THE PELVIS. 163 



The movement of the Ribs and Sternum in resjriration. 



The ribs and their cartilages are articulated to the spine 

 behind, and the sternum before, in a way which admits of a 

 compound motion. 



They are drawn from a position which slopes obliquely 

 downwards and forwards, into one which is more horizontal ; 

 and the posterior extremity of each rib, which is the centre of 

 this motion, is moved very little, while the anterior extremity 

 moves much more. 



At the same time, the ribs perform a rotation outwards, 

 upon their extremities connected with the spine and sternum ; 

 in consequence of which, the middle of each rib is moved out- 

 wards to a considerable extent. 



It is very obvious, that, by these motions, the thorax must be 

 enlarged from side to side, and from behind forwards. 



As the ribs are raised from the oblique towards the horizontal 

 position, the sternum is necessarily moved forward by them ; 

 and, if this bone does not move upon the first rib, the rib must 

 move to accommodate it : a small motion at the articulation of 

 the rib with the spine, being sufficient to produce considerable 

 motion at the lower end of the sternum. The sternum, there- 

 fore, vibrates forward when the ribs are elevated, and backward 

 when they are depressed. 



In easy respiration, these motions are not very great, for then 

 the enlargement of the thorax appears to be produced by the 

 increase of its vertical diameter, in consequence of the descent 

 of the diaphragm ; but when the inspirations are very large, 

 and when the descent of the diaphragm is impeded, as in 

 pregnancy, and in ascites, these motions are very considerable. 



It ought to be observed, that the first rib has very little motion, 

 except the rotation which favors the motion of the sternum ; and 

 that the lower ribs, having no support at their anterior extremi- 

 ties, have no rotation. 



The Pelvis. 



The pelvis is the cavity at the lower part of the trunk, formed 

 by the os sacrum, os coccygis, and ossa innominata. 



