THE FIBULA 



117 



patellar ligaments. A semicircular smooth notch, the sulcus muscularis, separates 

 the tuberosity from the lateral condyle, and gives passage to the common tendon 

 of origm of the extensor digitalis longus and the peroneus tertius. 



The distal extremity (Extremitas distalis) is much smaller than the proximal 

 one; it is quadrangular in form and larger medially than laterally. It presents an 

 articular surface (Cochlea tibia), which is adapted to the troclilea of the tibial 

 tarsal bone, and consists of two grooves separated by a ridge. The ridge and 

 grooves are directed obliquely forward and laterally, and are bounded on either 

 side by the malleoli, to which the collateral ligaments of the hock joint are at- 

 tached. A shallow synovial fossa is usually present on the middle of the articular 

 ridge. The lateral groove is wider and shallower than the medial one; it is fre- 

 quently marked by a line or groove which indicates the former demarcation between 

 the tibia and fibula. The medial malleolus (Malleolus medialis) is the more prom- 

 inent of the two, and forms the anterior boundary of a groove for the tendon of the 

 flexor digitalis longus. The lateral malleolus (Malleolus lateralis) is broader, and 

 is marked by a vertical groove for the passage of the lateral extensor tendon. 



Development. — The tibia has the usual three chief centers of ossification and 

 supplementary ones for the tuberosity and the lateral malleolus. The latter is 

 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 articular groove. The proxi- 

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

 about two years of age. 



Fig. 98. — Proximal Extremity of Right Tibia of 

 Horse; End View. 

 1, Medial condyle; 2, lateral condjie; 3, groove 

 on 2 for popliteus tendon; 4, popliteal notch; 5, tuber- 

 osity; 6, groove for middle patellar ligament; 7, tuber- 

 cles of spine; S, 'sulcus mu.scularis; Z.c.a,, /.c. p., depres- 

 sions for attachment of anterior and posterior cruciate 

 ligaments; I, m, m, depressions for attachment of 



Fig. 99. — Distal [Extremity of Right Tibia (and 

 Fibula) of Horse; End \'iew. 

 1, 1', Articular grooves; 2, intermediate ridge and 

 synovial fossa; 3, line of fusion of primitive distal end 

 of fibula with tibia; 4, medial malleolus; 5, 5', lateral 

 malleolus; 6, posterior border. 



THE FffiULA 



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

 border of the tibia. 



The shaft (Corpus fibulae) is a slender rod which forms the lateral boundary of 

 the interosseous space of the leg; it usually terminates below in a pointed end about 

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



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

 flattened transversely. Its medial surface presents a narrow area (Facies articu- 

 laris capituli) along the upper border for articulation with the lateral condyle of 

 tibia. The lateral surface is rough and gives attachment to the lateral ligament 

 of the stifle joint. It has rounded anterior and posterior borders. 



