MUSCLES OF THE LEG AND FOOT 367 
osseus at the distal end of the first phalanx, and may detach a tendon to the second 
digit. The lateral tendon is smaller. Its branches end on the third phalanges of 
the accessory (second and fifth) digits and on the lateral chief (fourth digit); there 
may be a branch to the third digit, and the branch for the fourth may go to the 
corresponding branch of the central tendon. Other variations occur. 
The synovial sheath for the tendons of the extensor longus and peroneus tertius at the hock 
extends nearly half an inch (ca. 1 em.) above the proximal annular ligament and an inch or more 
(ca. 2 to 3 cm.) below the distal annular ligament in a large adult. 
The lateral digital extensor lies on the lateral face of the leg, behind the per- 
oneus longus. It arises from the lateral surface of the fibula, the lateral femoro- 
tibial ligament, and the intermuscular septum between this muscle and the deep 
digital flexor. It consists of two parts. The larger anterior muscle (extensor 
digiti quarti) has a tendon which appears a little distal to the middle of the leg, de- 
scends on the grooved lateral surface of the fibula, inclines forward, crossing under 
the tendon of the peroneus longus, and ends on the extensor process of the third 
phalanx of the lateral chief (fourth) digit. It receives an interosseus tendon at 
the first phalanx. The tendon of the posterior muscle (extensor digiti quinti) 
accompanies that of the anterior one to the tarsus and descends to the lateral ac- 
cessory (fifth) digit. 
The two tendons are bound down at the lateral malleolus by an annular ligament. The 
anterior tendon may receive a branch of the long extensor tendon and send a tendon to the fifth 
digit. .The posterior tendon may send a reinforcing branch to the tendon of the long extensor for 
the fifth digit. There may be a third small head which arises from the middle of the fibula and 
sends a delicate tendon to join that of the posterior head. 
The extensor hallucis longus is a small, fusiform muscle which is covered by the 
extensor longus and peroneus longus. It arises from the proximal end of the fibula 
and its delicate tendon descends at first under that of the peroneus tertius, inclines 
medially at the hock, and ends on the medial accessory (second) digit. 
The extensor digitalis brevis is a well-developed muscle which lies on the dor- 
sal face of the distal part of the tarsus and on the chief metatarsal bones. It arises 
from the neck of the tibial tarsal and the body of the fibular tarsal bone, and is 
partially divided into three parts. The tendon of the superficial part joins that of 
the long extensor for the chief digits. The deep part has two tendons which join 
those of the long extensor for the accessory digits. 
The gastrocnemius has short but wide and thick heads. The lateral one is 
the larger and is united with the superficial digital flexor to the distal third of the 
leg. The tendon forms a groove for the superficial flexor tendon above the hock 
and is inserted chiefly into the prominences on each side of the notch of the tuber 
calcis. 
The soleus is thick and wide and blends with the lateral head of the gastroc- 
nemius. It arises from the lateral epicondyle of the femur and the deep fascia 
at the stifle. Its tendon joins that of the gastrocnemius. 
The popliteus presents nothing remarkable. 
The superficial digital flexor has a belly of considerable size. It arises with the 
lateral head of the gastrocnemius, with which it is fused to the distal third of the 
leg. The tendon is almost entirely enclosed by the twist of the gastrocnemius in 
the lower part of the leg. At the tuber calcis it is thick and largely cartilaginous, 
and is molded on the groove and ridges of the bone. It is attached by a strong 
band to each side of the tuber calcis. A large bursa under the tendon extends up- 
ward in the groove formed by the gastrocnemius almost to the muscular part and 
downward to the middle of the fibular tarsal bone. The tendon divides distally 
into two branches which go to the chief digits. It also detaches from its plantar 
urface two bands which join the fascia of the accessory digits. 
The deep digital flexor presents three distinct heads: (1) The tibialis pos- 
