GENERA L CHA RA C TERIS TICS 



1467 



FRONT VIEW OF THE RIGHT 

 HUMERUS OF A GULL. 



With regard to the limbs, the bones of a bird's wing correspond generally 

 to those of the arm or fore-leg of a Mammal; the arm bone or humerus having 

 distinct condyles (a, b) for the articulation of the bones of the fore-arm (radius and 

 ulna), and being sometimes furnished with a projecting process above the outermost 

 of these two condyles. The two bones of the fore-arm always remain separate from 

 one another. In the wrist the numerous bones found in Mammals are, however, 

 reduced to two, and the metacarpus and hand are flattened and specially modified. 

 Thus there are never more than three digits, which are usually without claws 

 although among recent birds two may be thus 

 armed. The thumb, or first digit, is represented 

 merely by one or two joints (as shown in the 

 skeleton on p. 1464), and carries the so-called 

 bastard wing; while the other two digits repre- 

 sent the index and middle fingers of the human 

 hand. Their respective metacarpals, as seen in the figure cited, are united at their 

 two ends, so as to form a single bone; while the index finger has two flattened 

 joints, and the third finger (not present in the figure) but one. 



In the hind-limb there is a still wider departure from the Mammalian type. 

 The uppermost bone in a bird's leg (A of the accompanying figure) is the thigh 

 bone, or femur; below this comes the tibia, or larger bone of the lower leg, on the 

 outer side of which is a small splint (not shown in the figure) representing the 



fibula. Below the tibia comes another long bone, 

 terminating (except in the ostrich, where they 

 are reduced to two) in three pulley-like surfaces, 

 known as trochleae, to which are articulated the 

 toe bones. Obviously, then, this third long bone 

 corresponds to the metatarsus of a Mammal, con- 

 sisting in fact of the three middle metatarsals of 

 the typical five-toed limb welded together, in the 

 same manner as two such metatarsals are united 

 in the hind-limb of a ruminant Mammal. It 

 may, therefore, be called either the metatarsus or 

 the canon bone. The reader will, however, now 

 ask what has become of the ankle or tarsus in 

 the bird's leg. To this it may be replied that its 

 upper bones have united to the lower end of the 

 tibia; while the lower row has joined the upper 

 end of the canon bone. The figure on p. 1468 

 exhibits the lower end of the tibia of an adult 

 crane and of a young ostrich; and it will be seen 

 that in the latter the upper ankle bone is still 

 distinct, while in the former it has become com- 

 pletely united with the tibia. A precisely similar 



state of things takes place in the formation of the canon bone. It will, therefore, 

 be apparent that the tibia of a bird corresponds to the tibia, plus the upper half of 



A. BONES OF THE RIGHT LEG OF A 

 MOA; B. CANON BONE OF SAME 

 ON A LARGER SCALE. 



