ELASTICITY 169 



Let P be the pull, a the cross-sectional area, I the length of the 

 fibre, and e the elongation produced ; then : 



stress P/a PI PI 



^=Y= - L JT= or 6=-=^. 

 strain e/l ea Ya 



Wertheim gives the moduli of the following substances in 

 grams weight per sq. cm. 



TABLE XXV. 



Bone - - 2304-1 x 10 6 



Tendon 16341 x 10 6 



Nerve 18-89 x 10 6 



Muscle (resting) 0-95 x 10 6 



Vein 0-87 x 10 6 



Artery - 0-052 xlO 6 



The above values are given in order of increasing " perfection " 

 and decreasing " strength " of elasticity. The figure last given, 

 that of arterial walls, may be taken as substantially that of 

 elastic fibrous tissue. 



Now strains may be applied in two ways. They may press 

 or they may pull. A pressing or crushing force is called a thrust 

 and a pulling or tearing force is called a stretch. 



The stress set up in- the tissues is a function of the colloidal matter 

 which we have seen gives rigidity to the tissues. Water and 

 aqueous solutions of crystalloids are practically incompressible, 

 i.e. their volume elasticity is negligible. It is obvious that they 

 cannot have any elasticity of shape. The dispersion of less than 

 a gram of gelatine in a hundred cubic centimetres of water endows 

 the solution with rigidity. 



The emulsoid gel has elasticity of shape, i.e. it returns to its 

 original shape after the application of a strain by thrusting, 

 stretching or bending provided it has not been strained beyond 

 the elastic limit. 



A moment's thought will convince one that a quite different 

 structure is required to meet strains of the stretching and of 

 the thrusting varieties, (a) The material used in the building 

 of struts to withstand thrust must have a high crushing limit, 

 while that going to form ties requires high resistance to tearing. 

 In the following table drawn up by Sir Donald MacAlister, are 

 given the approximate values of the crushing and tearing limits 

 of some building materials. 



