THE HUMAN FRAME. 7S 



vertebrsu shifting upon one another, so as to break the line 

 of the canal as often as the body moves or twists, or the 

 joints gaping externally whenever the body is bent forward 

 and the spine thereupon made to take the form of a bow. 

 These dangers, which are mechanical, are mechanically j ro- 

 V'ided against. The vertebrsB, by means of their procssses 

 Kzid projections, and of the articulations which some of these 

 form with one another at their extremities, are so locked in 

 and confined as to maintain, in what are called the bodies 

 or broad surfaces of the bones, the relative position nearly 

 unaltered, and to throw the change and the pressure pro- 

 duced by flexion almost entirely upon the intervening carti- 

 lages, the springiness and yielding nature of whose substance 

 admits of all the motion which is necessary to be performed 

 upon them, without any chasm being produced by a separa- 

 tion of the parts. I say, of all the motion which is neces- 

 sary ; for although we bend our backs to every degree al- 

 most of inclination, the motion of each vertebra is very 

 small : such is the advantage we receive from the chain 

 being composed of so many links, the spine of so many 

 bones. Had it consisted of three or four bones only, in bend- 

 ing the body the spinal marrow must have been bruised at 

 every angle. The reader need not be told that these inter- 

 vening cartilages are gristles, and he may see them in per- 

 fection in a loin of veal. Their form also favors the same 

 intention. They are thicker before than behind ; so that 

 when we stoop forward, the compressible substance of the 

 cartilage, yielding in its thicker and anterior part to the force 

 which squeezes it, brings the surface of the adjoining verte- 

 brae nearer to the being parallel with one another than they 

 were before, mstead of increasing the inclination of their 

 I'lanes, w^hich must have occasioned a fissure or opening 

 b<:lween them. Thirdly, for the medullary canal, givnig out 

 in its course, and in a convenient order, a supply of nerves 

 to diflerent parts of the body, notches are made in the upper 

 and lower edge of every vertebra, two on each '^dge, equi- 



Nal. T ...,1. 4 



