OSSEOUS SYSTEM OF AVES. 77 



VYhen the femur is pneumatic the proximal orifice is commonly 

 anterior, near the trochanterian ridge : but in the Ostrich it is 

 behind : the distal orifices when present are in the popliteal fossa. 

 Of the two condyles the outer one is most prominent posteriorly, 

 and, when the femur is held vertically, descends the lowest. In 

 the flexion of the leg on the thigh this puts the ligaments on the 

 stretch ; and, as they are partly elastic, the fibula enters its fossa 

 at the conclusion of the bend with somewhat of a jerk. 



The tibia, fig. 34, 66, is the chief or longest bone of the hind limb, 

 showing its extreme character in this respect in most Stilt-birds, 

 especially the Argala and Flamingo, fig. 14, and its smallest propor- 

 tions in Volitores, fig. 18, and the Frigate-bird, in Avhich the tibia is 

 not lialf the length of the skull. The shaft is straight and mainly 

 subtriedral, expanded at both ends and most so at the upper one. 

 This presents a semi-oval surface not quite transverse to the 

 shaft, but with the truncate margin raised toward the fore part of 

 the bone, and more or less developed above the level of the un- 

 dulating articular part. Of this the least marked is the almost 

 flat reniform ^ entocondylar ' surface for the inner condyle, feebly 

 hollowed near its back part which projects in that direction over 

 the shaft ; it is divided from the smaller and less defined ' ecto- 

 condylar ' surface for the outer femoral condyle by an ' inter- 

 condylar ' convexity. In advance of these the head of the tibia 

 extends into a ' rotular ' process, usually extended transversely 

 and truncate. From the fore or outer part of this process there 

 descend two vertical ridges or plates : the one from near the inner 

 or tibial angle of the rotular process is the ^ procnemial ridge,' 

 the other from the outer or fibular angle is the ' ectocnemial 

 ridge ; ' they subside more or less gradually upon the shaft, and 

 intercept a deep triangular concavity. On the outer side of the 

 intercondylar tuberosity is a single surface for ligamentous union 

 with the head of the fibula, and a little way below this there is a 

 vertical ridge for close attachment to part of the shaft of that bone : 

 below and behind the ' fibular ' ridge is the orifice of the medul- 

 lary artery. From this point the tibia maintains a uniform size 

 usually to its lower third, where along a rough tract, in a line 

 Avith the above ridge, the styliform end of the fibula terminates 

 by close union or anchylosis. There the tibia begins to gain 

 in transverse breadth, exchanging the triedral for a transversely 

 oval section, and it gradually expands in both directions to the 

 distal condyles, which are most developed from behind forward, 

 in advance of the shaft. The inner condyle is the largest, usually 

 in fore-and-aft, sometimes {Aquila) in transverse, extent. A 



