92 THE HUMAN MOTOR 



The resistance to fracture by compression is higher than in 

 the case of traction ; it is R = 14 kilogrammes on an average. 

 The values change according to the section and length of the 

 part. 



Hodgkinson's formula for wood ( 44) is applicable here by 

 writing : 



R = 2,700 x-g- 



But h must not exceed ^ decimetre, and the prisms must 

 approach a cube, cubic forms being very resistant. Under 

 compression : 



For a cube of 3 millimetres ... R = 16 kg. per mm z 

 5 R = 15 



On prisms only 10 to 12 kilogrammes are obtained, f 1 ). 



The rate of application of the compressive stress is of great 

 importance, because the pressure in shock attains a high value. 



It will be remembered that the impulse is : 

 Ft = mv ; whence F = , 



for a very short duration, ^F may reach a very high value, 

 several hundreds of kilogrammes. 



68. In " flexion " and " torsion/' the resistance of the long 

 bones is increased because they are hollow ( 45). Lesshaft ( 2 ) 

 and Otto Messerer ( 3 ) determined the resistance of bones of normal 

 dimensions. The weight needed to produce initial fracture was : 



To produce a complete fracture needs 2,900 kilr grammes for 

 the femur, and at least 4,100 for the tibia. 



(*) H. Triepel, Die Stossfestigkeit des Knochen (Arch.f. Anat., 1900, p. 229) 

 and Einfiihrung in die Physikalische Anal., Wiesbaden, 1902. 



( 2 ) P. Lesshaft : we quote him according to Triepel: 



(') O. Messerer, Ueber Elasticitat und Fe*tigkeit der Menschlichen Knochen, 

 Stuttgart, '1880. 



