446 MUSCULAR MOTION. 



menced by bending the thigh upon the pelvis, and, at the same time, 

 the leg upon the thigh. The extension of the thigh on the pelvis suc- 

 ceeds, and the whole limb is carried backward; the leg is afterwards 

 extended upon the thigh, the point of the foot is brought to the ground, 

 and the remainder of its under surface in succession. The other foot 

 is then raised on its point, by which the corresponding limb is elon- 

 gated; the pelvis, being pushed backwards, makes a rotation on the 

 limb which is behind, and is, by the action of appropriate muscles, car- 

 ried on a level with, or behind, the other, to afford a new pivot in its 

 turn. Walking laterally is different from the two last in no arcs being 

 described. In this case, one of the thighs is first slightly bent upon 

 the pelvis, in order to detach the foot from the ground; the whole limb 

 is then moved away by the action of the abductors, and is brought 

 down to the ground. The other limb follows. 



If we walk up hill, the fatigue is much augmented; because the 

 flexion of the limb, first carried forward, has to be more considerable; 

 and the limb, that remains behind, has not only to cause the pelvis to 

 execute the movement of rotation, but it has to raise the whole weight 

 of the body, in order to transport it upon the limb which is in advance. 

 To aid in throwing the weight forward, the body is bent forward, so 

 that the centre of gravity may be as favourably disposed as possible ; 

 and the extensor muscles of the leg carried forward are powerfully con- 

 tracted to raise the trunk; hence, the feeling of fatigue, which we expe- 

 rience in the knee and anterior part of the thigh, on ascending a long 

 flight of stairs. Fatigue is likewise felt in the calf of the leg, on ac- 

 count of the strong efforts developed in extending the foot, and pro- 

 jecting the body forward. Walking down hill is more fatiguing than 

 on level ground. In this case, there is a tendency in the body to fall 

 forward; great effort is, consequently, required to keep the vertical 

 line within the base of sustentation ; and, accordingly, the muscles, 

 employed in the extension of the head and vertebral column, experience 

 fatigue. 



In all these kinds of progression, the character of the soil is a mat- 

 ter of importance. It must be firm enough to afford support to the 

 limb that presses upon it, otherwise fatigue is experienced, and pro- 

 gression slow and laborious. This occurs, whenever the soil is too soft 

 or too smooth; the former yielding to the foot, and the latter present- 

 ing no inequalities to which the foot can attach itself. The soil, too, 

 has some influence, in particular cases, by virtue of its elasticity. Such, 

 at least, is the opinion of Borelli; 1 but Barthez 2 thinks, that the influ- 

 ence of the soil is limited to the degree in which it furnishes a firm 

 support. If the soil, again, be movable, as the deck of a vessel, the 

 line of gravity is apt to fall outside the base of sustentation; and to 

 avoid this, the base is enlarged by separating the legs so as to give a 

 characteristic air to the gait of the mariner ; and, lastly, if the base 

 be very narrow, as on the tight-rope, the steps are obliged to be rapid, 

 and the arms are aided in modifying the centre of gravity as may be 

 required, by the use of a long and heavy pole. 



1 De Motu Animalium, &c., Lugd. Bat, 1710. 



a Nouveaux Elemens de la Science de 1'Homme, Paris, 1806. 



