The Frog 83 



is inserted into the inner side of the tibia, just below its head. Its action 

 is the same as for gracilis major. 



Innervation: Branches and twigs from the sciatic nerve and 

 plexus. 



(2) Superficial muscles of the Extensor Surface of the Thigh. 



1. Triceps extensor femoris, or cruris, a very large muscle 

 inserted into the front of the tibia just below the head of the latter, but 

 arising from the pelvic girdle as three separate muscles, the rectus ami- 

 cus femoris ( ), vastus externus ( ), 

 and vastus internus, or crureus ( ). All these lie 

 on the front of the thigh, and their action is to straighten the knee. 



Innervation : Branches and twigs from the sciatic nerve and 

 plexus. 



(3) Superficial muscles of the Postaxial Surface (apparent dorsal) 

 of the Thigh. 



1. The gluteus (iliacus externus), already mentioned, lies in 

 the thigh between the rectus anticus femoris and the vastus externus. 

 It draws the thigh forward. 



2. The biceps (ileo-fibularis) is a long slender muscle which 

 arises from the crest of the ilium just above the acetabulum. It lies in 

 the thigh along the inner border of the vastus externus, and is inserted 

 by a flattened tendinous expansion into the distal end of the femur and 

 the head of the tibia-fibula. It draws the thigh dorsally and flexes the 

 leg. 



3. The semimembranosus is a stout muscle lying along the 

 inner side of the biceps, between it and the rectus internus minor. It 

 arises from the dorsal angle of the ischial symphysis just beneath the 

 cloacal opening, and is inserted into the back of the head of the tibia. 

 It is divided about its middle by an oblique tendinous intersection. It 

 adducts the thigh and flexes or extends the leg according to whether 

 the leg is in a flexed or extended position, 



4. The pyriformis is a slender muscle which arises from the 

 tip of the urostyle, passes backward and outward between the biceps 

 and the semimembranosus, and is inserted into the femur at the junction 

 of its proximal and middle thirds. It pulls the urostyle to one side and 

 draws the femur dorsally. 



Innervation : Branches and twigs from sciatic nerve and 

 plexus. 



(4) Deep muscles of the Thigh. 



1. The semitendinosus is a long thin muscle which arises by 

 two heads ; an anterior one from the ischium close to the ventral angle 

 of the ischial symphysis and the acetabulum ; and a posterior one from 



