THE ANATOMY OF A BItih 15 



of the ox or horse, upon which the creature walks, are one and the 

 same thing. It might be supposed that the hand of the bird, which 

 is not an organ of offence or meant to walk with, might be shorn of 

 these appendages. But this is not the case : every bird has at 

 least two nails (fig. 9), of a long and rather claw-like form when well 

 developed, and sometimes three nails, that is, one to each of its fingers. 

 It looks, therefore, very much as if the wing of the bird had been 

 formed out of a limb that was once an organ for climbing or walking 

 with. There is a curious bird, found in British Guiana, which is 

 known as the Hoatzin (figs. 9, 11). In the very young nestlings of the 

 hoatzin the claws of the fingers are so conspicuous that they are 

 actually used by the callow chick to climb with, before the feathers 

 of the wings have grown sufficiently to enable them to use their wings 

 in the proper way in which a bird should ; it has been said also, that 



III 



< 6 5 ' 



FIG. 9. WING OF NESTLING OPISTHOCOMUS. (After Pycraft in 

 ' Natural Science.') 



The second digit (II) is free, being prolonged beyond ala membrane (P.m.), 

 and remiges 8-10 are not developed. 



other birds scramble about and use their claws when they are young. 

 In the case of the hoatzin, it is stated that the thumb and the first 

 finger can be brought together so as to lay hold definitely of an 

 object. A very important thing to notice about the wing bones is 

 that they are capable of but little movement upon each other. There 

 are two hinges, one at the elbow, and the other at the wrist ; but the 

 radius and ulna cannot move round each other, as they can in our 

 arms, and the fingers are fixed and rigid. This would be most un- 

 fortunate if the wing had to be used as a walking or climbing limb ; 

 but it is most useful in relation to the function which the wing has 

 to perform that of flight. The strength of the downward stroke 

 would be enfeebled if the bones were in a limp condition and moved 

 upon each other. They offer, too, a firm foothold for the thick 

 quills of the big feathers of the wing. It has been mentioned that 



