qQ of mechanical arrangement 



settled posture is inconsistent with its use, a great difficul- 

 ty still remained, which was to prevent the vertebrae shift- 

 ing 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 provid- 

 ed against. The vertebrae, by means of their processes 

 and 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 po- 

 sition nearly unaltered ; and to throw the change and 

 the pressure, produced by flexion, almost entirely upon the 

 intervening cartilages, the springiness and yielding nature 

 of whose substance admits of all the motion which is ne- 

 cessary to be performed upon them, without any chasm 

 beincr produced by a separation of the parts. I say of all 

 the motion which is necessary ; for although we bend our 

 backs to every degree almost of inclination, the motion of each 

 vertebrae is very small ; such is the advantage which 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 bending the body the spinal marrow 

 must have been bruised at every angle. The reader need 

 not be told that these intervening cartilages are gristles ; 

 and he may see them in perfection in a loin of veal. Their 

 form also favours 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, 

 brino-s the surfaces of the adjoining vertebrae nearer to the be- 

 ing parallel with one another than they were before, instead 

 of increaing the inclination of their planes, which must have 

 occasioned a fissure or opening between them. Thirdly, for 

 the medullary canal giving out in its course, and in a conve- 

 nient order, a supply of nerves to different parts of the body, 

 notches are made in the upper and lower edge of every 

 vertebrae, two on each edge ; equi-distant on each side 

 from the middle line of the back. When the vertebrae are 

 put together, these notches,' exactly fitting, form small holes, 

 throuo'h which the nerves at each articulation, issue out in 

 pairs, in order to send their branches to every part of the 

 body, and with an equal bounty to both sides of the body. 

 The fourth purpose assigned to the same instrument, is the 



