ANIMAL AND VEGETABLE 177 



direction in which the particle will move, a parallelogram is 

 constructed of which OA and OB form two sides, and then 

 the diagonal OR of the parallelogram is drawn. It gives 

 the direction which the particle takes ; it is the resultant of 

 the two forces OA, OB ; and if the lines OA and OB repre- 

 sent by their lengths the magnitude of the forces, then the 

 diagonal will represent by its length the magnitude of the 

 resultant force. This is the parallelogram of force. 



In a similar way one force may be made to take the 

 place of several forces. Let a parallelogram be constructed 

 on the lines representing two of the forces. Take the 

 diagonal, and with it and the line representing the third 

 force construct another parallelogram. Its diagonal is 

 the resultant of the three forces ; with it and the line 

 representing the fourth force, the resultant of four forces 

 may be found, and so on. 



" It is different in the case of the more free ball -sockets, 

 which permit movement of the bone in any direction 

 within certain limits. When a socket of this sort is sur- 

 rounded by many muscles, each of the latter, if it acts alone* 

 sets the bone in motion in the direction of its own action. 

 If two or more of the muscles assume a state of activity at 

 the same time, then the action will be the resultant of the 

 separate tensions of each. 



" There is yet another way in which the work performed 

 by the muscles is conditioned by their attachment to the 

 bones. The latter must be regarded as levers which turn 

 on axes, afforded by the sockets. They usually represent 

 one-armed, but sometimes two-armed levers. Now, the 

 direction of the tension of the muscles is seldom at right 

 angles to that of the movable bone lever, but is usually at 

 an acute angle. In this case, again, the whole tension of 

 the muscle does not take effect, but only a component, 

 which is at right angles to the arm of the lever. Now, it is 

 noticeable that in many cases the bones have projections 



