546 MOVEMENTS OF THE RIBS. [BOOK n. 



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 experi- 

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

 contraction of the internal intercostal s, 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 held 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 

 the lower cervical and upper dorsal regions to the second, third, 

 fourth, and fifth ribs, by its contractions raises those ribs. In 

 laboured breathing a function of the lower false ribs, not very 

 noticeable in easy breathing, comes into play. They are depressed, 

 retracted, and fixed, thereby giving increased support to the 

 diaphragm, and directing the whole energies of that muscle to 

 the vertical enlargement of the chest. In this way the serratus 

 posticus inferior, which passes upward from the lumbar aponeurosis 

 to the last four ribs, by depressing and fixing those ribs becomes 

 an adjuvant inspiratory muscle. The quadratus lumborum and 

 lower portions of the sacro-lumbalis may have a similar function. 



All these muscles may come into action even in breathing 



