THE SKELETON 



79 



From the lateral aspect projects the external malleolus, on the 

 caudal side of which is a groove for the peroneus brevis muscle, 

 and on the cranial side is a slight depression for the peroneus 

 longus (Fig. 58). 



In many of the Mammalia the two bones of the crus are more 

 or less united. In most of the Chiroptera 

 and Ungulata the hbula is rudimentary. 

 The middle portion of the fibula is ab- 

 sent in the horse and its distal portion 

 is fused with the tibia (Fig. 36). The 

 ancestor of the Equidae in Eocene times 

 had a well-developed fibula, but in the 

 evolution of the horse this bone has 

 gradually diminished in size^ as shown 

 in the figure. 



The tarsus is composed of seven bones 

 in two rows. The proximal row contains 

 the OS calcis, talus, and navicular or 

 scaphoid; the distal row the cuboid, and 

 three cuneiform bones. The os calcis, or 

 calcaneum, is the largest bone of the 

 tarsus, being twice as long as broad (Fig. 

 44) . Its caudal end presents a groove for 

 the tendon of the plantaris muscle. The 

 peroneal tubercle is on the outer side of 

 the distal extremity. The sustentaculum 

 tali, a shelf-like process projecting from 

 near the middle of the inner side, sup- 

 ports a facet for the tendon of the flexor 

 longus digitorum muscle (Fig. 59). 



The talus lies on the inner or medial 

 side of the calcaneum and articulates 

 with the tibia, fibula, calcaneum, and scaphoid. The bone is 

 composed of a body, neck, and head. The body is proximal 

 and bears a trochlear surface for articulation with the tibia. 



OR 

 OF 



r 



Fig. 43. — Inner 

 Medial Aspect 

 Right Fibula. 



a, Facet for tibia; or, 

 facet for talus; ex, ex- 

 ternal malleolus; h, head; 

 s, shaft; /, articular sur- 

 face for tibia; gr, groove 

 for peroneus longus 

 muscle. 



