Fig. 2.
“In the third order of lever the power acts between the prop and the resistance (fig. 3), where also P : W :: BF : AF, and the pressure on the fulcrum is P - W, or the power less the weight.
Fig. 3.
“In the preceding computations the weight of the lever itself is neglected for the sake of simplicity, but it obviously forms a part of the elements under consideration, especially with reference to the arms and legs of animals.
“To include the weight of the lever we have the following equations: P.AF + AF. 1/2AF = W.BF + BF. 1/2BF; in the first order, where AF and BF represent the weights of these portions of the lever respectively. Similarly, in the second order P.AF = W.BF + AF. 1/2AF and in the third order P.AF = W.BF + BF. 1/2BF.
“In this outline of the theory of the lever, the forces have been considered as acting vertically, or parallel to the direction of the force of gravity.
“Passive Organs of Locomotion. Bones.—The solid framework or skeleton of animals which supports and protects their more delicate tissues, whether chemically composed of entomoline, carbonate, or phosphate of lime; whether placed internally or externally; or whatever may be its form or dimensions, presents levers and fulcra for the action of the muscular system, in all animals furnished with earthy solids for their support, and possessing locomotive power.”[11] The levers and fulcra are well seen in the extremities of the deer, the skeleton of which is selected for its extreme elegance.