MECHANICS OF BONE. 



105 



bones. There may be one or several in each end. The part formed around each 

 of these secondary centres is called an epiphysis. Growth takes place chiefly in the 

 cartilage between the epiphyses and the shaft. When, therefore, a joint is resected 

 in childhood the surgeon tries to leave a part of the epiphysis in place. A curious 

 relation exists between the course of the chief medullary artery of the shaft of a long 

 bone and the behavior of the epiphyses. The epiphysis towards which the vessel is di- 

 rected is the last to appear and the first to unite. (The fibula furnishes an exception. ) 

 As a rule, also, the largest epiphyses appear first and unite last. In long bones with 

 an epiphysis at one end only, the nutrient canal leads towards the opposite extremity. 



Mechanics of Bone. — A long bone has a hollow shaft containing ^narrow, 

 the wall being of compact bone. The hollowness of the shaft takes from the weight, 

 and, moreover, conforms to the well-known law that a given quantity of matter is 

 much stronger, both lengthwise and crosswise, when disposed as a hollow cylinder 

 than as a solid one of equal length. The proportion of the central or medullary 

 cavity is not the same in all bones. Perhaps, as an average, its diameter may be 

 said to equal one-third of that of the bone. In the shaft this cavity is crossed by a 

 few bony trabeculae, almost all of which are destroyed in maceration. Towards the 

 ends, as the outer wall becomes thinner, large numbers of thin plates spring from its 

 inner surface and incline towards one another in graceful curves, until at last the 

 expanded end of the bone consists of spongy or cancellated tissue enclosed within a 

 delicate wall of compact substance. The arrangement of these plates is distinctly pur- 

 poseful, since it has been shown that they are so disposed as to correspond with the 

 stress-lines an engineer would construct for the special purpose served by the end of 

 the bone. None the less, it would be unwarranted to maintain that mathematical 

 correctness is always to be found, or that there are not other modifying influences. 

 The internal structure of all bones, excepting, perhaps, those of the skull, is of this 

 nature, so that the following remarks apply to spongy bone in general. 



The delicate cancellated structure is for the most part in thin plates. The sim- 

 plest arrangement occurs in a short bone exposed to pressure only at two opposite 

 surfaces ; in such cases the plates run between these surfaces with few and insignifi- 

 cant cross-pieces. Where severe pressure may come in almost any direction, as in 

 the case of the globular heads of the humerus and femur, the round-meshed pattern 

 predominates, producing a very dense spongy structure which may be represented 

 diagrammatically by drawing lines crossing at right angles and by enlarging every 

 point of intersection. In the midst of this round-meshed type there is very fre- 

 quently a central core with stronger plates and larger spaces. The vaulted system 

 is found at the projecting ends of bones, and between the round-meshed cancellated 

 substance and the shaft. Several special arrangements will be described in connec- 

 tion with the bones in which they occur. An epiphysis, until it has fused, shows the 

 mechanical structure of a separate bone. K process for the attachment of muscles 

 or ligaments generally contains a very light internal structure, the surface of the shaft 

 of the bone being rarely continued under it. The continuation of the fibres of 

 attached tendons is not represented by internal plates of bone, although the oppo- 

 site opinion has supporters. 



Certain of the bones of the cranium and the face are in parts hollowed out into 

 mere shells bounding a cavity lined with mucous membrane continuous with that of 

 the nose or the pharynx. 



The elasticity of bones is enhanced by curves. The long bones very usually 

 present a double curve. It has been maintained that these curves form a spiral 

 structure. There are striking instances of it, but the universality of the law is not 

 proved ; although shocks are thus lessened, the passage of one curve to another is 

 a weak point in the bone. 



The ends of the long bones are enlarged for articulation with their neighbors. 

 The greater part of this enlargement forms the joint, the various shapes of which 

 will be discussed later. Besides this, there are usually at the ends prominences foi 

 muscles. The shaft generally bears ridges, which in some cases are made of dense 

 bone and materially add to the strength of the bone. A ridge or prominence 

 usually implies the insertion of a fibrous aponeurosis or a tendon. Muscular fibres, 

 however, may spring from the periosteum over a flat surface. 



