70 GENERAL ZOOLOGY 



origin of the tibiofibula is indicated by deep grooves in either 

 end of the shaft, the tibial component being preaxial and the 

 fibular postaxial. The ankle or tarsus of the frog is highly 

 modified. Adjoining the distal end of the tibiofibula are two 

 long bones, united at either end but separate in the middle, 

 called the tibiale or astragalus and the fibiale or calcaneum, 

 preaxial and postaxial, respectively. Beyond these are two or 

 three smaller tarsal bones, the largest of which represents the 

 fusion of two bones. There are five metatarsal bones, correspond- 

 ing to the five metacarpal bones. There are five toes of which 

 digits I and II contain two -phalanges each, digits III and V three 

 each, and digit IV, four. On the preaxial side of the first digit 

 is the prehallux or calcar, composed of one or two parts, and 

 regarded by some as a rudimentary digit. 



Limbs in General. — The similarity in the structural plan of the 

 fore- and hindlimbs of the frog is also true of the limbs of other 

 vertebrates. The limbless condition of snakes is secondary and 

 is the result of loss of limbs. All vertebrates above fishes are 

 believed to have had limbs at some time. If one wishes to get 

 some idea of what the primitive vertebrate limb was like, one 

 should study an amphibian in which the limbs are not highly 

 developed. The frog does not serve for this purpose because 

 of the obvious structural and functional specialization of its 

 limbs. Instead, one should turn to a tailed amphibian, such as 

 Amby stoma microstomum, which has relatively simple limbs. Its 

 locomotion consists of slow crawling movements in which the 

 body is raised just high enough to clear the ground, the tail 

 dragging. The limbs move principally in a horizontal plane, the 

 entire volar or plantar surface of the foot touching the ground 

 at each step. This type of foot posture is known as plantigrade. 

 In more rapid movements the body is thrown into snakelike, 

 side-to-side undulations by contractions of the trunk and tail 

 muscles, which accompany the movements of the legs. The 

 undulatory movements resemble the swimming movements of 

 these animals, except that in swimming the legs are not used. 



The proximal element of the limb skeleton of the salamander is 

 a single bone, the humerus in the forelimbs and the femur in the 

 hindlimb. The radius and ulna of the forelimb are separate 

 bones as are also the tibia and fibula of the crus. The carpal and 

 tarsal bones are relatively unspecialized and vary from seven to 



