244 THE BIOLOGY OF THE FROG CHAP. 



ing the tibio-fibula the tarsus and foot, corresponding to the 

 carpus, and hand. The femur is an elongated, cylindrical, 

 very slightly sigmoid bone ; it articulates by its expanded 

 and rounded head with the acetabulum above, forming the 

 hip joint. 



The tibio-fibula is an elongated, very slightly bent bone 

 somewhat expanded and flattened at either end, where it is 

 marked on both sides by a groove which indicates its forma- 

 tion from two bones, the tibia and the fibula, which were 

 originally separate. The tibia is preaxial in position and 

 corresponds to the radius of the forearm ; the fibula is post- 

 axial and corresponds to the ulna. Near the middle the 

 tibio-fibula is perforated by a foramen for the anterior tibial 

 artery. 



The tarsus of the frog is peculiarly modified in that the 

 proximal portion is much elongated and consists of but two 

 bones ; the preaxial bone next to the tibia is called the tibi- 

 ale, the postaxial one next to the fibula, the fibitlare ; the 

 two bones are united at their ends, inclosing a narrowly 

 oval space between them. The centrale is represented by 

 a small bone on the preaxial side of the distal end of the 

 tibiale ; at its distal end it supports the prehallux. The 

 tars alia (which correspond to the carpalia of the hand) are 

 much reduced both in size and number. There is a small 

 first tarsal behind the base of the first metatarsal bone. 

 Behind the second and third metatarsals is a small bone 

 which represents the fused second and third tarsalia. The 

 fourth and fifth tarsalia are absent. There are five meta- 

 tarsal bones, all of which are elongated and cylindrical. 

 Of the phalanges the first and second toes contain 

 two each, the third and fifth toes three each, and the 

 fourth toe four. The small prehallux is composed usually 

 of two pieces. 



