The Vertebrate Skeleton

by Sidney H. Reynolds

This is divisible into three portions, the thigh, the crus or shin, and the pes.

The thigh includes a single bone, the femur.

The femur (fig. 40, B, 12) is a short thick bone, with a prominent rounded head articulating with the acetabulum. Behind this head is a deep pit, beyond which is a roughened area corresponding with the great trochanter of mammals. The distal end is expanded and somewhat convex.

The bones of the crus or shin are the tibia and fibula. These are both straight rod-like bones with expanded terminations which closely approach one another, while elsewhere the bones diverge considerably.

The terminations of all three of the leg bones are formed by epiphyses.

The Pes consists of the tarsus or ankle, and the foot, which is made up of five digits.

The tarsus. The tarsal bones of the Turtle do not retain their primitive arrangement to such an extent as do the carpals. They are arranged in a proximal row of two and a distal row of four. Of the bones in the proximal row the postaxial one is much the smaller and is the fibulare; the larger pre-axial one (fig. 40, B, 15) represents the tibiale, intermedium, and centrale fused, and articulates with both tibia and fibula. The first three distal tarsalia are all small bones and are very similar in size, and each articulates regularly with the corresponding metatarsal. The fourth bone (fig. 40, B, 19) is much larger, and represents tarsalia 4 and 5 fused. The first two distal tarsalia articulate with the pre-axial tarsal of the proximal row, the third only with its neighbours the second, and the fused fourth and fifth. The latter articulates with both bones of the proximal row.

Each digit consists of a metatarsal and of a varying number of phalanges.

The metatarsals. The first metatarsal (fig. 40, B, 20) is broad and flattened, the second, third and fourth, are all elongated bones with nearly flat terminations formed by small epiphyses. The fifth is large and flattened, and the articular surface for the phalanx is situated somewhat laterally.

The phalanges. The first digit has two phalanges and is the stoutest of them all; its distal phalanx is sheathed in a large horny claw. The other digits, of which the third is the longest, have each three phalanges. The distal phalanges of the second and third digits are flattened and pointed and bear small horny claws.