200 A MANUAL OF PHYSIOLOGY 



tendency during its contraction for these to be drawn in- 

 wards and upwards ; but this is opposed by the pressure of 

 the abdominal viscera, and by the action of the quadratus 

 lumborum, which fixes the twelfth rib, and of the serratus 

 posticus inferior, which draws the lower four ribs backward. 

 When these and the other inspiratory muscles that act 

 especially upon the ribs are paralyzed by injury to the spinal 

 cord, and respiration is carried on by the diaphragm alone, 

 the line of its attachment to the ribs is distinctly marked 

 during inspiration by a shallow circular groove. 



The antero-posterior and transverse diameters of the 

 thorax are enlarged by the action of certain muscles that 

 elevate the ribs. Of these, the most important are the 

 levatores costarum twelve in number on each side. They 

 arise from the transverse processes of the last cervical and 

 first eleven dorsal vertebrae, and, passing obliquely down- 

 wards and outwards, are inserted between the tubercle and 

 the angle into the first or second rib below their origin. 

 The scalene muscles, which may in a lean person be felt to be 

 tense during inspiration, fix the first and second ribs (scalenus 

 anticus and medius, the first ; scalenus posticus, the second 

 rib), and so afford a fixed line for the intercostal muscles to 

 work from on the lower ribs. 



The action of the intercostals has been much debated ; but it 

 seems to be certain that the external intercostals do aid to a 

 slight extent in raising the ribs when the upper two have 

 been fixed by the contraction of the scaleni. The inter- 

 cartilaginous portion of the internal intercostals also con- 

 tracts simultaneously with the diaphragm, and may there- 

 fore be reckoned in the list of inspiratory muscles ; but the 

 function of the interosseous portion is still in doubt. It is 

 probable that the chief importance of the intercostal muscles 

 (both external and internal) is not so much to act upon the 

 ribs, as to increase by their contraction the rigidity of the 

 intercostal spaces, and so prevent them from being drawn in 

 when the chest is expanded by the action of the diaphragm, 

 the> levatores, and the scaleni. Since the ribs slant down- 

 wards and forwards to their sternal attachments, the sternum 

 is raised when they are elevated ; or, rather, since the upper 



