THE FROG 63 



homologous structures are found in the vertebral column, 

 and in the limbs. 



4. The hind-limb. The bones have the same general 

 character as those of the fore-limb, with which they are 

 homologous. 



The thigh. The femur or thigh-bone is a long bone with 

 a slight sigmoid curve. The proximal end or head is sphe- 

 roidal and fits into the acetabulum, forming the hip-joint; 

 the distal end is somewhat broadened for articulation with 

 the tibio-fibula. 



The leg or crus. The tibio-fibula is slightly longer than 

 the femur, slightly curved, and broadened at each end. 

 Like the radio-ulna, this is a compound bone, formed of two 

 bones usually separate in other animals (compare the human 

 skeleton). Near each end, a longitudinal groove indicates 

 the line of fusion of the two bones. 



The ankle or tarsus contains five tarsal bones. The two 

 proximal ones are long bones united at each end by common 

 epiphyses. The preaxial one is known as the tibiale or as- 

 tragalus, the postaxial and larger one is the fibulare or 

 calcaneum. The centrale is represented by a small bone 

 a mere nodule on the preaxial side of the distal end of 

 the tibiale; it supports the prehallux (see below). Between 

 the two proximal tarsal bones and the metatarsals are two 

 very small irregular tarsal bones corresponding to the distal 

 row of carpal bones in the wrist. Of what advantage to the 

 frog is the elongation of the two proximal tarsal bones ? 



The foot or pes exclusive of the tarsus has five complete 

 digits and a small supernumerary digit. The supernu- 

 merary digit is on the preaxial side of the foot and consists 

 of from one to three small bones ; it is sometimes called the 



