THE FROG 83 



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

 the thigh between the rectus anticus and the vastus externus. 



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 aiSc 

 the head of the tibia-fibula. 



3. The semimembranosus is a stout muscle lying along tlu 

 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 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 backwards and outwards between the biceps 

 and the semimembranosus, and is inserted into the femur at the junc- 

 tion 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 arise* 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 

 the ischial symphysis. The anterior head passes through a slit in the 

 adductor magnus and unites with the posterior head in the distal third 

 of the thigh. The tendon of insertion is long and thin, and joins that 

 of the rectus internus minor to be inserted into the tibia just below its 

 head. It adducts the thigh and flexes the leg. 



2. The adductor brevis is a short wide muscle, lying beneath 

 the upper end of the adductor magnus. It arises from the pubic and 

 ischial symphyses, and is inserted into the preaxial surface of the proxi- 

 mal half of the femur. 



3. The pectineus ( ) is a rather smaller 

 muscle, lying along the outer (extensor) side of the adductor brevis. 

 It arises from the anterior half of the pubic symphysis in front of the 

 adductor brevis, and is inserted like it into the proximal half of the 

 femur. 



4. The ilio-psoas (iliacus internus) arises by a wide origin 

 from the inner surface of the acetabular portion of the ilium ; it turns 

 round the anterior border of the ilium, and crosses in front of the hip- 

 joint, where, for a short part of its course, it is superficial between the 

 heads of the vastus internus and of the rectus anticus femoris ; it then 



