52 THE FROG CHAP. 



frog's leaping powers : those of the distal row are very 

 small. 



The metatarsals are five in number : those of the first 

 and second digits (I, II) bear two phalanges each, 

 those of the third and fifth, three each, and that of the 

 fourth, four. Attached to the inner side of the tarsus 

 is a little claw-like structure (C) composed of two or 

 three bones and called the calcar or spur. 

 . Notice the striking correspondence in structure be- 

 tween the fore- and hind-limbs, a correspondence which 

 extends also, though less obviously, to the limb-girdles. 

 The humerus corresponds or is serially homologous 

 (p. 39) with the femur, the radius with the tibia, the ulna 

 with the fibula, the carpals with the tarsals, the meta- 

 carpals with the metatarsals, and the phalanges of the 

 fingers with those of the toes. Then in the limb-girdles 

 the glenoid cavity corresponds with the acetabulum, 

 the scapula and supra-scapula (being above the articular 

 cavity) with the ilium, the precoracoid and clavicle 

 (being ventral and anterior in position) with the pubis, 

 and the coracoid with the ischium. Thus not only are 

 the limbs and limb-girdles serially homologous structures, 

 but their several parts are also serially homologous, 

 each to each. 



Nature of Bone. It is a mistake to suppose that 

 bones are made exclusively of hard mineral matter, like 

 rocks or stones. If one of the long bones, for example, 

 is put into weak acid, bubbles of gas will rise from the 

 bone, showing that the phosphate and carbonate of lime, 

 of which it is partly composed, is being decomposed 

 with the liberation of carbonic acid gas. When the 

 liberation of bubbles is over, the bone will be found to be 

 unaltered in form, but to be quite flexible instead of 

 hard and rigid. It can be bent in any direction, and a 

 bone of sufficient length, such as a sheep's rib, can be 



