12 THE EXTERIOR OF THE HORSE. 



Of the two forces which act upon the lever, one, which is called the power, 

 is destined to equalize the other, which is called the resistance, or to overcome 

 its action. The object of the lever is to favor one of these forces at the expense 

 of the other. We will see later on what its consequences are. In the animal 

 economy, these forces are represented by the muscles, and the levers by the bones. 



For convenience of demonstration, we will suppose that the two forces which 

 incite the lever are situated in the plane of the latter. In most instances it is 

 not thus : the forces and the lever are placed in different planes. 



An example will explain this better. Suppose it to be a question of the adductor muscle 

 of the arm. The lever upon which it is inserted is the humerus, the resistance which it must 

 overcome is the weight of the member applied to the articulation of the elbow. It is then easy 

 to determine that the humeral and vertical axis passing through the centre of this articulation, 

 forms a plane in which the adductor muscle of the arm is not situated. If it were located there, 

 it would determine flexion alone of that bone, which is not the case, since it promotes adduction. 



Among the muscles of the members, it is only the direct extensors and 

 fiexors that are situated in the plane of their respective levers. It is the same 

 for the muscles of the spinal column. All the others act in different planes. 

 This does not mean that the conditions of the equilibrium of the lever are not 

 applicable to the former, but the developments into which we must enter in order 

 to resolve particular cases would lead us too far away. 



In mechanics, the moment of a force in relation to an axis is the product of 

 the projection of this force upon a plane perpendicular to the axis, through the 

 distance of this force to the axis. 



When the idea of moment is applied to the study of the lever, it may be 



denned thus : The product 

 of the force by the arm of the 

 lever, because the forces, 

 being situated in the same 

 plane, are themselves their 

 projection ; as to the axis, 

 it is supposed to pass 

 through the point of 

 support. 



Suppose the two forces F 

 and F' inciting the lever AB 

 (Fig. 4). The whole system is 

 situated in the plane of the ac- 

 tion. The forces then project 

 FIG. 4. themselves following FA and 



F'B. Let us suppose now an 



axis perpendicular to this plane and piercing it at the point 0. It is evident that the distances 

 of these two forces to this axis are measured by the perpendiculars OC and OD. The moment 

 of force /'will be FX OC; that of the force F . F' x OD. 



The perpendiculars OC and OD are called lever-arms extending from the 

 point of support in the direction of the forces .Fand F'. 



Whence it results that in the lever the moment of a force is the product of 

 that force by its lever-arm. It is demonstrated in the same manner that the 

 lever is in equilibrium when the moments of the two forces are equal. We will 



therefore obtain ; 



FXOC=F'XOD. 



