254 THE BIOLOGY OF THE FROG chap. 



anterior side of the leg, partly covered by the tibialis anticus 

 longiis. It arises by a slender tendon, from the distal end 

 of the femur. It is inserted directly upon the bone, along 

 the anterior side of the tibio-fibula. It extends the leg. 



The tibialis anticus brcvis is a short muscle lying close 

 alongside of the extensor cruris. It arises from the distal 

 third of the tibio-fibula where it is attached directly to the 

 bone. Uistally it ends in a tendon which is inserted in the 

 proximal end of the tibiale. It flexes the foot. 



There are numerous smaller muscles for moving the dif- 

 ferent parts of the foot, whose description we shall omit. 



When a muscle works in company with others, its action 

 is often quite different from that produced when it acts 

 alone. This fact has been well illustrated by Dr. Lombard,^ 

 who has worked out the double action of the muscles of the 

 frog's leg in a very thorough manner. The triceps femoris, 

 for instance, when working alone acts as an extensor of the 

 leg ; but since it pulls the thigh forward at the same time, 

 it causes the flexor muscles on the back of the thigh to 

 become tense, and thereby indirectly brings about a flexion 

 of the leg. The muscles of the frog's leg are so arranged 

 that any force which pulls the leg forward causes the crus to 

 flex against the thigh and the foot to flex against the crus, 

 and any force which pulls the leg backward extends the crus 

 and foot. The movements of the frog's leg in jumping and 

 swimming consist mainly in alternately bringing the leg up 

 against the side of the body, folding the various parts 

 together, and pulling it backward and extending or unfold- 

 ing its difl'erent parts. Any muscle, such as the iliacus 

 internus, which pulls the femur backward, brings about a 

 flexion of the crus through the tension produced on the 



1 " Contributions to Medical Research, dedicated to V. C. Vaughan," 

 p. 260, 1903. 



