3'4 



AMPHIBIA 



XII. 10 



are put. Evidently similar morphogenetic processes are at work in 

 both limbs. There are nearly always three main joints in each limb, 

 at shoulder (hip), elbow (knee), and wrist (ankle). The hand and foot 

 provide basically similar five-rayed levers, with several joints in the 

 digits (Figs. 185 and 186). 



The bones of the limbs can be plausibly derived from those of a 

 crossopterygian fin, and indeed the condition in *Eusthenopteron 

 already distinctly suggests that of the limb of an early amphibian (Figs. 

 179 and 180). We know less about the origin of the hind than of the 

 front leg, but the two are so similar that they may be treated together 

 for elementary analysis. There is a basal humerus (femur), articulating 

 distally with two bones in each case, a more anterior (pre-axial) radius 

 (tibia) and a posterior (post-axial) ulna (fibula). These bones articu- 

 late at the wrist or ankle with a carpus or tarsus, consisting, in the fully 

 developed condition, of three rows of little bones, namely 3 in the 

 proximal row, about 3 centrals, and 5 distals. Each of the latter carries 

 a digit, composed of numerous jointed phalanges. In naming these 

 bones of the carpus and tarsus it is convenient to call the proximal 

 carpals by their position radiale, intermedium, and ulnare and the 

 tarsals tibiale, intermedium, and fibulare. The centrals and distal 

 carpals may then be numbered beginning with 1 at the pre-axial 

 border in each case. Unfortunately other less explicit systems of 

 naming are in use, as shown in the following table. 



Plan of the Tetrapod Carpus and Tarsus. {The names used for the bones 

 in man are shown in brackets.) 



Carpus 



The plan of the carpals and tarsals can well be imagined to have been 

 derived from that of a fin such as is seen in the fish *Eusthenopteron 

 (Fig. 179), which might be said to have humerus, radius, and ulna, 



