MUSCULAR SYSTEM. 27 



or featherfomi, is characterized by fibers attaching to two sides 

 of a continuous tendon. 



They are also classified according to function into flexors, 

 extensors, abductors, adductors, and rotators. 



The belly is the active working part (lean meat). The ten- 

 don merely serves to give connection with a distant object, usu- 

 ally a bone. Tendons are strong but have no power of contraction. 

 The strength of a muscle depends upon the thickness of the belly. 

 The extent of its movement depends upon the length of the belly. 



The two ends of a muscle are defined as origin and insertion. 

 The origin is the less movable end ; insertion the more movable 

 end. It may happen at one time that one end is the insertion, 

 at another time the origin. 



FIG. 14. MUSCLE FIBERS. 

 A, Bundle of voluntary fibers, .side view. B, 0, D, Three involuntary fi- 

 bers, spindle-shaped cells with nuclei. 



A muscle produces motion by pulling upon some bone which 

 acts as a lever with the fulcrum at a joint. 



The muscle fibers may attach directly to the bone, or indi- 

 rectly through tendon fibers. The connection between the muscle 

 fiber and the tendon fiber is by insertion of the conical point of 

 the muscle fiber into a conical cavity at the end of the tendon 

 fiber. 



