6o THE BIOLOGY OF BIRDS 



marked contrast to the cruro-tarsal ankle-joint of mammals. 

 For when a mammal moves its ankle-joint, the lower end 

 of the tibia works against one of the upper tarsals, the 

 astragalus, and to a slight extent against the other, the 

 calcaneum or os calcis. The last-named bone which 

 projects so conspicuously at the heel of the mammal is 

 equally conspicuous by its absence in the bird. Yet it is 

 there all the time. 



The adaptations in this part of the skeleton are three. 

 (a) The suppression of the separate tarsals, which usually 

 occur in two mobile rows, makes it easier for the bird to 

 recover its balance when it suddenly alights on the ground 

 or on a perch, (b) The almost vertical position and the 

 length of the tarso-metatarsal region (half of the ankle and 

 the bulk or the whole of the instep) facilitate the sudden 

 upward and forward spring when the bird launches itself 

 into the air. (c) The fusion of three metatarsals (two in 

 the ostrich) into one long bone is comparable to the fusion 

 of the third and fourth metatarsals (or metacarpals) in the 

 leg of a cow ; it makes a firm step and a strong spring 

 easier, it simplifies the disposition of the tendons, and it 

 lessens the risk of dislocation and sprain. It is interesting 

 to notice the partial separateness of the lower ends of the 

 three metatarsals in the penguins, where the hind leg 

 serves in great part as a propeller. Hilzheimer points out 

 that this is a secondary peculiarity, for the fossil penguins 

 are nearer the usual type as regards their metatarsals. In 

 divers and grebes the tarso-metatarsus is laterally compressed 

 like a blade, an adaptation to reduction of friction in swim- 

 ming, and concentrating the stroke in the toes which form 

 the instrument that strikes the water. 



The maximum number of toes in a normal bird is 

 four, of which the first is the hallux or big toe. If there be 

 four, the metatarsal of the hallux is free from the other 

 three fused metatarsals and is turned backwards. If there 

 are only three toes, the hallux has been suppressed. The 

 obvious general adaptation in the majority of cases is the 

 elongation and horizontal spreading out of the toes, affording 



