On Respiration 191 



more then the ribs arched over the said plane (or 

 what is the same thing, over the spine and sternum, 

 which are in that plane) approach to right angles, so 

 much the greater will be the space which lies 

 between the raised ribs and the mediastinum, as we 

 have already shown. And thus it is clear that one 

 half of the chest is expanded by the ribs being raised 

 towards right angles, and it is evidently the same 

 with the other side. As for the false ribs, although 

 their extremities are connected not with the sternum 

 but with the diaphragm, they have notwithstanding the 

 same motion, and in like manner dilate the chest. But 

 since the ribs when drawn upwards approach nearer 

 to right angles with the spine, and the ribs when 

 raised to right angles open up a space in the thorax, 

 it follows that when the ribs are drawn upwards they 

 dilate the chest, which is what we undertook to prove. 

 Nay, any one can experience in himself that the ribs 

 are drawn upwards in inspiration and the dilatation of 

 the chest, but that they descend in expiration and the 

 contraction of the chest. 



This premised, if the ribs are raised by the inter- 

 costal muscles, even the internal ones (which has next 

 to be proved), it follows necessarily that the chest is 

 dilated by their contraction. 



Whenever, I say, a muscle attached to two bones 

 contracts, the bone which is less fixed moves towards 

 the other which is more fixed. Wherefore, since 

 every lower rib is less fixed than the one above it, 

 each of the lower ribs must be elevated when the 

 intercostal muscles, even the internal ones, contract. 

 For a quite similar reason holds with the internal as 

 with the external muscles ; nor is it an objection to 

 this that the former are attached to the ribs in a 

 different position, as is obvious from Plate II., Fig. 2, 



