LOCOMOTION. 659 
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. It follows that a walking person 
describes a series of vertical curves with the head, and of. hori- 
zontal ones with the body, the resulting total being complex. 
The peculiarities of the gait of different persons are natu- 
rally determined by their 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 
period of the cycle off the ground at the same time, owing to 
a very energetic action of the foot acting as a fulcrum. 
Jumping implies the propulsion of the body by the impulse / 
given by both feet at the same moment. 
Hopping is the same act accomplished by the use of one 
leg. 
Comparative—The movements of quadrupeds are naturally 
very complicated, but have now been well worked out by 
Fias. 496 and 497.Showing the more or less perpendicular direction of the stroke of the wing 
in the flight of the bird (gull): how the wing is gradually extended as it is elevated (e, f, 9); 
how it descends as a long lever until it assumes the position indicated by h :-how it is 
flexed toward the termination of the down-stroke, as shown at A, i, j, to convert it into a 
short lever (a, b) and prepare it for making the up-stroke. The difference in the length of 
the wing during flexion and extension is indicated by the short and long levers a, b and 
e,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 use of instantaneous photography. Even the bird’s flight 
is no longer a wholly unsolved problem, but is fairly well 
understood. The movements of centipeds and other many- 
legged invertebrates are highly complicated, while their rapid 
