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 Equidz 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. Theos calcis,or calcaneuwm,is 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 
caleaneum 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 
