PHYSICAL PEOPEBTIES OF TISSUES. 



65 



arrangement of the compact substance of the bone is especially fitted to 

 overcome these direct or indirect pressures (Fig. 43). 



2. Elasticity. — The elasticity of the tissues varies in the same way 

 as their cohesion. The ' moist tissues have, as a rule, a very slight elas- 

 ticity ; that is, they otter slight resistance to external forces which tend 

 to change their form, and in most of the tissues which are rich in water, 

 as the brain and glands, the elasticity is incomplete ; that is, the original 

 form is not regained after the distorting force ceases to act. On the 

 other hand, in the elastic tissues and muscles the force must be excessive 

 to produce permanent distortion. 



The cohesion of a body is its resistance to tearing forces ; elasticity is 

 developed as resistance to alteration in form, and refers to the property by 

 which the original form is regained. The elasticity of a bod}' is therefore 

 great when a great force is required 

 to produce change in form, and 

 vice versa; while the completeness 

 of the elasticity is expressed by the 

 perfection with which the original 

 form is regained after the distorting- 

 force ceases to act. Thus, the elas- 

 ticity of lead is great but incom- 

 plete ; of rubber, is small but 

 perfect. 



Elasticity cannot be measured 

 by stretching force alone, but com- 

 pressing, twisting, and bending 

 forces must also be considered. 

 The resistance to each of these 

 forces is the same. The less exten- 

 sible a boc^y is, the less compressible 

 is it also, and the more rapidly it 

 vibrates when bent from its position of equilibrium. 



Organic tissues which are poor in water, as wood and bone, and 

 which possess high elasticity, behave to stretching weights like inorganic 

 bodies, i.e., the increase in length is proportionate to the weight. In 

 the soft tissues, of less but more complete elasticity, the increase in 

 length produced by heavy weights is proportionately less than that due 

 to smaller weights. The cause of this lies in the greater extensibility of 

 such tissues, through which they are more stretched by small weights 

 than is possible in rigid bodies, because in the latter a much smaller ex- 

 tension would exceed the limit of cohesion ; though the use of weights 

 of very great diiference shows that the extensibility of rigid bodies is 

 probably also governed by the same laws. 



5 



Fig. 43.— Diagram of the Structure 

 of the Neck of the Human Femur. 

 (Ward.) 



The fibres, A. by their rigidity, and the blues, B, by 

 their tenacity, tend to the support of the weight, as 

 illustrated iri the bracket, while the latter fibres inter- 

 lace with the areiform fibres, F. 



