ORNITHOLOGY. 271 



are protected by a naked, scaly skin which, in some cases, ex- 

 tends above the tarsus, and partly up the leg. The soles of the 

 toes are covered with a granulated and callous modification of 

 this skin. (Plates IX. and X.) 



In Birds, particularly those of extended and powerful flight, 

 the greatest part of the muscular force centers in the wings. 

 The muscles which produce the downward stroke of the wing, 

 are enormous; for their attachment, the breast-bone is greatly 

 enlarged, and its surface is also increased by having its middle 

 portion raised into a perpendicular ridge, the two faces of which, 

 from their direction, afford an advantageous point of resistance, 

 or purchase. A bird can move its wings with a degree of 

 strength which, when compared with the animal's size, is almost 

 incredible. The flap of a Swan's wing has power sufficient to 

 break a man's leg ; and a similar blow from an eagle has been 

 known to produce instant death. 



The powerful muscular action involved in flight, would nat- 

 urally tend to draw the shoulders together; b it this tendency is 

 resisted by the insertion between the two bones (coracoids) to 

 which the shoulder bones (H) are joined, of a singular arched 

 bone, called the wish-bone or merrythought, (G.) In the domes- 

 tic fowl, the bone is feeble; but in birds of powerful flight, as 

 the Hawks, the Swallows, and the Humming-Birds, it is very 

 strong and elastic. On the other hand, when the bird never rises 

 upon the wing, as in the case of the Ostrich and Emu, this bone 

 is reduced to a mere rudiment. The bones of the lower, or pos- 

 terior extremities also differ materially in structure from those 

 of quadrupeds. These consist of (1) a thigh-bone, (or femur,) R. ; 

 leg. bones, (tibia and fibula,) S. ; (these leg-bones are really two, 

 but the fibula is very small, and becomes anchylosed to the 

 tibia; i. e., immovably fixed by a continuation of bony secre- 

 tion ;) (2) the metatarsal,or shank-bones, U. U., at the lower end of 

 which there are as many processes as there are toes, each pro- 

 cess being furnished with a pulley for moving its corresponding 

 toe ; (3) the toes, of which the usual number is four, a number 

 never exceeded ; while a few birds have only three ; and the 

 Ostrich only two. The three toes are directed forwards, and 

 one, answering to the great toe, backwards. This, at least, is the 

 general rule. The back, or great toe, is wanting in some birds. 

 In the Swallows it is directed forwards ; but in the Climbing 

 Birds, as the Parrots and Woodpeckers, the outer toe and back 

 toe are both directed backwards; while the Swifts have all the 

 four toes directed forwards. 



As the upper limbs, or anterior extremities, are exclusively for 



