UPPER AND LOWER LIMBS. 



55 



protects its mechanism, but also avoids the grave injuries 

 which might be produced in certain organs, such as the brain 

 and the liver, by the rebound. 



Fig. 19. The foot. 



If we compare the upper and the lower limbs taken as a 

 whole, we shall remark among their principal distinguishing 

 characteristics, that the flexion of the fore-arm upon the arm 

 takes place in a forward direction, while that of the leg is 

 backward. M. Martins has demonstrated that this opposi- 

 tion in their movements, necessitated by their destination to 

 different functions, is due to the twisting of the humerus. 

 The femur is a straight bone and not twisted upon its axis. 

 The humerus, on the contrary, is turned on itself 180, or 

 half the circumference. This is not the result of the me- 

 chanical action of the muscles and of the function of the 

 upper limb ; although the form of the bone does not permit 

 this peculiarity to be anatomically recognized till about the 

 second year, it exists virtually as soon as the bone is 

 developed. Muscles, vessels, and nerves all follow this 

 rotating movement indicated by the spiral disposition of the 

 humerus, and most anatomists have pointed it out without 

 hinting at the physiological consequences which M. Martins 

 has shown result from it. This learned observer has shown 

 that the humerus, artificially untwisted, corresponds in all 

 points with the femur of the same side, and by this manoeuvre 



