CHAPTER XXVI. 



THE APPLICATION OF SKELETAL MUSCLES. 



The consideration of the many varieties of muscles, and the 

 various modes in which they are attached to the bones that they 

 are destined to move, belongs to the department of practical anat- 

 omy, and needs no mention here. As a general, but by no means 

 universal rule, a muscle has one attachment which is fixed, com- 

 monly spoken of as its origin, and a second, called its insertion, 

 upon which it acts by approximating it to the origin. Muscles 

 mostly pass in a straight line between their two attachments, but 

 sometimes they act round an angle by sliding over a pulley, or 

 by means of a small bone in the tendon, like the patella. 



The muscles are so attached that they are always slightly on 

 the stretch, and thus at the moment they begin to contract they 

 are in an advantageous position to bring their action to bear on 

 the bones which they move. When the contraction ceases, the 

 bones are drawn back to their former position without any sudden 

 jerk or jar. 



The muscles commonly act upon the bones as levers by working 

 upon the short arm of the lever, so that more direct force is re- 

 quired on the part of a muscle than the weight of the body 

 moved ; but from this arrangement considerable advantages are 

 gained, viz., that a small contraction of the muscle causes an 

 extensive excursion of the part moved, and much greater rapidity 

 of motion is attained. 



All the three orders of levers are met with in the movements 

 of the different bones of the skeleton ; often, indeed, all three 

 varieties are found in the same joint, as the elbow, where the 

 simple extension and flexion motions of the biceps and triceps 

 muscles give us good examples (Fig. 194). 



The first order of lever is used when the triceps is the power 

 and draws upon the olecranon, thus moving the hand and 

 forearm around the trochlea which acts as the fulcrum. This 



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