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 e.xtremity; 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 reduction in the number of parts, 

 either by fusion or by absolute loss. Fusion accounts for the fact 

 that with complete pentadactyly the number of carpalia is usually less 

 than the ten, to be expected from the schema. Degeneration and loss 

 explain the existence of animals with four, three, two, or even one digit, 

 and one can say with certainty that the missing parts are in most cases 



30 



Fig. 522. — Schema 

 of a pentadactyle 

 appendage (after Ge- 

 genbaur). The dot- 

 ted lines indicate the 

 lateral rays; the 

 names for the hinder 

 extremities in paren- 

 theses. H, humerus 

 (femur); t'.ulna (hb- 

 ula) ;i?, radius (tibia). 

 Carpus (tarsus) con- 

 sisting of two rows 

 and two central por- 

 tions: Row I; r, 

 radiale (tibiale); z, 

 intermedium; », ul- 

 na re (iibulare:) c, 

 centraUa. Row II: 

 1-5, carpalia (tar- 

 salia) ; the metacar- 

 pjals (metatarsals) 

 and phalanges not 

 lettered. 



