I.] THE FROG. 79 



tendon of insertion passes obliquely forwards and 

 downwards immediately beneath that of the glu- 

 teus, to reach the head of the femur. 



7. The muscles of the leg. 



a. Turn the frog on its back, and remove the skin 

 from the foot: fasten the same down, dorsal sur- 

 face upwards. The os cruris will now be seen 

 running down the middle of the leg. Lying on 

 its inner side are two muscles, viz. : 



a. The gastrocnemius : a muscle with a great fleshy 

 belly; it arises above by two tendons; one 

 (much the larger) starts in the region of the 

 knee-joint partly from the femur, partly from 

 the os cruris ; the other arises from the apo- 

 neurosis adjacent. Below, the muscle ends in 

 a great tendon (tendo Achillis\ which termi- 

 nates in an aponeurosis on the plantar surface 

 of the foot. 



/?. The tibialis posticus : a slender muscle covered 

 in the main by the gastrocnemius; it arises 

 from the posterior surface of the os cruris and, 

 passing to the inner side of and through the 

 ankle-joint (cf. p. 72, leg-bone), is inserted into 

 the astragalus. 



b. a. and /3. act as flexors of the foot ; on the 

 opposite side of the leg lie the following ex- 

 tensors. 



a. The peroneus : the largest and most external; 

 it arises from the outer side of the distal ar- 

 ticular end of the femur and, running across 

 the ankle-joint, is inserted into the calcaneum. 



