OF THE ATTITUDES AND OF LOCOMOTION*. 141 



stand on one foot nor even on the two hinder limbs ; it is 

 impossible for them to maintain, excepting for a few seconds, 

 such a position; the base of sustentation is too narrow; their 

 centre of gravity is towards the chest, and the muscles are 

 not strong enough to maintain the attitude. But in man this 

 position is easy, natural, and belongs to him by the character 

 of his organization. 



286. In the vertical position it is chiefly the extensor 

 muscles which are called into play ; the limbs forming broken 

 levers, become fixed ; the knee and ankle become immovable. 



287. Sitting is less fatiguing than standing ; but the 

 horizontal position is that alone which gives absolute rest to 

 the body. 



288. Walking. When we walk, one of our feet is 

 carried forwards, whilst the other is extended on the limb ; 

 and as this forcibly extended foot rests or presses against a 

 resisting soil, its elongation displaces the pelvis, and thus 

 projects the whole body forwards ; at the same instant, the 

 pelvis turns a little on the femur of the opposite side, and the 

 leg which at first remained behind, bends, is carried forward, 

 and in its turn is placed on the soil, to carry forward the 

 whole body by the extension of the foot. By the aid of these 

 alternate movements of flexion and extension, each limb carries 

 in its turn the weight of the body, and at each step the centre 

 of gravity is pushed forwards. For an instant the body is 

 carried alternately on one foot, and the centre of gravity is 

 carried flexuously from side to side at each step, and this in 

 proportion to the width of the pelvis. 



289. As the functions of every apparatus are always in 

 relation with the structures, so the limbs of various animals 

 show great variety in their disposition. Thus, amongst 

 mammals, some are destined to move in water or on land, 

 that is, to swim or walk, as suits them, or to swim only; 

 others have the limbs formed for speed ; others, as bats, fly 

 like birds ; some use their fore limbs only as instruments of 

 prehension or touch ; and yet the limbs in all these animals 

 are formed precisely on one plan. In the swimming limbs 

 of the seal, the wing of the bat, the fore limbs of the squirrel 

 or mole, we find the same number and arrangement of the 

 bones (Fig. 81) as in the human arm. 



290. In mammals organized for speed, the limbs are 

 slender and the feet small ; we see this in the horse, stag, and 

 camel, in which the toes are but little divided (Fig. 82), and 



