MECHANISM OF THE SPINE 27 



the edge or border of one vertebra to the border 



o 



of the one next it. When a weight is upon the 

 body, the soft gristle is pressed out, and the 

 membrane yields : the moment the weight is re- 

 moved, the membranes recoil by their elasticity, 

 the gristle is pressed into its place, and the bones 

 resume their position. 



We can readily understand how great the influ- 

 ence of these twenty-four joinings must be in 

 giving elasticity to the whole column ; and how 

 much this must tend to the protection of the 

 brain. Were it not for this interposition of elas- 

 tic material, every motion of the body would pro- 

 duce a jar to the delicate texture of the brain, 

 and we should suffer almost as much in alighting 

 on our feet as in falling on our head. It is, as 

 we have already remarked, necessary to interpose 

 thin plates of lead or slate between the different 

 pieces of a column to prevent the edges (techni- 

 cally called arrises) of the cylinders from coming 

 in contact, as they would, in that case, chip or 

 split off. 



But there is another very curious provision for 

 the protection of the brain : we mean the curved 

 form of the spine. If a steel spring, perfectly 

 straight, be pressed betwixt the hands from its 

 extremities, it will resist, notwithstanding its elas- 

 ticity, and when it does give way, it will be with 

 a jerk. 



