RIBS. [Book ii. 



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

 resenting the ribs, moving vertically by means of their articula- 

 tions with an upright representing the spine, and connected at 

 their free ends by a piece representing the sternum, it is un- 

 doubtedly true that stretched elastic bands attached to the bars 

 in such a way as to represent respectively the external and in- 

 ternal 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 elas- 

 ticity, capable moreover of rotation on their own axes, and hav- 

 ing their movements determined by the characters of their 

 vertebral articulations. 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 

 experiments on the cat and dog tend to shew that in these ani- 

 mals 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 rais- 

 ing 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 move- 

 ments. 



Additional space in the transverse diameter is afforded prob- 

 ably 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. 



§ 265. Laboured Inspiration. When respiration becomes 

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



