166 THE VERTEBRATE SKELETON. 



D. THE POSTERIOR LIMB. 



This corresponds closely to the anterior limb and, like it, 

 is divisible into three parts, the thigh, the shin or cms and 

 the pes. 



As was the case with the anterior limb, all the long bones 

 have their ends formed by prominent epiphyses which do not 

 unite with the shaft till late in life. 



In the thigh there is only a single bone, the femur. 



The femur is a moderately long, slender bone with a well- 

 ossified hollow shaft slightly curved in a sigmoid manner. 

 Both ends are expanded, the proximal end is hemispherical 

 and articulates with the acetabulum, the distal end is larger 

 and more laterally expanded. 



The shin likewise includes a single bone, the tibio-fibula, 

 but this, as can be readily seen by the grooves at the proximal 

 and distal ends of the shaft, is formed by the fusion of two 

 distinct bones, the tibia and fibula. The tibio-fibula is longer 

 and straighter than the femur. 



The pes consists of two parts, the ankle or tarsus and the 

 foot. 



The tarsus consists of two rows of structures, very dif- 

 ferent in size. The proximal row consists of two long bones, 

 the tibiale and fibulare, which are united by common epi- 

 physes at the two ends, while in the middle they are widely 

 separated. The tibiale lies on the tibial or pre-axial side, 

 and the fibulare which is the larger of the two bones on 

 the fibular or postaxial side. The distal row of tarsals 

 consists of three very small pieces of calcified cartilage. The 

 postaxial of these is the largest, it articulates with the second 

 and third metatarsals and is probably homologous with tarsalia 

 2 and 3 fused. The middle one is very small, it articulates 

 with the first nietatarsal and is probably tarsale 1. The pre- 

 axial one articulates with the nietatarsal of the calcar, a 

 structure to be described immediately, and has been regarded 

 as a centrale. 



