RESPIRATION IN MAN 203 



tubercle of the rib glides downwards and forwards on the flat 

 upper facets on the transverse processes. The axis of the move- 

 ment in the lower set does not correspond to the neck of the rib ; 

 it corresponds to its spinal segment and is determined by the action 

 of the muscles rather than by the shape of the articulations and 

 ligaments. Anteriorly the diaphragmatic set of ribs is inserted 

 to the lower end of the sternum by the strong peculiar complex 

 formed by the costal cartilages of the sixth, seventh, eighth, and 

 ninth ribs. As the diaphragm and external intercostals elevate 

 the lower costal series, they also raise the sternum upwards through 

 the cartilage complex. The ilio-costalis, rising from the iliac crest, 

 steadies the spinal segments of the ribs, or during an energetic 

 inspiration actually draws them downwards. The result of a 

 movement of the lower ribs or better, of the action of the 

 diaphragmatic mechanism is to increase the transverse and back- 

 to-front diameter of the lower thorax and the vertical diameter of 

 the whole cavity. 



RESPIRATORY MOVEMENTS OF THE FLOATING RIBS 



As already mentioned, the eleventh and twelfth ribs are 

 functionally parts of the belly wall. Sibson observed that the 

 tenth and eleventh intercostal spaces widen during inspiration, 

 and diminish during expiration, an observation which the writer 

 has frequently verified. The opposite is true of the other spaces. 

 The twelfth rib is not only controlled by the quadratus lumborum 

 and erector spinae muscles, but also by a strong and constant 

 ligamentous membrane, the function of which has been entirely 

 overlooked. This membrane, really an extension of the middle 

 layer of the lumbar fascia, anchors the lower border of the twelfth 

 rib to the transverse processes of the first and second lumbar ver- 

 tebrae ; it is continued up between the spinal parts of the lower 

 five ribs. Thus the lower five ribs at and near their angles are 

 bound to the spinal column in such a way as to strictly limit the 

 upward movement of the spinal segments of the ribs. On the 

 other hand, the lateral and anterior parts of the ribs are not so 

 limited ; the whole arrangement is designed to secure the move- 

 ments of the lower ribs round an axis, not corresponding to their 

 necks, but to their spinal segments, the parts to which the erector 

 spinae is attached. 



