64 THE MUSCULAR SYSTEM OF THE FROG 



a. The rectus anticus femoris forms the middle 

 division of the triceps : it arises from the ven- 

 tral border of the posterior third of the ilium, 

 in front of the acetabulum : about half-way 

 down the thigh it joins the next division. 



ft. The vastus interims, the preaxial division of 

 the triceps, is a large muscle arising from 

 the ventral and anterior border of the 

 acetabulum, and lying in the thigh between 

 the sarfcorius and the rectus anticus. 



y. The vastus externus, the postaxial division of 

 the triceps, arises from the posterior edge 

 of the dorsal crest of the ilium, and joins 

 the other two divisions of the triceps about 

 the junction of the middle and distal thirds 

 of the thigh. 



c. Superficial muscles of the postaxial (apparent dorsal) 

 surface of the thigh. 



i. The glutseus has been already noticed : it lies in 

 the thigh between the rectus anticus and the 

 vastus externus. 



ii. The biceps 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. 



iii. 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. 



iv. The pyrifonnis is a slender muscle which arises from 

 the tip of the urostyle, passes backwards and 

 outwards between the biceps and the semi- 

 membranosus, and is inserted into the femur at 

 the junction of its proximal and middle thirds. 



