56 



ANIMAL LOCOMOTION". 



The greatest angle made at the knee-joint is equal to the 

 sums of the angles made by the hip and ankle joints when 

 these joints are simultaneously flexed, and when the angle of 

 inclination made by the foot with the ground equals 30°. 



From this it follows that the trunk maintains its erect 

 position during the extension and flexion of the limbs. The 

 step in walking was divided by Borelli into two periods, the 

 one corresponding to the time when both limbs are on the 

 ground ; the other when only one limb is on the ground. In 

 running, there is a brief period when both limbs are off the 

 ground. In walking, the body is alternately supported by 

 the right and left legs, and advanced by a sinuous movement. 

 Its forward motion is quickened when one leg is on the 

 ground, and slowed when both are on the ground. When 

 the limb (say the right leg) is flexed, elevated, and thrown 

 forward, it returns if left to itself (i.e. if its movements are 

 not interfered with by the voluntary muscles) to the position 

 from which it was moved, viz. the vertical, unless the trunk 

 bearing the limb is inclined in a forward direction at the 

 same time. The limb returns to the vertical position, or 

 position of rest, in virtue of the power exercised by gravity, 

 and from its being hinged at the hip by a ball-and-socket 

 joint, as explained. In this respect the huma,n limb when 

 allowed to oscillate exactly resembles a pendulum, — a fact first 

 ascertained by the brothers Weber. The advantage accruing 

 from this arrangement, as far as muscular energy is concerned, 

 is very great, the muscles doing comparatively little work.^ 

 In beginning to walk, the body and limb which is to take 

 the first step are advanced together. When, however, tlie 

 body is inclined forwards, a large proportion of the step is 

 performed mechanically by the tendency which the pendulum 

 formed by the leg has to swing forward and regain a vertical 

 position, — an eff'ect produced by the operation of gravity alone. 

 The leg which is advanced swings further forward than is 

 required for the step, and requires to swing back a little 

 before it can be deposited on the ground. The pendulum 



1 The brothers Weber found that so long as the muscles exert the general 

 force necessary to execute locomotion, the velocity dej)ends on the size of the 

 legs and on external forces, but not on the strength of the muscles. 



