278 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



FIG. 166. 



direction works on a longer lever arm than the force tending to depress 

 the rib, the total result of the contraction of the muscle must be to raise 

 the lower rib and with it the upper rib. 



Hamberger showed, moreover, that an elevation of the ribs must be 

 associated with a widening of the thorax. Thus, in Fig. 166, it is evident 



that the distance from ~b to the vertical line gf 

 is greater than the distance from the point I* to 

 the vertical line cd, i.e., that in the inspiratory 

 condition, the distance between sternum and 

 spine, that is to say, the antero-posterior 

 diameter of the thorax, is increased. Again, 

 since ~bh is smaller than l>a, the elevation of 

 the ribs is attended with a widening of the 

 intercostal spaces. It is also obvious that al is 

 shorter than cik ; in other words, the contracted 

 condition of the muscular fibres corresponds 

 with a raised condition of the ribs. 

 Analogous reasoning applied to the internal intercostal muscles 

 shows that these, so far as they lie between the bony ribs, must be 

 expiratory in function and tend to depress the ribs. The inter- 

 cartilaginous portions, on the other hand, although they continue the 

 general direction of the internal intercostal muscles, yet bear a 

 different relation to their attachments, i.e. the cartilages, since these are 

 directed upwards and inwards. As , , #, 



will be seen from Fig. 167, the 

 action of these fibres on the rib 

 cartilages appears to resemble the 

 action of the external intercostals 

 on the bony ribs, namely, to raise 

 the cartilages and to flatten out the 

 angle between them and the bony 

 ribs. They must, therefore, it is 

 argued, be inspiratory in function. 

 The views of Hamberger as to 

 the action of the external intercostal 

 muscles have been confirmed by 

 physiological experiment. Newell FIG. 167. ac, he, two ribs with their car- 



tilages eel, ef. kl, a tibre of the external 

 intercostal. %, a fibre of the inter- 

 cartilaginous muscles. The tendency of 

 these two muscles acting together to de- 

 press the angle c, is counteracted by the 

 next set of muscles k'l', tiif, which act 

 at a greater mechanical advantage, and 

 so tend to raise the point c. 



Martin and Hartwell x recorded the 



contraction of a segment of the 



internal intercostal muscles after 



they had been separated from the 



rest of the body by dividing the 



ribs and intercostal spaces on each 



side of the muscle, leaving only the 



intercostal artery and the nerve supply to the internal intercostal 



intact. They found that these muscles acted alternately with the 



diaphragm, and were therefore expiratory. On the other hand, the 



segment of the muscles between the costal cartilages acted synchronously 



with the rest of the internal intercostals, and were therefore also 



expiratory. Since this result with regard to the intercartilaginous 



muscles is difficult to understand from a mechanical point of view, it 



has been put to the test of a further experimental investigation by Masoin 



1 Journ. Physiol., Cambridge and London, 1879, vol. ii. p. 24. 



