“In straight levers of the _first kind_, the fulcrum is between the power and the resistance, as in fig. 1, where F is the fulcrum of the lever AB; P is the power, and W the weight or resistance. We have P : W :: BF : AF, hence P.AF = W.BF, and the pressure on the fulcrum is both the power and resistance, or P + W. “In the second order of levers (fig. 2), the resistance is between the fulcrum and the power; and, as before, P : W :: BF : AF, but the pressure of the fulcrum is equal to W - P, or the weight less the power. [Illustration: 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. [Illustration: 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}.(AF)/2, and in the third order P.AF = W.BF + {BF}.(BF)/2. “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. [11] Bishop, _op. cit._ [Illustration: FIG. 4. Skeleton of the Deer (after Pander and D’Alton). The bones in the extremities of this the fleetest of quadrupeds are inclined very obliquely towards each other, and towards the scapular and iliac bones. This arrangement increases the leverage of the muscular system and confers great rapidity on the moving parts. It augments elasticity, diminishes shock, and indirectly begets continuity of movement, _a._ Angle formed by the femur with the ilium, _b._ Angle formed by the tibia and fibula with the femur, _c._ Angle formed by the cannon bone with the tibia and fibula, _d._ Angle formed by the phalanges with the cannon bone. _e._ Angle formed by the humerus with the scapula. _f._ Angle formed by the radius and ulna with the humerus.]