The gastrocnemius, when it contracts, extends the foot on the leg.
It serves to maintain the tibio-tarsal angle in the standing position, and during walking, to determine the steadying of the hind-limbs, which then, after the fashion of a spring, project the body forward.
By an analogous movement they take part in the posterior projection of the hind-limbs in the act of kicking; but they are not the only ones to act in this case, the muscles of the buttock and thigh also being brought into play.
Soleus ([Fig. 83], 10; [Fig. 86], 8).—This muscle, much less developed in quadrupeds than in man, does not exist in the dog.
With regard to the soleus in the pig, Professor Lesbre says: ‘Meckel denied its existence; we, however, believe that it is united to the external head of the gastrocnemius, its origin being transferred to the femur.’[32]
[32] F. X. Lesbre, ‘Essai de Myologie comparée de l’homme et des mammifères domestiques en vue d’établir une nomenclature unique et rationelle,’ Lyon, 1897, p. 169.
But in animals in which it exists, this muscle, of but little importance, occupies the outer side of the leg. It arises above from the external tuberosity of the tibia, and terminates below in a tendon which is united with that of the gastrocnemius.
The soleus has the same action as these latter.
Plantaris.—In quadrupeds this muscle is blended with the superficial flexor of the toes, which we will study afterwards.
Popliteus ([Fig. 87], 16; [Fig. 88], 14).—In man, this muscle, which occupies the posterior surface of the tibia, above the oblique line, is completely covered by the gastrocnemius.