THE INTEBNAL SKELETOIf. 201 



Bony supports for them may exist anchylosed to the inner side 

 of the tarso-metatarsal bone. 



The articulations of the leg-hones 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 fingets by their mere flexion and extension. 

 Many birds, especially the long-legged wadgrs, can sleep securely 

 on one leg owing to an arrangement of the bones which does not 

 allow them to be flexed without an efiort. 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. 



