116 



ESSENTIALS OF ZOOLOGY 



supinators, levators, depressors, sphincters, dilators. A fiexor mus- 

 cle is one wliich bends the part on itself. An extensor is one which 

 by its action tends to extend or straighten the part. An adductor 

 muscle is one attached to a long bone in such a way as to move it 

 toward the ventral midline of the body. An ahductor opposes the 

 action of the adductor. Pronator muscles rotate the part forward 

 toward the ventral side while supinators rotate the part back toward 

 the dorsal side. A levator (elevator) raises or elevates, and a de- 

 pressor lowers the part concerned. A sphincter muscle is one encir- 

 cling an aperture which it closes by contraction. The orhicuUrts ons 

 in the lips around the mouth is an example. A dilator is antagonistic 



sTeTnomasToIdeus----' 



clavotTafsezius--' 

 levaToTclaviculse 

 acTomiodelToideus- 

 sfiinodelToldeus- 



K.. 38.-Superflcia. -f - »' ^ Tmn^'cTionfortte ImericTn' Pwt^sipS 

 (From Greene, Anatomy of the Rat, iiansacuons oi uie u^ 



Society, New Series, Vol. XXVII, 1935.) 



to a sphincter muscle. Protractor muscles cause the part concerned 

 to be thrust out as in the case of the tongue while the action of retrac- 

 tors is to withdraw the part. 



Muscles are usually attached to bones in such a way that the 

 bones serve as levers with the articulations serving as fulcra. Ad- 

 vantage of power is usually gained in machines by causing a small 

 force to act through a great distance by use of a long lever thereby 

 moving a heavy load a short distance. It has been observed by 

 students of myology that most skeletal muscles operate m an almost 

 opposite manner, in that usually a powerful muscle acts on a shorter 

 lever to produce a rapid movement of a small load at the end of a 

 longer lever. This arrangement produces more speed but it is less 



