The pelvic girdle differs from the pectoral in the fact that the dorsal bones--the ilia--are nearly always firmly united to transverse processes of the sacral vertebrae, by means of rudimentary ribs. The pubes and ischia generally meet in ventral symphyses.
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.
Similarly there are nine bones in a simple =tarsus= such as that of _Salamandra_. They form a proximal row of three, the tibiale, intermedium and fibulare, and a distal row of five, called respectively tarsale 1, 2, 3, 4, 5, beginning on the tibial side. Between the two rows there is a centrale as in the carpus, or there may be two.
The following names derived from human anatomy are commonly applied to the various carpal and tarsal bones: