66 MODES OF PROGRESSION. 



terms have been employed to express the various modes of 

 progression, according to the rapidity or the succession in 

 which the limbs are advanced. 



185. 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 accomplished by two limbs only, as in man, the body 

 is inclined forwards, and carries the centre of gravity in 

 that direction, and while one leg sustains the body, the 

 other is thrown forwards to prevent it from falling, and to 

 sustain it in turn. For this reason, walking has been de- 

 fined to be a continual falling forwards, continually inter- 

 rupted by the projection of the legs. 



186. 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, so that the pressure of the atmosphere outside is 

 sufficient of itself to maintain them in place, without the 

 assistance of the muscles. This may be proved by experi- 

 ment. If we cut away all the muscles around the hip-joint, 

 the thigh-bone still adheres firmly to the pelvis, but sepa- 

 rates the moment a hole is pierced, so as to admit air into 

 the socket. 



187. In ordinary walking, the advancing leg touches the 

 ground just 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 be- 

 come simultaneous. The walking of quadrupeds is a simi- 

 lar process, but with this difference, that the body always 

 rests on two legs at least. The limbs are raised in a deter- 

 minate order, usually in such a manner that the hind-leg of 

 one side succeeds the fore-leg of the opposite side. Some 



