GENERAL CHARACTERISTICS. 
295 
one another. In the wrist the numerous hones found in Mammals are, how¬ 
ever, 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 repre¬ 
sented merely by one or two joints (as shown 
in the skeleton on p. 292), and carries the so- 
called bastard-wing; while the other two digits front mew op the right humerus 
represent 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 sur¬ 
faces, known as trochlese, to which are articu¬ 
lated the toe-bones. Obviously, then, this third 
long bone corresponds to the metatarsus of a 
mammal, consisting 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 cannon-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 
cannon-bone. The figure on p. 296 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 completely 
united with the tibia. A precisely similar state of things takes place in the for¬ 
mation of the cannon-bone. It will, therefore, be apparent that the tibia of a bird 
corresponds to the tibia, plus the upper half of the ankle, of a mammal; while the 
cannon-bone represents the metatarsus, plus the lower half of the ankle. Hence, 
while the ankle-joint in a mammal occurs between the tibia and the upper row of 
ankle-bones, in a bird it is placed between the upper and lower rows of the ankle. 
The bony bridge seen at a in the tibia of the crane is very commonly present in 
birds; it acts as a pulley for the tendons of the muscles of the front of the leg 
A B 
A, BONES OF THE RIGHT LEG OF A MOA ; 
B, CANNON - BONE OF SAME ON A 
LARGER SCALE. 
