Its fleshy body arises above from the external lateral ligament of the knee-joint, and from the whole length of the fibula. In the middle third of the leg it is narrowed; lower down it is replaced by a tendon. This is lodged in a groove hollowed on the external surface of the inferior extremity of the tibia; then after passing along the external surface of the tarsus, it is directed forward, and proceeds to blend towards the middle of the canon-bone with the tendon of the long extensor of the toes, or anterior extensor of the phalanges, of which it shares the insertions. It extends the phalanges into which it is inserted. It also flexes the foot.
Muscles of the Posterior Region
It will not be unprofitable to recall to mind that, in man, the muscles of this region are arranged in two layers: a superficial layer consisting of the gastrocnemius and soleus, to which is added a muscle of little importance, the plantaris, and a deep layer formed by four muscles—the popliteus, flexor longus digitorum, tibialis posticus, and flexor longus pollicis.
The gastrocnemius and soleus, independent in their upper portion, unite below in a common tendon; they thus form also a triceps muscle, which we designate under the name of the triceps of the leg, or triceps suralis, because it forms the elevation of the calf of the leg (from sura, calf).
Gastrocnemius ([Fig. 83], 9, 11; [Fig. 84], 13, 14; [Fig. 86], 7, 9; [Fig. 88], 13).—The external and internal heads of the gastrocnemius, distinct from one another only in their upper portion, arise from the shaft of the femur, above the condyles, on the borders of the popliteal surface, to a relatively considerable extent in the great quadrupeds.
At this level they are situated in the popliteal region—that is to say, in the space limited externally by the biceps, and internally by the semi-tendinosus. But as they descend to a rather low level on the leg in quadrupeds, and especially in carnivora, they do not, properly speaking, determine a projection of the calf of the leg. However, they pass from this region but to be soon continued by a tendon—the tendo-Achillis, which is inserted into the calcaneum.
Now, the region of the tarsus is called by veterinarians the ham, the posterior surface of which is angular, because of the oblique direction of the leg with regard to the vertical direction of the metatarsus and the presence of the calcaneum; the prominence which this surface presents has received the name of the point of the ham, and the tendon which ends there that of the cord of the ham.
But the tendo-Achillis does not alone form this cord. Indeed, as we will soon see, the tendon of the superficial flexor of the toes takes part in its formation.
We may add, with regard to the tendo-Achillis, that it is more clearly perceived as an external feature, because the skin sinks in front of it, as it does in man, over the lateral parts of the region which it occupies.