84 ELEMENTS OF MAMMALIAN ANATOMY. 



notch. The sharp cephalic border of the bone forms the 

 crest. On the distal extremity is a process, the internal 

 mallcolus, bearing two grooves, the more caudal of which 

 shelters the tendon of the flexor longus digitorum muscle 

 while the other is for the tibialis posticus (Fig. 52). A 

 facet is present on the lateral aspect of this extremity for 

 articulation with the fibula. 



The fibula in proportion to its length is the slenderest 

 bone in the skeleton. It is the outer or lateral bone 

 of the crus, and articulates proximally with the tibia 

 and distally with the tibia and astragalus (Fig. 21). 

 The proximal extremity is the head, upon which is a 

 circular facet for articulation with the tibia. The sharp 

 edge along the cephalic aspect of the shaft furnishes 

 attachment to the interosseous membrane uniting the 

 two bones of the crus. The mesal 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 cephalic side is a slight depression for the pero- 

 neous 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 

 Ungulata 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 

 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, astragalus, and 

 navicular or scaphoid; the distal row the cuboid, ecto- 



