104 THE SKELETON OF THE HORSE 



really the distal end of the fibula; it is a separate piece at birth, and the line of 

 union is commonly quite evident in the adult in the external articular groove. The 

 proximal end unites with the shaft at about three and a half years, and the distal 

 end at about two years of age. 



THE FIBULA 



The fibula of the horse is a much reduced long bone, situated along the outer 

 side of the tibia. 



The shaft or body (Corpus fibulae) is a slender rod which forms the outer bound- 

 ary of the interosseous space; it usually terminates below in a pointed end a]:)out 

 one-half to two-thirds of the way down the external border of the tibia. 



The proximal extremity or head (Capitulum fibulae) is relatively large, and is 

 flattened transversely. Its internal surface presents a narrow area along the upper 

 border for articulation with the tibia. The external surface is rough and gives 

 attachment to the external lateral ligament of the stifle joint. It has rounded 

 anterior and posterior borders. 



Poplitenl 

 Groove for poplitois tendon 't:' ', <^- 



External condijle — 4" 



,^V -' '§ Internal condyle 



Ic Q \ External and internal 



/ tubercles of spine 



Groove for tendon of extensor ^'' 

 longits and peroneus tertius 



\ (iroorc for middle patellar ligament 



Tuberosity 



Fig. 71. — Proxi.mal Extremity of Right Tibia of Horse, Exd View. 

 I. CM., I.e. p.. Depressions for attachment of anterior and posterior crucial ligaments; I, m, in, depressions for at- 

 tachment of semilunar cartilages. 



The distal extremity is fused with the tibia, constituting the external malleolus. 



Development. — This resembles that of the ulna. The embryonic cartilaginous 

 fibula extends the entire length of the leg, but does not articulate with the femur. 

 The lower part of the shaft is usually reduced to a fibrous band. Three centers of 

 ossification appear, one each for the shaft and the extremities. The distal end 

 unites early with the tibia, forming the external malleolus. 



It is interesting to note that in some oases the entire shaft of the tibula develops, a reversion 

 to the condition in the Miocene ancestors of the present horse. 



THE PATELLA 



The patella is a large sesamoid bone which articulates with the trochlea of 

 the femur. It presents for description two surfaces, two borders, a base, and an 

 apex. 



The anterior or free surface (Facies libera) is irregularly quadrilateral, convex, 

 and rough for muscular and ligamentous attachment. 



The posterior or articular surface is smaller and is triangular in outline. It 

 presents a vertical rounded ridge, which corresponds to the groove on the trochlea 

 of the femur, and separates two concave areas. Of the latter, the inner cavity is 



