XII 



MECHANICS OF EESPIEATION 



413 



It is incontrovertible that the direction of the fibres in the 

 external intercostals between two ribs is identical with that of the 

 leva tores costarum, and that the shortening of any one fibre can 

 only occur, notwithstanding the extension of the intercostal space, 

 when both ribs are raised. On the other hand, it is a fact that the 

 fibres of the internal interosseous intercostals, which run in the 

 opposite direction, can only shorten when the intercostal spaces 

 are reduced by the lowering of the ribs. The tenability of this 

 view is apparent if we expose the intercostal muscles of the thorax 

 of a dead body, and imitate the inspiratory process, by drawing the 

 sternum upwards with a hook, introduced through a hole in the 

 manubrium. It will then be seen that as the intercostal spaces 

 widen, the external intercostal muscles relax, and the internal 

 interosseous intercostals contract ; this shows that during life the 

 insertions of the external intercostals come together during 

 inspiration, and those of the internal in- ^ ^ 



tercostals separate. 



Since the muscles shorten actively 

 during their contraction, and are passively 

 elongated by the action of the antagonists, 

 it follows that the external intercostals 

 must be inspiratory and the internal inter- 

 costals expiratory. Hamberger's schema 

 and machine (1751), however imperfect 

 and inadequate as an exact reproduction 

 of the physiological process, still serves to 

 elucidate the mechanics of this fact (Fig. 

 179). When the ribs ac and Id pass into 

 the inspiratory position ag and If, the 

 intercostal space dilates (bk<^ab) ; the sternum gf moves away 

 from the vertebral column ab (bf>be); the fibres ^of the external 

 intercostals ak shorten (ak>al), and those of the internal inter- 

 costals ck lengthen (ck<lg). The reverse occurs when the system 

 passes from the inspiratory position a b g f to the expiratory 

 position abed. 



In regard to the inspiratory function of the intercartilaginous 

 muscles which form the anterior prolongation of the internal 

 intercostals, Hamberger's explanation is less convincing, but it is 

 intelligible by the help of the following schema (Fig. 180). When 

 the ribs are curved, they may be regarded as rods bent at an 

 angle acd and bef, in which the articular points c and e represent 

 the symphysis between the bony and cartilaginous parts on which 

 the traction is exerted. During inspiration the fibres of the inter- 

 cartilaginous muscles, which have the direction gh, move the 

 sternum df away from the vertebral column 'ab. like the fibres of 

 the external intercostals, which run in the direction kl. During 

 this double action the angles c and e must get blunted, because the 



FIG. 179. Hamberger's schema 

 to demonstrate the functional 

 antagonism of internal and ex- 

 ternal intercostals (reproduced 

 from Fig. 2 of his dissertation). 



