. A manual of zoology. Zoology. IV. VERTEBRATA 465. terminal portions of the principal row produce the bones of the fifth, the ac- cessory rows those of the other fingers or digits. Then there is an unequal growth of parts; the metapterygium, already in Elasmobranchs a consid- erable element, increases in size and forms in the fore limb the humerus, in the hind limb the femur. In like manner the second clement of the principal row and the first of the first accessory row increase and form respectively ulna and radius in front, fibula and tibia behind. Then follow parts which remain small and s
. A manual of zoology. Zoology. IV. VERTEBRATA 465. terminal portions of the principal row produce the bones of the fifth, the ac- cessory rows those of the other fingers or digits. Then there is an unequal growth of parts; the metapterygium, already in Elasmobranchs a consid- erable element, increases in size and forms in the fore limb the humerus, in the hind limb the femur. In like manner the second clement of the principal row and the first of the first accessory row increase and form respectively ulna and radius in front, fibula and tibia behind. Then follow parts which remain small and some- what cubical, carpal bones in the fore limb, tarsals in the hinder ; they bear in turn slender bones, the metacarpals or metatarsals, and these at last the phalanges. (For the nomenclature of carpals and tarsals see the explanation of fig. 522.) A process on the upper end of the ulna is the olecranon; in the hind limb there is an analogous 'knee-pan' or patella in the tendon passing over the knee. The third and most important modification is brought about by the development of joints. So long as the appendage serves as an oar it must act as a single plate with its parts firmly held. On the other hand, when it must act as a system of levers to support and move the body, as is necessary in a terrestrial animal, it must be divided into sections, jointed to each other. By this there are developed two joints of importance in both fore and hind limbs; the elbow (knee) joint between humerus (femur) on the one hand and radius and ulna (tibia and fibula) on the other; and the wrist joint (ankle) between the bones of the fore arm (shank) and the carpals (tarsals). Less important are the joints of the fingers and toes. If the limbs of terrestrial vertebrates be compared with this primary form, variations are seen in two directions. Rarely are there more bones than in the schema; then there occur remnants of a sixth or even a seventh row or finger. More frequently there is a
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