62 THE HUMAN BODY. 



necessary, and consequently fatigue is sooner induced. When 

 resting on one foot only the body departs laterally from the 

 vertical and leans a little backward, the leg which supports 

 only its own weight rests on the ground, with the muscles 

 completely relaxed, acting as a support and counterpoise. 

 This attitude when standing is the least fatiguing, firmest, 

 and also the most elegant; it is the one preferred by painters 

 and sculptors, and was considered by Leonardo de Vinci as 

 the most natural. 



When the body moves it is divided into two quite distinct 

 sections: the one, comprising the head, trunk, and upper 

 limbs, representing the mass to be transported; the lower 

 limbs are at once the movable supports of the superior parts 

 of the body, and the agents of propulsion which communi- 

 cate to them the movement of translation. In all move- 

 ments of this nature the trunk inclines forward at an angle 

 which varies according to the quickness, from 5 7' in the 

 slowest walk to 22 5' in the fastest running. From this 

 position there is a constant tendency to fall forward, which 

 is neutralized by the moving of the lower limbs in such a 

 direction as that the heads of the femurs shall always serve 

 as the point of support for the body. M. Longet compares 

 this unstable equilibrium of the body upon the femurs to 

 that of a rod on the end of a finger, so inclined that the 

 only means to prevent its fall is to carry the finger forward 

 in the same direction as the inclination, more rapidly as this 

 inclination becomes greater. 



By their alternate flexion and extension the lower ex- 

 tremities give an impulsion to the trunk, they lengthen and 

 contract in a direction inclined to the horizon, since it is 

 forward and not vertically that they push the body; the re- 

 sult is that the centre of gravity sinks toward the ground just 

 in proportion to the rapidity of the mode of progression. 



Each extremity props itself by turns on the ground, and 

 then the impulse being given the knee bends, the heel rises, 

 the foot is lifted, the limb, shortened by flexion and sus- 

 pended from the pelvis, is directed from behind forward, 

 and is again placed upon the ground. 



In this movement the leg, according to the brothers 



