52 THE FROG chap. 



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 Httle clavv-hke structure (C) composed of two or three 

 bones and called the calcar or spur. 



Notice the striking correspondence in structure 

 between 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 metacarpals 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 procoracoid 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 i)ut 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, ^\'hen the liberation of 

 bubbles is cner, the bone will be found to be unaltered 

 in form, but to be quite flexible instead of hard and rigid. 

 It <:an be bent in any direction, and a bone of sufficient 

 length, such as a sheep's rib, can be tied in a knot. This 

 shows that the bone contains a large amount of organic 

 or animal matter. On the other hand, a bone may be 



