THE FROG 67 



movements. In the hind limb the ball-and-socket joint at 

 the head of the femur allows great freedom of motion, but 

 other joints permit of movement approximately in only one 

 plane. 



With respect to the kind of motion produced the muscles 

 of the limbs of the frog may be classified as abductors, ad- 

 ductors, rotators, flexors, and extensors. When the thigh 

 of the frog is pulled forward (i. e., away from the median 

 line), it is said to be abducted; when it is pulled backward 

 (i. e., toward the median line), it is said to be adducted. 

 When a muscle of the thigh is placed obliquely its effect is 

 generally to rotate the thigh on its long axis. When the leg 

 (crus) of the frog is bent so as to form an angle with the 

 thigh, the leg is said to be flexed ; when it is brought in line 

 with the thigh, the leg is said to be extended. The leg is in- 

 capable of rotation. The foot may be flexed, extended, or 

 slightly rotated. 



The "ventral" and "dorsal" surfaces of the hind leg are 

 such in consequence of the extreme obliquity of the pelvic 

 girdle; they are really preaxial and postaxial, and correspond 

 respectively to the inner and outer surfaces of the human 

 leg. 



The muscles should be studied in a frog that has been 

 hardened in formalin. Aid in the identification of the mus- 

 cles may be obtained by consulting Fig. 70 of Holmes' Biol- 

 ogy of the Frog, or Fig. n of Marshall's The Frog. The 

 origin and insertion of each muscle should be carefully de- 

 termined, and then located on the skeleton of the frog. The 

 action of the muscle may sometimes be tested by pulling it 

 gently, but this is often impracticable in preserved material ; 

 the action of a muscle may generally be inferred from its 



