xii. io AMPHIBIAN LIMBS 317 



carpals, and 7 or 8 digits. In the amphibian *Eryops most of the digits 

 radiate from the radius, in later forms mostly from the ulna. Moreover, 

 in the hand of *Eryops there seem to have been six digits and it is 

 usually stated that the first of these is a pre-pollex 'not comparable 

 with the pollex of higher forms'. 



The effect of this system is to provide a lever that can be held 

 firmly against the ground while it is moved by the muscles running 

 from the girdles to the humerus or femur. In addition the lever is 

 itself extensible by means of its own muscles. Whatever may have 

 been their origin in fishes these muscles in tetrapods work in such a 

 way as to bend each segment up and down. The shoulder and thigh 

 joints usually allow movement in several planes, both towards and 

 away from the midline (adduction and abduction), and forwards and 

 backwards (protraction and retraction). As we have seen, the animal 

 balances at these joints by muscles arranged round them. Movements 

 of rotation are also possible at these, and sometimes at other joints, 

 the distal bone turning about its own axis on the proximal one. Such 

 movements may be very important for the proper placing of the limbs 

 in walking. Pronation is the rotation of the radius about the ulnar 

 bone, so that the manus is directed caudally, supination being the 

 opposite movement. The terms flexion and extension are convenient 

 at certain joints (e.g. the elbow), but have no consistent meaning with 

 reference to the main axes of the body. 



The limbs of the earlier amphibians were ponderous affairs, with 

 large bones and widely expanded hands and feet (Figs. 181 and 182). 

 It is not certain exactly how they were used ; probably they were held 

 out sideways, giving a wide base on which the somewhat precarious 

 balance was maintained, the body being often slumped on to the 

 ground. In modern urodeles the limbs retain the full pattern of parts, 

 but with imperfect ossification, as would be expected since they carry 

 little weight. 



In frogs, specialized for jumping, the radius and ulna are united and 

 the carpals are reduced in number. There are only four true digits, the 

 first digit (thumb or pollex) being reduced. There is, however, a small 

 extra ossification, the pre-pollex, which becomes well developed as a 

 copulatory organ in the male and may be compared with a similar 

 digit found in some stegocephalians. It is to be expected that in a 

 system of repeated parts, such as a tctrapod limb, multiplications and 

 reductions will be common. It can be imagined that they can be 

 produced by changes in the rhythm of morphogenetic processes, and 

 it is surprising that there is such constancy in number of digits. 



