2G2 ANATOMY OF VERTEBRATES. 



a multiplication of its segments with mobility of its ribs But the 

 verteljrao are specially modified, to compensate, by the strength of 

 their numerous articulations, for the weakness of their manifold 

 repetition, and the consequent elongation of the slender column. 

 As serpents move chiefly on the surface of the earth, their danger 

 is greatest from pressure and l)lows from above ; all the joints are 

 fashioned accorchngly to resist yielding, and sustain pressure in a 

 vertical direction. 



§ .50. Locomotion of limbed Reptiles. — The fish-like Batrachia 

 move in water by means of the lateral inflections of the hinder- 

 half of the trunk, which is compressed and extended vertically 

 by a marginal tegumentary fin. The pjarial limbs are small and 

 feeble : they are limited, in the amphibious Siren, to the pectoral 

 region, and to the function of raising the head and fore-part of 

 the trunk upon the Ijank or shore. In the rest of the order 

 both pairs arc present : in the Amphiumc they are too feeble to 

 suggest any particular locomotive function ; hut they subserve, 

 when somewhat more developed, a slow and awkward reputation, 

 as in the Menopome and Newt. In the Land-Salamander, fig. 

 140, they acquire the due strength for terrestrial progression, and 

 the tail is shortened and rounded. In the Toads and Frogs the 

 tail is absorbed, and the legs lengthened and strengthened, espe- 

 cially the hinder pair ; but with an outward direction from the 

 body, and a position too horizontal to enable them to raise or 

 support it alaove the ground. 



The Frog, in repose, assumes a sitting posture, the thighs 

 turned outward and forward, the legs bent backward, and the 

 lengthened tarsi and feet directed forward. The fore-part of 

 the trunk is propped up by the fore limbs, at an angle of 45^, 

 with the base between the hind limbs, which, in their state of 

 flexion, are ready on the least alarm to project the body forward 

 by their sudden extension. The shoulder-joints of the limbs 

 that receive the shock on alighting from the leap are strength- 

 ened by an interarticular ligament. The great Bull-Frog may 

 clear six feet at a leap, and repeat them so rapidly as to escape a 

 l)ursuer, unless chased at a great distance from the water. Both 

 fore and hind feet arc webbed for swimming, which is chicflv 

 effected by strokes of the strong hind liml)s. Tiie large Indian 

 frog {Rana tigrinci) is said to be able to run along the surface of 

 the water for a short distance. 



Tlic Tree-Frogs (Tli/la) have a concave disc at the end of 

 each toe, for clhnljlug and adhering to the bark and lca\cs of 

 trees. 



