334 THE MECHANICAL FUNCTIONS. 



from the continual separation of the plates, which 

 bend inwards, and crossing each other, leave a 

 multitude of irregular spaces or cells, which are 

 termed cancelli. The plates, proceeding 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 mechanical 

 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 materials could not possibly have 

 been disposed in a manner better calculated for 

 such resistance than when in the form of a tube, 

 or hollow cylinder.* To this mechanical principle 

 I have already had occasion to advert, when speak- 

 ing of the hollow stems of vegetables, which derive 

 their chief strength from their possessing this 

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

 structure of bones, which by having been made 

 hollow, are rendered considerably stronger than if 

 the same materials had been collected into a solid 

 cylinder of the same length. We may farther 

 remark, that as it is in the middle of the shaft that 

 the strain is greatest, so it is here that the cavity 

 is largest, and the resistance most effectual. 



* An elaborate mathematical demonstration of this proposition 

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

 first volume of Medical Essays and Observations, published by a 

 Society in Edinburgh, p. 95. 



t Pages 70 and 71. 



