SKELETON 865 



third and fourth metatarsal bones, which are fused together and 

 bear on their proximal end an epiphysis-like mass containing several 

 of the distal tarsal bones. Except in Sphenisci and to a certain 

 extent in Psittaci, the three metatarsals do not lie in the same plane, 

 the middle one, which is the third, having its upper end thrust 

 backward and its lower forward, consequently the malleolus of the 

 third toe is more prominent on its anterior surface than its neigh- 

 bours, while on the same surface of the upper half or more of the 

 " metatarsus " there is a deep longitudinal furrow, and on the 

 posterior surface of the proximal end is found the " Hypotarsus.'' 

 Articulation with the tibio-tarsus is effected by an inner and an outer 

 concave facet for the corresponding " tibial " condyles, while these 

 two facets are separated in front by a knob Avhich fits into the inter- 

 condylar sulcus. Two rough surfaces near the upper end of the 

 anterior metatarsal sulcus indicate the position of a transverse liga- 

 ment, beneath which passes the common extensor tendon of the toes. 

 In one section of Owls (page 672) and in the Osprey (page 660) 

 this ligament becomes a strong bony bridge. A little lower than 

 the inner end of this ligament or bridge is a small tuberosity in- 

 dicating the insertion of the muse, tibialis anticus. 



The Hypotarsus is of some taxonomic value from its ridges which 

 end in furrows or canals for the passage of the several flexor 

 tendons, but in regarding its characters we must remember that the 

 conversion of a furrow into a canal is often due to extended ossi- 

 fication progressing with age. The Hypotarsus is simple, possessing 

 only one wide groove lying between two low ridges, in Sphetiisci, 

 Ciconiinse, Plataleinse, Phanicopteri, Palamedeas, Accipitres (excl. Pan- 

 dion), Crypturi, Cariama and Ehinochetus. It is also simple, though 

 with an additional median ridge, in Striges, Cypseli and Trochili ; but 

 complex, with high ridges and a canal or more than one in most 

 other birds, exhibiting numerous modifications, each of them directly 

 connected with the arrangement of the flexor tendons, and the size, 

 position and function of the toes. Intermediate forms are of 

 course numerous, and their careful study gives a valuable hint as to 

 the group to which any particular form belongs : in the Colymhi, 

 for instance, the two lateral ridges are very high and almost 

 completely enclose a wide triangular space. 



In the Sphenisci the three tarso-metatarsal bones are almost in 

 the same plane and are incompletely co-ossified, a feature frequently 

 regarded as primitive because of its likeness to embryonic and 

 therefore phylogenetically ancestral conditions. It may, however, be 

 reasonably conjectured that we have here a case of relapse, indeed a 

 pseudo-archaic formation produced in adaptation to the peculiar 

 plantigrade functions of the feet in these birds. An analogous con- 

 dition obtains in Fregata.^ 



^ The morphological meaning of the bones of the Bird's foot is best understood 



55 



