THE INTERNAL SKELETON, 20. 
Bony supports for them may exist anchylosed to the inner side 
of the tarso-metatarsal bone. 
The articulations of the leg-bones are so formed that they 
mainly move with a hinge-like joint one on the other. This is 
especially the case with the joint between the tibia and the tarso- 
metatarsal bone. The articulation of the femur with the pelvis 
is more free, though not so free as that of the humerus with the 
thoracic girdle. The knee is capable of a slight rotation, espe- 
cially in aquatic birds, and in them again we find such a dispo- 
sition of the joints between the metatarsal condyle and the 
proximal phalanges as determines the spreading out or ap- 
proximation of the fingers by their mere flexion and extension. 
Many birds, especially the long-legged waders, can sleep securely 
on one leg owing to an arrangement of the bones which does not 
allow them to be flexed without an effort. A rounded prominence 
on the front of the proximal part of the tarso-metatarsal bone 
passes up, and locks into a depression on the front of the distal 
end of the tibia when the leg is straightened, and firmly main- 
tains the leg in this position, <A slight voluntary effort, how- 
ever, serves to unlock this junction, and allows the prominence 
to pass into the fossa between the condyles when the leg is 
bent, which forms a sort of socket for it, though a sharp promi- 
nence at the lower end of that ‘* socket ” prevents the process 
passing into it without such effort. This is well seen in the 
Stork. The connection of the head of the fibula with the side 
of the outer condyle of the femur also serves to maintain the 
limb in firmness and stability; although the knee-joint is 
almost constantly more or less flexed during sleep, the weight 
of the body keeping it so, 
