158 FOSSIL BIRDS. 



points ; namely, that of the greatest flexion, and that of the 

 most perfect extension. Those are the only points in which the 

 ligaments are not stretched, and in which the bones are pre- 

 served in their proper places by the simple action of these 

 ligaments, unless the bird makes an effort to displace them. 

 The head of the peroneum produces this effect, by its mode of 

 catching in a particular fossa of the femur. 



This head enlarges very much from front to rear, and its 

 upper edge is nearly a straight line, which mounts obliquely 

 behind, thus rendering its posterior extremity more elevated 

 than the other. The femur rests upon this right line by a 

 projecting line drawn over its external condyle, the middle of 

 which forms an almost semicircular convexity, while the two 

 ends, on the contrary, are a little concave, and the two bones 

 are attached in this place by an elastic ligament which goes 

 from one to the other, crossing almost perpendicularly the line 

 by which they touch. 



It is clear, then, that this ligament will be more stretched, 

 according to the degree in which the femur shall touch the 

 peroneum by the convexity of the projecting line which we 

 have described : that is, so long as the leg is neither completely 

 extended, nor completely bent. But in these two extreme po- 

 sitions the peroneum will re-enter into one of the concavities 

 placed at the two ends, and will be retained there by the elastic 

 contraction of the ligament. 



The femur, then, of birds is distinguished from that of qua- 

 drupeds in this, — that its external condyle, instead of presenting 

 a simple convexity behind, for the external fossa of the head 

 of the tibia, presents two projecting lines : one stronger, which 

 is the true condyle, and which corresponds to the upper exter- 

 nal facet of the tibia, and the internal facet of the peroneum ; 

 and another more exterior, descending less, and resting on the 

 upper edge of the peroneum. The external condyle, therefore, 

 of birds is forked, or hollowed into a canal more or less deep 

 behind. 



