CHAPTER XXXVI 



THE MECHANISM BY WHICH THE CHANGES IN CAPACITY OF 

 THE THORAX AND LUNGS ARE BROUGHT ABOUT 



BY R. G. PEARCE, B.A., M.D. 



The changes that take place in the form and the dimensions of the 

 thorax during respiration are brought about by movements of the ribs, 

 diaphragm, sternum, and vertebras. The share which each plays must 

 be considered separately. 



The Movements of the Ribs 



The first seven pairs of ribs progressively increase in length, and are 

 attached directly to the sternum by cartilaginous bands. The eighth to 

 the twelfth pairs progressively decrease in length, and as far as the 

 tenth they are indirectly attached to the sternum by cartilages which join 

 the seventh. The eleventh and twelfth have their anterior ends free, and 

 may be considered a part of the abdominal wall and not an intrinsic part 

 of the thoracic cage. 



Each pair of ribs, together with its articulating cartilage and vertebrae, 

 forms a ring, the plane of which is directed forward and downward. 

 The spinal articulations of the upper ribs differ from those of the lower 

 ones. In the former the articulations on the tubercle exist as convex 

 ovoid facets, which fit into corresponding hollow facets on the transverse 

 processes of the vertebrae, while the corresponding facets of the lower 

 ribs are flat. Each transverse process from above downward is tilted a 

 little more backward than the one above, so that the angle at which the 

 ribs are set to the spine increases from above downward. This manner 

 of articulation of the upper ribs with the vertebrae prevents any rotation 

 in the spinosternal axis, so that there can be no so-called bucket-handle 

 movement in this region (Keith). The articulation, however, allows the 

 neck of the rib to rotate in an axis approximately transverse to the body. 

 The angle which the shaft of the rib makes near its neck, together with 

 the arch of the shaft, which is directed downward and forward, has the 

 effect of causing the transverse rotation of the neck of the rib to be 



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