248 THE BIOLOGY OF THE FROG CHAP, xn 



of some of the muscles of the hind leg. If the skin be 

 stripped off from the leg and the frog laid on its back, the 

 following muscles may be seen on the ventral side of the 

 thigh : 



The sartorius, a narrow muscle having its origin on the 

 ileum, just in front of the pubis. It crosses the thigh ob- 

 liquely and is inserted by a tendon a short distance below 

 the head of the tibia. When it contracts, it flexes the leg on 

 the thigh and pulls the whole limb forward and ventrally. 



The adductor magniis, a large thick muscle lying behind 

 the sartorius, which it crosses at its lower end. Its origin 

 is from the pubis and ischium, and it receives a small slip 

 which originates on the tendon of one of the heads of the 

 semitendinosus. It is inserted' into the distal end of the 

 femur. Its action is to bend the thigh ventrally and to pull 

 it anteriorly or posteriorly according to the position of the 

 limb. When the thigh is pulled forward so that a line con- 

 necting the centers of the origin and insertion of the muscle 

 lies in front of the head of the femur, the contraction of the 

 adductor magnus has the effect of moving the limb still 

 farther forward while pulling it ventrally. When the thigh 

 is bent back so that the head of the femur lies in front of a 

 line connecting the centers of the origin and insertion, the 

 contraction of the muscle has the opposite effect. The 

 adductor magnus thus acts as an adductor or an abductor 

 according to circumstances. 



The adductor longus, a narrow muscle arising from the 

 ventral part of the ileum just dorsal to the head of the 

 sartorius ; distally it joins the adductor magnus. It is partly 

 covered by the sartorius, but a small portion of it is exposed 

 along the preaxial side of that muscle. It pulls the thigh 

 forward and ventrally. 



The triceps femoris, a very large muscle covering the 



