<^0 MODES OF PROGRESSION. 



backwards, and the fore-arm forwards. Different terms have 

 been employed to express the various modes of progression, 

 accoiJ!ng 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 pacea 

 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 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 defined to be a continual 

 falling forwards, continually interrupted by the projection of 

 the legs. 



186. The throwing forwards of the leg, which would 

 require -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 suffi- 

 cient of itself to maintain the limbs in place, without the as- 

 sistance of the muscles. This may be proved by exi)eri- 

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

 thigh bone still adheres firmly to the pelvis, but separates 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 

 become simultaneous. The walking of quadrupeds is a 

 similar process, but with this difference, that the body alwa3'S 

 rests on at least two legs. 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 



