154 THE MODES OF PROGRESSION. 



Moreover, tlie muscles of the toes are so disposed that the 

 weight of the bird causes them to contract firmly, hence birds 

 are enabled to sleep standing, in perfect security, upon their 

 perch, and without effort. 



292. In quadrupeds, the joints at the junction of the limbs 

 with the body bend freely in one direction only, that is, to- 

 wards the centre of gravity ; so that if one limb yields, the 

 tendency to fall is counteracted by the resistance of the limbs 

 at the other extremity of the body. The same antagonism is 

 observed in the joints of the separate limbs, which are flexed 

 alternately in opposite directions. Thus the thigh bends 

 forwards, and the leg backwards ; while the arm bends back- 

 wards, and the fore-arm forwards. Different terms have been 

 employed to express the various modes of progression, accord- 

 ing to the rapidity or the succession in which the limbs are 

 advanced. 



293. PROGRESSION is a forward movement of the body, 

 effected by successively bending and extending the limbs. 

 WALKING is the ordinary and natural gait, and other paces 

 are only occasionally employed. When walking is accom- 

 plished by two limbs only, as in man, the body is inclined 

 forwards, carrying the centre of gravity in that direction, and 

 whilst one leg sustains the body, the other is thrown for- 

 wards to prevent it from falling, and to sustain it in turn. 

 For this reason, walking has been defined to be a continual 

 falling forwards, interrupted by the projection of the leg. 



294. The throwing forwards of the leg, which would re- 

 quire a very considerable effort were the muscles obliged to 

 sustain the weight of the limbs also, is facilitated by a very 

 peculiar arrangement ; that is, the joints are perfectly closed 

 up, so that the external pressure of the atmosphere is sufficient 

 of itself to maintain the limbs in place, without the assistance 

 of the muscles. This may be proved by experiment. If we cut 

 away all the muscles around the hip-joint, the thigh-bone still 

 adheres firmly to the pelvis, but the moment a hole is pierced, 

 so as to admit air into the socket, it separates. 



295. In ordinary walking, the advancing leg touches the 

 ground before the other is raised ; so that there is a moment 

 when the body rests on both limbs. It is only when the 

 speed is very much accelerated, that the two actions become 

 simultaneous. The walking of quadrupeds is a similar process, 



