MECHANISM OF THE SPINE 



31 



n 



FIG. 6. 



The spine forms here a semicircle which has 

 this effect : that, whether by the exertion of the 

 lower extremities, the 

 spine is to be carried for- 

 ward upon the pelvis, or 

 whether the body stops 

 suddenly in running, the 

 jar which would neces- 

 sarily take place at the 

 lower part of the spine 

 A, if it stood upright 

 like a mast, is distributed 

 over several of the bones 

 of the spine, 1, 2, 3, 4, 

 and, therefore, the chance 

 of injury at any particular part is diminished. 



For example, the sacrum, or centre bone of the 

 pelvis, being carried forward, as when one is 

 about to run, the force is communicated to the 

 lowest bone of the spine. But, then, the surfaces 

 of these bones stand with a very slight degree of 

 obliquity to the line of motion ; the shock com- 

 municated from the lower to the second bone of 

 the vertebra} is still in a direction very nearly per- 

 pendicular to its surface of contact. The same 

 takes place in the communication of force from 

 the second to the third, and from the third to the 

 fourth ; so that before the shock of the horizon- 

 tal motion acts upon the perpendicular spine, it 



