OSTEOLOGY OF BIRDS 



77 



slightly elevated, we find the broad articular surface for the anti- 

 trochauter of the pelvis. Its breadth increases as it recedes from 

 the head of the bone, and is carried slightly over the summit of the 

 ridge. 



This bone is gently arched throughout its length, the concavity 

 being upon its posterior aspect. Its shaft is very smooth, for the 

 most part cylindrical on section, and but faintly shows the usual 

 muscular lines adown its length ; the most distinct one passing from 

 the forepart of the trochanterian ridge to the outside boundary of 

 the inner condyle. At the anterior aspect of the lower third of the 

 shaft, we observe that the ridges which are the beginnings of the 

 condyles are parallel with each other until they disappear at the 

 lower end of the bone. They are quite prominent and thus give 

 rise to a well marked " rotular channel." A very noticeable thing 

 about the femur of Circus is that the condyles are about on the same 

 level at their lowest points. If the bone is held vertically against a 

 plane surface tangent to these points, the axis is very nearly per- 

 pendicular to the plane. This is by no means the rule with a great 

 majority of birds, where the inner condyle is produced beyond the 

 outer one. It is well shown in a specimen of Geococcyx. 



The intercondyloid fossa is broad and fairly divided from the 

 popliteal depression by a low transverse bar. As usual, the outer 

 condyle is vertically cleft behind to afford an articular cavity for the 

 head of the fibula. The little tuberosities for muscular and liga- 

 mentous insertion about this end of the femur in Circus are well 

 marked, and the foramen for the entrance of the medullary artery 

 occupies its usual site on the posterior aspect of the shaft below the 

 juncture of the upper and middle thirds. 



In the skeleton of Circus, as it is ordinarily prepared for study, 

 the tibia and fibula are highly characteristic of the nonpneumatic 

 class of bones, being dark, and for the most part of a deep amber 

 color and greasy. The former is but little curved forward, as we 

 sometimes see it, the shaft being very straight from any point of 

 view [fig. 30]. Seen directly from above, the proximal articular 

 surface for the condyles of the femur is nearly square [fig. 32]. 

 The intercondylar convexity is but feebly pronounced, and the rotular 

 crest of -the bone rises but slightly above the general undulating 

 articular surface. The apex of the ectocnemial ridge points directly 

 outward, the opposite or procnemial ridge being developed as a 

 crest parallel with the outer surface of the fibula, and produced some 

 distance down the shaft of the bone. Below, and on the outer side 



