66 THE SURGICAL ANATOMY OF THE HORSE 



bone. Its external surface is slightly convex, and roughened for the 

 attachment of the external lateral ligament of the stifle. Its inner 

 surface is also roughened. It is somewhat flattened, and is accom- 

 modated in the depression on the outer aspect of the external tuberosity 

 of the tibia. 



The body is a slender rod of bone, which tapers to a point inferiorly. 

 For two or three inches at its inferior end it is closely applied to the outer 

 edge of the tibia, but above this portion there is an interval between 

 the two bones which in the living animal accommodates an interosseous 

 membrane which binds the two bones together. 



The fibula develops from two centres of ossification, one for the head 

 and the other for the body. 



This bone is very rarely fractured. The peroneus muscle affords it a 

 considerable degree of protection. In cases where fracture does occur 

 diagnosis is difficult, owing to its concealed position, the head being the 

 only part which can be felt in the living animal. Moreover, fracture of 

 this bone is not of great consequence, since it does not materially affect 

 the action of the limb, and the tibia performs the function of a natural 

 splint in maintaining the parts in position. The most serious com- 

 plication which might possibly occur is injury to the vessels passing 

 through the tibio-fibular arch when the bone is fractured just below 

 the head and there is inward displacement. 



THE BONES OF THE TARSUS 



The Astragalus. — In shape this bone is very irregular, but for 

 purposes of description it may be said to possess six surfaces, which 

 may be named anterior, posterior, superior, inferior, external lateral, 

 and internal lateral. 



The superior and anterior surfaces are blended, and present two 

 parallel articular ridges, separated from one another by a deep articular 



