262 



THE MECHANICAL FUNCTIONS. 



greater thickness in the middle of the shank or shaft of the 

 column, and hecome thinner as we follow them towards 



either of the ends. This gradual diminu- 

 tion in the thickness of the walls 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, 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 extremi- 

 ties of the bone, where this spongy or can- 

 cellated 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 cal- 

 culated for such resistance than v/hen in the form of a tube, 

 or hollow cylinder.* 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;! and we now find it again ap- 

 plied 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. Porterfiekl, in a paper contained in the first volume of 

 Medical Essays and Observations, publislied by a Society in Edinburgh, 

 p. 95. 



t P. 70. 



