THE BACK AND LOINS. 13 



the elasticity of the material (very old seasoned deal) of which it 

 is manufactured. 



From their mobility the arches of the spine not only derive the 

 requisite elasticity, but likewise that self-regulating power which, 

 under all changes, preserves their equilibration or undiminished 

 strength. Dr. Hutton, in his " Principles of Bridges," tells us that 

 the arch of equilibration, i. e. the one which is in equilibrium in all 

 its constituent parts, is the only one which admits of a horizontal 

 line across its top, and, in addition to being both graceful and con- 

 venient, possesses the advantage of being made higher or lower 

 at pleasure, with the same span or opening. He adds, that the 

 elliptical form of arch approaches the nearest to that of equilibra- 

 tion for strength and convenience. How strikingly and beautifully 

 is all this verified in the fabric of the spine, and how wonderful it 

 is that the structure should be capable of preserving its equilibra- 

 tion under all the various changes of figure and span which its 

 arcs must, in the movements of the animal, be continually under- 

 going! Who but " the Divine Architect of the Body" could have 

 framed and finished such a piece of workmanship ] 



It must be manifest that the mechanical powers of the spine, 

 regarded either as a fixed or moveable body, must be affected by 

 its length: and that there exist considerable variations in the 

 longitude of spines, must appear self-evident to any observer of 

 horses : not that there are any additional bones in a spine of un- 

 usual length ; but that the vertebra? are of larger size, and their 

 bodies and intervening cartilages thicker. Some horses have 

 very long backs and necks ; some very short ones. Hardly any 

 two have backs and necks of the same length; from all which 

 result certain advantages and disadvantages. We will first con- 

 sider the longitude of that part of the spine that forms the back, 

 and in two points of view : — 1st, as it affects the support of weight; 

 2dly, as it mechanically affects progression. 



Regarding the dorsal portion of the spine with its super-imposed 

 burthen as a pole or lever supported in front by the fore limbs 

 and behind by the back limbs, after the manner of a barrel of 

 beer or a sedan between its bearers, it is manifest that the greater 

 its length the greater must be the leverage, and consequent re- 

 duction of the weight of the burthen. On this principle, the legs of 



