42 



ANIMAL MECHANICS 



ening of the bone in the direction on which the 

 weight bears. If we resume the experiment with 

 the piece of timber, we shall learn why the spine 

 is harder than the rest of the bone. If a portion 

 of the upper part of the timber be cut away, and 

 a harder wood inserted in its place, the beam 



will acquire a new 

 power of resisting 

 fracture, because, as 

 we have stated, this 

 part of the wood 

 does not yield but by 

 being crushed, and 



the insertion of the harder portion of wood in- 

 creases this property of resistance. With this fact 

 before us we may return to the examination of 

 the spine of bone. We see that it is calculated to 

 resist pressure, first, because it is farther removed 

 from the centre of the cylinder ; and, secondly, 

 because it is denser, to resist compression, than 

 the other part of the circumference of the bone. 1 

 This explanation of the use of a spine upon a 

 bone gives a new interest to osteology. 2 The 

 anatomist ought to deduce from the form of the 

 spine the motions of the limb ; the forces bearing 



1 As the line A B extends farther from the centre than B C, 

 on the principle of a lever, the resistance to transverse fracture 

 will be greater in the direction A B than B C. 



2 Osteology, from the Greek words, signifying discourse on 

 bone, being the demonstration of the forms and connection of the 

 different bones. 



