60 



ANIMAL LOCOMOTION. 



(fig. 18, p. 37), the fish when swimming (fig. 32, p. 68), and 

 the bird when flying (figs. 73 and 81, pp. 144 and 157). 



The alternate rotation of the trunk upon the limb and the 

 limb upon the trunk is well seen in fig. 27, p. 59. 



At A of fig. 27 the trunk {g) is observed rotating on the 

 left foot (/). At D of fig. the left leg (Ji) is seen rotating on 

 the trunk {a, i) : these, as explained, are complementary move- 

 ments. At A of fig. the right foot (c) is firmly placed on the 

 ground, the left foot (/) being in the act of leaving it. The 

 right side of the trunk is on a lower level than the left, which 

 is being elevated, and in the act of rolling over the foot. At 

 B of fig. the right foot (m) is still upon the ground, but the 

 left foot having left it is in the act of swinging forward. At 

 C of fig. the heel of the right foot (n) is raised from the 

 ground, and the left foot is in the act of passing the right. 

 The right side of the trunk is now being elevated. At D of 

 fig. the heel of the right foot {o) is elevated as far as it can 

 be, the toes of the left foot being depressed and ready to 

 touch the ground. The right side of the trunk has now 

 reached its highest level, and is in the act of rolling over the 

 right foot. The left side of the trunk, on the contrary, is 

 subsiding, and the left leg is swinging before the right one, 

 preparatory to being deposited on the ground. 



From the foregoing it will be evident that the trunk and 

 limbs have pendulum movements which are natural and 

 peculiar to them, the extent of which depends upon the 

 length of the parts. A tall man and a short man can con- 

 sequently never walk in step if both walk naturally and 

 according to inclination.^ 



In traversing a given distance in a given time, a tall man 



^ The number of steps which a person can take in a given time in walking 

 depends, first, on the length of the l^g, which, governed by the laws of the" 

 pendulum, swings from behind forwards ; secondly, on the earlier or later in- 

 terruption which the leg experiences in its arc of oscillation by being placed 

 on the ground. The weight of the swinging leg and the velocity of the ti'unk 

 serve to give the imi^ulse by which the foot attains a position vertical to the 

 head of the thigh-bone ; but as the latter, according to the laws of the pendu- 

 lum, requires in the quickest walking a given time to attain that position, 

 or half its entire curve of oscillation, it follows that every person has a 

 certain measure for his steps, and a certain number of steps in a given 

 time, which, in his natural gait in walking, he cannot exceed." 



