254 



ZOOLOGY 



SECT. 



the latter represents the pollex, and the complete digits are the 

 second to the fifth of the typical hand. Six carpals only are 

 present, the third, fourth, and fifth digits articulating with a 

 single bone which has apparently arisen by the fusion of the third, 

 fourth, and fifth distalia and of at least one centrale. 



The pelvic girdle (Fig. 874) is very peculiarly modified ; it re- 

 sembles in form a Bird's " merrythought," consisting of two long 

 curved bars articulating in front wjth the transverse processes of 

 the sacral vertebra (Fig. 870) and uniting posteriorly in an irregular 

 vertical disc of mingled bone and cartilage which bears on each side 

 a deep, hemispherical acetabulum (6r) for the articulation of the 

 thigh-bone. The curved rods are the ilia (II, , P) ; they expand 



posteriorly and unite with one another in 

 the median plane to form the dorsal 

 portion of the disc and about one-half of 

 the acetabulum. The posterior portions 

 of the disc and acetabulum are furnished 

 by the ischia (Is), fused with one another 

 in the sagittal plane, their ventral portions 

 by the similarly united pubes (Kri). The 

 ilium and ischium are formed of true 

 bone, the pubis of calcified cartilage ; 

 the union of the elements in the median 

 plane is called the symphysis. In the 

 larva the ilium is vertical, but during 

 development it becomes lengthened and 

 at the same time rotated backwards, thus 

 bringing the articulation of the hind 

 limbs as far back as possible. 



In the hind-limb the tibia and fibula 

 are fused to form a single tibio- fibula 



(Fig. 870, TI. Fi), and the two bones in the proximal row of the 

 tarsus probably the tibiale or astragalus (AST) and the fibulare or 

 calcaneum (CAL) are greatly elongated and provide the leg with 

 an additional segment. There are three tarsals in the distal row, 

 one of which appears to represent a centrale, another the first 

 distale, and the third the fused second and third distalia. There 

 are five well-developed digits, and on the tibial side of the first 

 is a spur-like structure or calcar (c), formed of three bones, a meta- 

 tarsal and two phalanges : such a rudimentary digit is called a 

 prce-halhix. 



All the long bones of the limbs consist of a shaft formed of 

 true bone and of extremities of calcified cartilage. The distinction 

 is a very obvious one, both in the freshly prepared and in the 

 dried skeleton. 



The muscular system has undergone great modifications in 

 correspondence with the complex movements performed by the 



FIG. 874. Rana esculenta. 



Pelvic girdle from the right 

 side. G, acetabulum ; II, P, 

 ilium ; Is. ischium ; Kn, 

 pubis. (From Wiedersheim's 

 Comparative Anatomy.) 



