20 THE PRI^XIPLES OF MYODYXAMICS. 



(i.) A fixed bone gives origin to the moving 

 muscle — but does not move. 



(2.) A movable bone gives insertion to the 

 moving muscle — and is moved. 



(3.) In some cases the movable bone becomes 

 fixed, then the fixed bone may move. 



(4.) When a muscle spans two joints and the 

 interjacent bones, there may be one or two 

 movable bones : — 



(i.) One of the adjacent bones may move, 

 when the other adjacent bone and the interjacent 

 bone will be fixed. 



(ii.) The two adjacent bones may move, when 

 the interjacent bone will be the fixed bone : — 



(iii.) One of the adjacent bones and the inter- 

 jacent bone may move, when the other adjacent 

 bone will be the fixed bone. 



13. A movable bone may be a lever of the 

 first, second, or third order : 



Examples, (i.) The skull acted on by the 

 posterior cervical muscles is a lever of the first 

 order. 



(2.) The foot acted on by the muscles of the 

 tendo-Achillis is a lever of the second order. 



