374 THE MECHANICAL FUNCTIONS. 



arises from the continual separation of the 

 plates, which bend inwards, and crossing each 

 other, leave a multitude of irregular spaces or 

 cells, which are called cancelli. The plates, pro- 

 ceeding from each side obliquely inwards, at 

 length meet each other in the axis of the 

 cylinder, so as to close the middle cavity near 

 the extremities of the bone, where this spongy 

 or cancellated structure is found to occupy its 

 whole diameter. 



Now if we consider that the principal me- 

 chanical property required in every cylindrical 

 lever is rigidity, and more especially the power 

 of resisting forces applied transversely, that is, 

 tending to break the cylinder across, we shall 

 soon perceive, that a given quantity of ma- 

 terials could not possibly have been disposed in 

 a manner better calculated for such resistance 

 than when in the form of a tube, or hollow cy- 

 linder.* To this mechanical principle I have 

 already had occasion to advert, when speaking 

 of the hollow stems of vegetables, which derive 

 their chief strength from their possessing this 

 form ; f and we now find it again applied in the 

 structure of bones, which by having been made 



* An elaborate mathematical demonstration of this proposi- 

 tion was long ago given by Dr. Porterfield, in a paper contained 

 in the first volume of Medical Essays and Observations, pub- 

 lished by a Society in Edinburgh, p. 95. 



f P. 81. 



