It will be most convenient to defer a discussion of the limbs of fishes to chap. VIII.
All vertebrates above fishes have the limbs divisible into three main segments:—
| Anterior or Fore limb. | Posterior or Hind limb. | |
| Proximal segment. | upper arm or brachium. | thigh. |
| Middle segment. | fore-arm or antibrachium. | shin or crus. |
| Distal segment. | manus. | pes. |
The proximal segments each contain one bone, the humerus in the case of the upper arm, and the femur in the case of the thigh. The middle segments each contain two bones, the radius and ulna in the case of the fore-arm, and the tibia and fibula in the case of the shin.
The manus and pes are further subdivided into
(a) two or three proximal rows of bones forming the wrist or carpus in the case of the manus, and the ankle or tarsus in the case of the pes.
(b) a middle row called respectively the metacarpus and metatarsus.
(c) a number of distal bones called the phalanges which form the skeleton of the fingers and toes, or digits.
Typically the manus and pes both have five digits (pentedactylate). The first digit of the manus is commonly called the pollex, and the first digit of the pes the hallux.
In a very simple carpus such as that of Chelydra, there are nine bones. They are arranged in a proximal row of three, the radiale, intermedium, and ulnare,—the first being on the radial side of the limb, and a distal row of five called respectively carpale 1, 2, 3, 4, 5, beginning on the radial side. Between these two rows is a single bone the centrale, or there may be two.