FREE APPENDAGES BIRDS 



289 



embryo on the ulnar side (fig. 312) of the last persisting digit, with which it soon 

 fuses, it follows that the numbering 3, 4, 5 cannot hold. Analogy with other 

 animals (p. 279) would tend to show that digit one often is the first to disappear, 

 thus affording support to the numbers 2, 3, 4 adopted above. 



Sometimes digit 3 has three phalanges, and then digit 2 has two. Archce- 

 opteryx (which possibly used its digits in part for grasping) has the phalanges 2, 3 

 and 4. Penguins lack digit 2, 4 being very long. Some Struthionines have only 

 digits 3 and 3, Apteryx and Dromcciis only 3. 



As it is the only locomotor appendage in walking or swimming, 

 the leg is usually strong and sometimes (wading birds) is very long. 

 The femur is relatively short and stout; its head and neck are at 

 right angles to the shaft, the hemispherical head having a groove for 

 the teres ligament. The shaft is directed forwards so as to bring the 

 centre of gravity over the feet. The upper end has both tibular and 

 fibular trochanters, which, owing to the torsion of the bone, face 

 backwards and forwards respectively. A patella is common. In 

 the crus the tibia is strong, the fibula reduced and often fused with 

 the tibia, its lower end never reaching the tarsus, its upper end always 

 articulating with the femur. The tibia usually has one 

 or two anterior (cnemial) ridges for origin of the foot 

 muscles and its lower end has two pulley-hke articular 

 surfaces, separated by a groove (sometimes bridged by 

 bone) for the passage of a tendon. 



The tarsus is reptihan in its intratarsal joint. The 

 embryo has three proximal tarsal cartilages which soon 

 fuse as the 'astragalus;' this has an ascending process, 

 the intermedium, extending up on the tibia. Soon the 

 astragalus fuses with the tibia to a tibiotarsus, the 

 astragalus remaining distinct only in Apteryx. From 

 one to four tarsaha appear in the embryo and soon 

 fuse with the proximal ends of the united meta- 

 tarsals, the result being a tarsometatarsus (fig. 313), 

 the tarsal bones thus being lost as separate bones in the stedt, '72, in 



° Bronn). 



adult. 



There are, at most, but two tarso-metatarsal bones in the adult. 

 The larger of these varies in length, being very long in wading birds, 

 shortest in penguins. Its ends are expanded, the upper having two 

 articular faces, the lower three pulleys (fig. 313), an evidence of the 

 compound nature of the bone. The other metatarsal, when present, 

 is at some distance down on the tibial side and disappears with the 



Fig. 313. — 

 Tarso - m e t a- 

 tarsus of swan 

 (Quenner- 



