LEPUS CUNICULUS 289 



The distal phalanges in both fore and hind limbs are of a 

 characteristic bent pointed shape, and are covered by the hollow 

 horny claws so that they are termed the ungual phalanges. 



The Pentadactyl Limb. 



Having now studied the fore and hind limbs of both frog 

 and rabbit we are in a position to consider the relation of one to 

 the other and to the limbs of air-breathing vertebrates in general. 



Fb Ce MT 



FIG. 96. Idealised diagram of pentadactyl limbs. 



_C., pre-axial centrale ; Ce., post-axial centrale ; D.C., distal carpalia ; D.T., distal tarsalia ; 

 F., fore limb ; Fb., fibulare ; Fe., femur ; F.H., head of femur ; Fi., fibulare ; H., hind limb ; 

 H.H., head of humerus ; Hu., humerus ; I., intermedium ; L., axis of h'mb ; M., main axis of 

 body ; M.C., metacarpal ; M.T., metatarsal ; Po., post-axial border of limb ; Pr., pre-axial border 

 of limb ; R., radius ; Ra., radiale ; T., tibia ; Ti., tibiale ; U., ulna ; Ul., ulnare ; I., pollex ; 

 I'., hallux ; II.-V., digits. 



Comparison will show that not only are the fore and hind limbs in 

 each animal composed of homologous parts, but also that there is a 

 fundamental similarity between those of Lepus and Rana. Indeed, 

 we can go further than this and sketch the plan of a primitive limb 

 from which most probably those of all vertebrates in the classes 

 Amphibia, Reptilia, Aves and Mammalia have been derived and 

 of which they can certainly be regarded as modifications. This 

 ideal type is known as the typical pentadactyl limb and is constituted 

 in the following way. The proximal element is a long cylindrical 

 bone, the humerus or femur, enlarged proximally to form a head 

 for articulation with the girdle and enlarged distally for articulation 

 with the succeeding elements. The next ''port ion is composed of 

 two bones, a radius and ulna or a tibia and fibula, which articulate 



u 



