300 THE THIGH AND LEG. [chap. 



to the side of the astragalus, giving additional strength to 

 the articulation, called "ankle-joint." 



The fibula has a slender and generally compressed shaft, 

 and is somewhat expanded at each extremity. Its upper 

 end usually takes no part in the knee-joint, being con- 

 nected, by a separate synovial joint, with the tibia just 

 below that articulation. The lower end, however, forms 

 the outer side of the ankle-joint, under the name of 

 external malleolus. 



In many Mammals the fibula is in a more or less rudi- 

 mentary condition, and it often ankyloses with the tibia at 

 one or both extremities. 



As a general rule each of these bones has a principal 

 centre of ossification for the shaft, and an epiphysis at either 

 extremity. 



In the neighbourhood of the knee-joint, certain sesamoid 

 bones are often found in connection with the tendons which 

 pass over the various bony prominences. 



The largest and most constant is the patella, placed on 

 the anterior surface of the joint, in the conjoined tendon 

 of the four great extensor muscles of the leg, and having 

 a smooth articular facet, which plays upon the narrow 

 anterior part of the inferior articular surface of the femur, 

 and forms part of the wall of the cavity of the knee-joint. 

 This bone varies considerably in form, being in some cases 

 broad, flattened, or lozenge-shaped, and in others, laterally 

 compressed or oval. It is found in an ossified condition 

 in all Mammals, with the exception of a few of the Mar- 

 supialia. 



There are also very frequently smaller ossicles, one 

 or two in number, situated behind the femoral condyles, 

 called fabellce; and occasionally there is a wedge-shaped 

 bone within the joint, lying on the articular surface of the 



