THE SKELETON. 69 



the head, upon which is a circular facet for articulation 

 with the tibia. The sharp edge along the cranial aspect 

 of the shaft furnishes attachment to the interosseous mem- 

 brane uniting the two bones of the crus. The medial 

 aspect of the distal extremity shows a prominent facet for 

 articulation with the astragalus. Immediately proximal to 

 this is the articular surface for the tibia. 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. 52). 



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

 more or less united. In most of the Chiroptera and Ungu- 

 lata the fibula is rudimentary. The middle portion of the 

 fibula is absent in the horse and its distal portion is fused 

 with the tibia (Fig. 35). The ancestor of the Equidse in 

 Eocene times had a well-developed fibula, but in the evolu- 

 tion 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, astragalus, and 

 navicular or scaphoid ; the distal row the cuboid, and three 

 cuneiform bones. The ay calcis, or calcaneum, \?, the largest 

 bone of the tarsus, being twice as long as broad (Fig. 43). 

 Its caudal end presents a groove for the tendon of the plan- 

 taris muscle. The peroneal tubercle is on the outer side of 

 the distal extremity. The sustentaculum, a shelf -like process 

 projecting from near the middle of the inner side, supports 

 a facet for the tendon of the flexor longus digitorum muscle 



(Fig- 53). 



The astragalus 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 



