44 PHYSIOLOGY. 



a. a. the extremities having a shell or thin plate of compact 

 texture, crowded with small cells, diminishing in size, but in- 

 creasing in number as they approach the articulation ; c, the 

 cavity for containing the marrow ; 6. b. the walls of the shaft 

 very firm and solid. The compact part is thickest near the 

 middle of the bone where the greatest strength is required. 

 This structure admirably combines the greatest degree of 

 strength, with the least degree of weight aud expense of ma- 

 terial. It tfan be mathematically demonstrated that the re- 

 sistance of a cylindrical body, such as a pillar or mast, to a 

 force applied transversely is increased in proportion to its 

 diameter. The same quantity of matter, therefore, placed in 

 the circumference of a circle produces a stronger bone than 

 if united in the centre with proportionally diminished diame- 

 ter. 



14. How admirably adapted is the arrangement of the 

 parts of the long bones to the purposes for which they are 

 destined. Their extremities are the fixed points from which 

 the muscles re-act, and where greater space was required for 

 the insertion of the tendons ; their diameter is on this ac- 

 count considerably increased, and their osseous matter*is 

 disposed in nearly an equal degree through their whole sub- 

 stance ; while in the middle of the bone, which is more 

 exposed to external violence^ and where nothing was want- 

 ing but mere strength, the bony plates are all consolidated 

 together into a compact dense ring, leaving the centre 

 nearly hollow. 



15. While the long bones of the lower extremities are 

 adapted for the support of the body, and its various progres- 

 sive motions, those of the upper extremities are equally 

 fitted for acting upon contiguous bodies, being so attached to 

 the trunk as to be easily applied to them in all directions. 



{The flat bones on the other hand offer an extensive sur- 

 face of defence, as those of the skull, or for the origin and 

 insertion of muscles, as the shoulder blade.^ The rounded 

 bones composing the spinal column and the angular bones of 



