250 THE BIOLOGY OF THE FROG chap. 



whole front of the thigh. It arises by three heads. The 

 anterior head {caput anticum or cruralis) arises from 

 the anterior border of the acetabulum at the junction of the 

 ileum and pubis. The middle head {caput mediuni) is a 

 short, flat muscle which arises from the ventral side of the 

 ileum near the middle and joins the fascia of the rest of the 

 muscle at about the proximal third of the anterior margin 

 of the thigh. The posterior head {caput posticum, vastus 

 externus, gluteus jnag?ms) arises from the posterior end of 

 the crest of the ileum ; distally its fibers join those of the 

 cruralis or anterior head. The whole muscle is inserted by 

 a very strong tendon which passes over the knee and joins 

 the upper end of the tibio-fibula. A tendinous connection 

 passes also to one head of the gastrocnemius. The action 

 of the whole muscle is to extend the crus and draw the whole 

 leg forward. 



The gracilis major, a large muscle lying along the poste- 

 rior side of the ventral surface of the thigh. It arises by a 

 small tendon from the posterior margin of the ischium. Its 

 distal tendon divides into two parts, the one passing beneath 

 the tendon of the sartorius to be inserted into the preaxial 

 side of the proximal end of the tibia, the other passing dorsal 

 to the tendinous insertion of the semitendinosus and joining 

 t"he posterior side of the head of the tibio-fibula. Near the 

 middle the muscle is crossed by an oblique tendinous in- 

 scription. The gracilis major pulls the femur backward and 

 either flexes or extends the crus according to the position 

 of the latter in relation to the femur. If the crus is flexed 

 so that it makes an angle of less than 90° with the femur, 

 the action of the gracilis is to flex it stiU more. If, how- 

 ever, the crus is partly extended so that it makes a con- 

 siderably greater angle than 90° with the femur, the contraction 

 of the gracilis still further extends it. The difference of the 



