90 STRUCTURE AND ATTACHMENTS OF MUSCLES. 



hand up with a sudden jerk. Voluntary motion is, 

 in fact, effected by the contraction of muscles acting 

 upon and changing the relative positions of the 

 bones or solid support of the system, and therefore 

 almost all muscles are attached to one bone by their 

 origin, and to another by their insertion ; the former 

 being merely the fixed extremity, towards which the 

 opposite and more moveable end, called the insertion, 

 is carried by the shortening of the intervening belly 

 of the muscle. 



The figure represents the bones of the arm and hand, having 

 all the soft parts dissected off except one muscle OBI, of 

 which the function is to bend the arm. O the origin of the 

 muscle. B the belly. 1 the insertion. T T the tendons? 

 S the shoulder-joint. E the elbow. When the belly con- 

 tracts, the lower extremity of the muscle, I, is brought 

 nearer to the origin or fixed point O, and, by thus bending 

 the arm at the elbow-joint, raises up the weight W placed 

 in the hand. 



If the muscles must be attached to bones, it may 

 be asked, how can the bones, which present com- 

 paratively so small a surface, afford space enough 

 for the attachments of muscles, which are so much 

 larger, and which even appear in successive layers 

 above each other 1 This difficulty is obviated in 

 two ways. In the first place, the heads and other 

 parts of bones to which muscles are attached are 



