402 VERTEBRATE LIFE AND ORGANIZATION 



'Femixr 



Gastrocnemius 

 ori 



Tibiofibula 



Tibialis 

 a.niicus 

 lon^us 



*^ Tibiofibtrla.-iarsai 

 ■joint = Fulcrum, (a) 



Fulcrum. 



Point of cLction 

 of muscles 



Point of 

 exertion of 

 force 



Gastrocnemius^ 

 insertionCb) 



Meta-taj-sals 



Distance aJp x pull of muscles 

 = Distance a.c x force exerted 



.Point of exertion 

 of force (c) 



Figure 21.6. A, A diagram to illustrate the antagonistic action of muscles on the 

 frog's right hind foot. The gastrocnemius moves the foot in the direction of the solid 

 arrow; the tibialis amicus in the direction of the shaded arrow. B, A comparable lever 

 system. 



The forward movement of the entire limb at the hip or shoulder is some- 

 times also called flexion, but protraction is a more appropriate term. 

 Retraction is the opposite movement. Adduction is a movement that 

 brings the distal end of an appendage toward the midventral line of 

 the body; abduction, away from the midventral line. 



Most of the muscles are attached to the bones in such a way that the 

 fulcrum is at one end of the lever, and the muscle attachment is nearer 

 the fulcrum than the point at which the lever exerts its force (Fig. 21.6). 

 Such levers are mechanically inefficient, but this arrangement provides 

 for compactness and speed of movement. 



The superficial skeletal muscles of the frog are shown in Figure 21.7. 



1 84. Body Cavity and Mesenteries 



The internal organs of the frog protrude into the body cavity, or 

 coelom, which contains a small amount of watery coelomic fluid. The 

 space and fluid facilitate the expansion, contraction, and slight move- 

 ment of the organs in relation to each other. The coelom is divided into 

 an anterior pericardial cavity containing the heart, and a posterior 



