102 CONTRACTILE ELEMENTS. 



head, that is to say, above and a little in front of the centre 

 of motion (in R). The power is represented by the muscles 

 of the nape of the neck, inserted in the lower half of the 

 occipital (in P). By uniting these different points we obtain 

 a bent lever of the first order, which can be easily transformed 

 into a direct lever. We see the same thing exemplified in 

 the preservation of the equilibrium of the trunk upon the 

 heads of the two femoral bones : the coxo-femoral joints 

 form the leaning-point of a lever of the first order, in which 

 the resistance (centre of gravity of the trunk) is placed be- 

 hind, and the power (anterior muscles of the thigh) is placed 

 before. A similar lever is found in the articulation of the 

 thigh with the leg, and of the leg with the foot (in the move- 

 ments of equilibrium to maintain the erect position). 



The two other kinds of levers are to be principally found, 

 not in the equilibrium of standing, but in the movements of 

 locomotion. 



The lever of the second kind, or inter-resisting lever, in 

 which the arm of the power is longer than the arm of the 

 resistance, and in which, consequently, speed is sacrificed to 

 strength, is found in man only under two circumstances, 

 when the upper part of the trunk is raised by pushing with 

 the palms upon a resisting plane, and when the whole weight 

 of the body is raised by standing upon the points of the toes, 

 which happens at each step, in the movement of walking, 



Fig. 29. Example of a lever of the second class, as shown in Fig. 28. 

 (Dalton.) 



fit the period when the foot, leaving the ground, oscil- 

 lates in the air, and places itself before the other. In this 

 case (Fig. 28) the point d'appui is upon the axis of the 

 transverse cylinder, formed by the junction of the series 

 of metacarpal bones with the phalanges. The power ia 

 represented by the muscles of the tendo Achillis, and its 

 point of application is found in the posterior extremity of 

 the os calcis. The resistance, that is to say, the weight of 

 the body transmitted by the tibia, lies in the upper facet 



