248 SKELETON. 



position, also, by advancing the bony points, from which arise 

 several of the muscles used in the extension of the thigh, removes 

 these muscles more from the line of their contraction, and there- 

 by enables them to act more advantageously and promptly. As 

 each pace on these occasions is taken to the fullest stretch, the 

 pelvis is rotated forwards from side to side, alternately upon the 

 head of the os femoris, which may be fixed at the time. The 

 face being directed forwards, whatever rotation in the vertebrae 

 can occur, is then performed. As the pelvis communicates its 

 motions to the trunk, so the latter carries its own to the upper 

 extremities; which are thereby slung, alternately, backwards 

 and forwards, and are brought, continually, to adjust the centre 

 of gravity, which is then more in danger of being lost than in 

 ordinary walking. 



The ascent of an inclined plane, either by walking or running, 

 is attended with unusual fatigue and difficulty, for the following 

 reasons: In order to advance the thigh, it is necessary to give it 

 great flexion at the hip joint, the knee must also be bent in an 

 equal degree, and the foot be flexed, in order to adjust it to the 

 surface against which it reposes. To bring forward the other 

 extremity, it requires an equal flexion at the hip and knee; be- 

 sides which, its heel being below the phalanges, the foot must 

 perform a full rotation at the ankle joint. The difficulty is 

 somewhat diminished by stepping only on the phalanges. As, in 

 these cases, the trunk of the body, to preserve its equilibrium, 

 must be inclined forwards, there are certain acclivities, which, 

 though they furnish a base sufficiently large for the foot, are 

 yet impracticable from not allowing the trunk to be thrown for- 

 wards. 



The descent of an inclined plane is more easy, because it re- 

 quires but little flexion, in the articulations mentioned, to bring 

 the extremity behind on a line with that in front; and its subse- 

 quent descent is produced by keeping it almost straight, and 

 shortening the extremity which is fixed. Running is then at- 

 tended with some inconveniences, for the impulsion, forwards, 

 which this motion communicates to the trunk, assisted by the in- 

 clination of the plane in that direction, determines a fall, inevita- 

 bly, without a successively accelerated advance of the hind leg. 

 We see frequently, in the descent of a very inclined hill, a step, 

 at first guarded and leisurely taken, converted, unavoidably, 



