8 ARBOREAL MAN 



is that seen in living forms among the tailed Amphibians, 

 and in some of the less specialized Reptiles. This we 

 may define as a limb of three segments: arm, forearm, 

 and hand; thigh, leg, and foot (see Fig. 1). The first 

 segment consists of muscles massed round one central 

 bone humerus or femur; the second segment of two 

 parallel bones ulna and radius, or fibula and tibia, and 



FIG. 1. DlAGEAMMATIC DRAWING TO SHOW THE CONDITION 



OF PRIMITIVE LIMBS. 



their regulating muscles ; and the third of a series of small 

 bones carpals or tarsals, with the muscles and bones of 

 the separate digits. This limb is possessed of a high 

 degree of mobility. It can move in all directions on the 

 trunk at the shoulder and hip joints; the second segment 

 is enabled to move on the first by bending or straighten- 

 ing of the elbow or knee; the two parallel bones of the 

 second segment may move upon each other, so that the 

 third segment may be moved with the second, and be 

 turned (at any rate to some extent) palm or sole up 

 supinated, or knuckles up pronated (see Figs. 2, 3). 

 Finally the third segment is free to move on the second 

 in a variety of ways at the wrist and ankle. Such is the 

 limb which is the heritage of all land-living Vertebrates, 

 and such a limb is beyond doubt the heritage of the an- 

 cestral Mammal. The functions of this primitive limb 

 are simple in the extreme; it enables the animal to walk 

 about under water, and it serves to drag the animal about 



