On Respiration 193 



equal force in this direction and in that, the ribs 

 meanwhile rise straight upwards, as is shown in 

 Plate II., Fig. 4, where, when the external muscle a^ a^ 

 and the internal c, c, contract together, the lower and 

 more mobile rib will rise not obliquely but straight 

 upwards, just as though it were pulled by a muscle 

 attached to it at right angles. So that clearly the 

 external and internal muscles contract simultaneously, 

 and by their united effort elevate the ribs and expand 

 the chest. Moreover, that the internal muscles do 

 not cause expiration, may be gathered from the fact 

 that the thorax in a dead animal is always con- 

 tracted, for to die and to expire mean the same 

 thing ; but in the dead, the action of the muscles 

 altogether ceases ; and so this contraction of the 

 chest cannot be caused by the internal muscles, since 

 they no longer contract. Should any one remark 

 here that that contraction of the thorax is caused 

 by the internal muscles immediately before the death 

 of the animal, I ask, in reply, how then is it that the 

 external muscles (since the two cases are similar) 

 never contract in the dying, so that the chest should 

 remain for some time dilated ? 



It is probable then that, in expiration, the parts of 

 the thorax return, by a movement of restitution, to 

 their natural position without any aid from the 

 muscles. For it is difficult and contrary to their 

 natural position for the ribs to be drawn upwards, so 

 that for this there is indispensable need of the two- 

 fold and united action of both sets of muscles, the in- 

 ternal and the external. But the ribs sink down again, 

 without any work, as is clear in the case of a dead 

 animal or a skeleton. Wherefore there is no reason 

 for saying that Nature has provided as much muscular 

 power for the latter action, which is clearly no 



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