PHYSICAL PROPERTIES OF BONE. 



85 



Fig. 



IT3- 



Physical Properties. — Rauber has shown that a five-millimetre cube of com- 

 pact bone of an ox when calcined will resist pressure up to 298 pounds ; when decal- 

 cified up to 136 pounds ; under normal conditions up to 852 pounds, the pressure 

 being applied in the line of the lamellae. 



It results from its composition that while bone is very hard and resistant to press- 

 ure, it is also somewhat flexible, elastic, and capable of withstanding a tearing strain. 

 It is remarkable that in many substances the power to resist a crushing strain is very 

 different from that of resisting a tearing one. Thus, cast iron is more than five times 

 as resistant to the former strain as to the latter, and wrought iron is nearly twice as 

 resistant to the latter as to the former. Neither of these materials, therefore, is well 

 fitted to resist both strains, since a much greater quantity must be used than would 

 be needed were either material to be exposed only to the strain it is best able to with- 

 stand. Bone, however, has the property 

 of resisting both strains with approximately 

 equal facility, its tearing limit being to its 

 crushing limit about as 3 is to 4. This has 

 the advantage that strength need not be 

 obtained by great increase of weight, con- 

 sequently the plan of bone structure com- 

 bines lightness and strength. 



Structure of Bone. — On sawing 

 through a bone from which the marrow and 

 other soft parts have been removed by ma- 

 ceration and boiling, the osseous tissue is 

 seen (Fig. 113) to be arranged as a pe- 

 ripheral zone of compact bone enclosing a 

 variable amount of spongy or cancellated 

 bone. In the typical long bones, as the 

 humerus or femur, the compact tissue al- 

 most exclusively forms the tubular shaft 

 enclosing the large marrow-cavity, the can- 

 cellated tissue occupying the expanded 

 extremities, where, with the exception of 

 a narrow superficial stratum of compact 

 bone, it constitutes the entire framework ; 

 the clefts between the lamellae of the spongy 

 bone are direct extensions of the general 

 medullary cavity and are filled with mar- 

 row-tissue. In the flat bones (Fig. 116), 

 as those of the skull, the compact substance 

 consists of an outer and inner plate, or 

 tables, enclosing between them the cancel- 

 lated tissue, or diplo'e, as this spongy bone 

 is often termed. Short and irregular bones 

 are made up of an inner mass of spongy 



bone covered by an external shell of compact substance which often presents local 

 thickenings in order to insure additional strength where most needed. 



The cancellated bone consists of delicate bars and lamellae which unite to 

 form an intricate reticulum of osseous tissue well calculated to insure considerable 

 strength without undue weight ; in many positions, conspicuously in the neck of the 

 femur (Fig. 374), the more robust lamellae are disposed in a definite manner with 

 a view of meeting the greatest strains of pressure and of tension. 



Although composed of the same structural elements, compact and spongy bone 

 differ in their histological details in consequence of the secondary modifications 

 which take place during the conversion of the spongy bone, the original form, into 

 the compact substance. To obtain the classic picture of osseous tissue, in order to 

 study its general arrangement in the most typical form, it is desirable to examine 

 thin ground sections of the compact substance cut at right angles to the axis of a 

 long bone which has been macerated and dried, and in which the spaces contain air. 



Section of upper end of humerus, showing the 

 external layer of compact bone surrounding the med- 

 ullary cavity below and the spongy bone above. 



