EXTERNAL PARTS OF BIRDS.— THE FEET. 
119 
scarcely ever have more, and the principal lessenings of the number result from the absence 
of one or two toes, or a slight reduction in the number of the joints of some toes, or absence of 
the knee-cap. Of the normal twenty, fourteen are bones of the toes ; one is an incomplete 
bone connecting the hind toe with the foot ; one is the knee-cap, and four are the principal 
bones of the thigh (1), leg (2), and foot (1). The fi^'st or uppermost is the thigh-bone or 
femur (Lat. femur ; adjective, femoral), fm, from hip to knee, ^ to 5 in the figure. It is 
ii rather short, quite stout, cylindrical bone, enlarging above and below. Above it has a 
globular head, a, standing off obliquely from the shaft, received in the acetabulum (Lat. aceta- 
bulum^ a kind of receptacle) or socket of the hip, and a prominent shoulder or trochanter, 
which abuts against the 
brim of the acetabulum. ^ 
Below, it expands into 
two coyidyles (Gr. Kovbv- 
Aoy, a knob), for articu- 
lation with both the 
bones it meets at the 
knee. It is the same 
bone as the femur of a 
quadruped or of man, 
and corresponds to the 
humerus of the wing. 
In the knee-joint, many 
or most birds have a 
small ossicle, and a few 
have two such bony nod- 
ules, not shown in the 
figure, but nearly in the 
position of the letter B : 
the knee-pan or knee- 
cap, patella (Lat. patel- 
la). The thigh is the 
first segment of the limb ; 
the next segment is the 
leg proper, or crus (Lat. 
-crus, the shin ; adjective, 
crural), 5 to C in the 
figure, or from knee to 
heel. This segment is 
occupied by two bones, 
the tibia (Lat. tibia, a 
tube, trumpet), tb, and 
fibula (Lat. fibula, a 
splint, clasp), fi. Of 
these the tibia is the 
principal, larger, inner 
l>one, running quite to the heel ; the fibula is smaller, and (with rare exceptions, as in some of 
the penguins) only runs part way down the outside of the tibia as a slender pointed spike, close 
pressed against or even partly fused with the shaft of the tibia. Above, at the knee, both 
bones articulate with the femur ; the tibia with both the femoral condyles, the fibula only with 
the outer condyle. Above, the tibia has an irregularly expanded head or cnemial process (Gr. 
Fig, 34. — Bones of a bird's hind limb : from a duck, Clangula islandica, | nat. 
size; Dr. R. W. Shufeldt, U.S.A. A, hip: B, knee: C, heel or ankle-joint; D, 
bases of toes. A to £, thigh or " second joint " ; -B to C, crus, leg proper, " drum- 
stick," often wrongly called " thigh " ; C to D, metatarsus, foot proper, correspond- 
ing to our instep, or foot from ankle to bases of toes ; in descriptive ornithology 
the tarsus; often called "shank." From D outward are the toes or digits, fm. 
femur ; tb, tibia, principal (inner) bone of leg ; fibula, lesser (outer) bone of 
leg; mf, principal metatarsal bone, consisting chiefly of three fused metatarsal 
bones; am, accessory metatarsal, bearing 1^, first or hind toe, with two joints ; 2t, 
second toe, with three joints; 3t, third toe, with four joints; 4/, fourth toe, with 
five joints. At C there are in the embryo some small tarsal bones, not shown in 
the figure, uniting in part with the tibia, which is therefore a tibio-tarsus, in part 
with the metatarsus, which is therefore a tarso-metatarsus ; the ankle-joint being 
therefore between two rows of tarsal bones, not, as it appears to be, directly be- 
tween tibia and metatarsus. 
