LOCOMOTION. 613 



various directions in which, an individual, when erect, may fall 

 according as one or the other line (center) of gravity is dis- 

 placed for a long enough time. 



Walking (man) implies the alternate movement of each leg 

 forward, pendulum-lite, so that for a moment the entire body 

 must be supported on one foot. When the right foot is lifted 

 or swung forward, the left must support the weight of the 

 body. It becomes oblique, the heel being raised, the toe still 

 resting on the ground ; and it is upon this as a fulcrum that 

 the body-weight is moved forward, when a similar action is 

 taken up by the opposite leg. 



It follows that to prevent a fall there must be a leaning of 

 the body to one side, so that the line of gravity may pass through 

 each stationary foot; hence a person walking describes a series 

 of vertical curves with the head and of horizontal ones with the 

 body, the resulting total being complex. 



Fios. 454 and 455. — Showing the more or less perpendicular direction of the stroke of 

 the wing in the flight or the bird (gull); how the wing is gradually extended as it 

 is elevated (e,/, q)\ how it descends as a long lever until it assumes the position 

 indicated by A; how it is flexed toward the termination of the down-stroke, as 

 shown at h, i.j, to convert it into a short lever (o, 6) and prepare it for making the 

 up-stroke. The difference in the length of the wing dnnng flexion and' extension 

 is indicated by the short and iong levers a, b and c, d. The sudden conversion of 

 the wing from a long into a short lever at the end of the down-stroke is of great 

 importance, as it robs the wing of its momentum and prepares it for reversing its 

 movements (Pettigrew). 



The pecuUarities of the gait of different persons are naturally 

 determined by thfeir height, length of leg, and a variety of other 

 factors, which are often inherited with great exactness. We 

 instinctively adopt that gait which economizes energy, both 

 physical and mental. 



Running differs from walking, in that both feet are for a 



