146 PELVIS AND BONES OF THE LEGS OF BIKDS. 



part of the trochlea is a deep depression, and in many 

 birds an osseous bridge extends across it. 



The third segment of the leg, 69, is a compound bone, 

 consisting originally of one proximal piece, short and 

 broad, presenting two articular concavities to the two thick 

 and round borders of the tibial trochlea, of three meta- 

 tarsals which coalesce with each other and with the above 

 tarsal piece, and of one or more bony processes which are 

 ossified from the back part of the proximal piece, or from 

 the proximal ends of the metatarsals, and which, from 

 their relations to the extensor tendons, are called "cal- 

 caneal" processes. In most birds, a small rudimental 

 metatarsal, supporting the innermost toe, or "hallux," z, 

 is articulated by ligament with the innermost of the 

 coalesced metatarsals, and is properly included in the 

 same segment of the limb. The three principal metatar- 

 sals are interlocked together before they become anch}^- 

 losed, the middle one being w^edged into the back part of 

 the interspace of the two lateral ones above, and into the 

 fore part below, passing obliquely between them. The 

 period at which these several constituents of the " tarso- 

 metatarse" coalesce is shorter in the birds that can fly 

 than in those that cannot ; and the extent of the coales- 

 cence is least in the penguins, in which the true nature of 

 the compound bone is best seen. 



The modifications of the tarso-metatarse are chiefly 

 manifested in its relative length and thickness, in the 

 relative length of the three metatarsals, and in the number 

 and complexity of the calcaneal processes. 



The inner of the two cavities for the condyles at the 

 proximal end of the bone is the "entocondyloid" cavity 

 or surface, the outer one the " ectocondyloid" surface ; they 

 are separated by an " intercondyloid" tract, from the fore- 



