302 MUSCULAR CONTRACTION. LiECT. XVI. 



where these movements take place ; and which furnish fixed 

 points, or points of support. Th%theories concerning the 

 composition of forces, the centre of gravity, levers, and the 

 resistance of media, necessarily apply, therefore, as well to 

 animal machines, as to any machine employed in the arts. 

 Oscillation of the Lower Extremities in Walking. We 

 are indebted to the brothers, Weber, for an important dis- 

 covery in the theory of walking and running in man, and 

 of which 1 must not leave you ignorant. It consists in 

 having demonstrated, by experiments, that the lower limbs, 

 when put in motion, oscillate in a pendulum-like manner 

 around the trunk, by the action of gravity. Limbs of dif- 

 ferent lengths, both of living men and of bodies, were made 

 to oscillate; and in every case it \vas found that the dura- 

 tions of these oscillations were proportional to the square 

 roots of the lengths of the oscillating limbs. These move- 

 ments, then, are effected independently of our will; a fact 

 which explains the perfect regularity with which the steps 

 succeed each other in a child as well as in an adult, in the 

 idiot as well as in him whose will and sensibility have re- 

 ceived full development. The action of the muscles, there- 

 fore, is little or nothing in the execution of these movements. 

 The leg raised and then left to itself, accomplishes the step 

 by the sole influence of gravity. The head of the thigh- 

 bone suffers a very slight friction only in rotating in the 

 cotyloid cavity, where it is retained by atmospheric pressure, 

 which thus assists in accomplishing these movements. The 

 whole weight of the limb does not press against the sides 

 of the hip-joint: the head of the thigh-bone remains fixed 

 in this cavity by atmospheric pressure, and hence the effect 

 of gravity upon the member is destroyed. For this fact 

 we are indebted to the researches of the brothers, Weber, 

 who have demonstrated this mode of action of atmospheric 

 .pressure. The tendons and the muscles which connect the 



