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Animal Mechanics

leg, muscles and body

ANIMAL MECHANICS.

Standing in the erect posture is the position which requires least muscular exertion, the different curves of the back-bone aiding in balancing the trunk, and the various joints being so constructed that only a certain amount of steadying action is required on the part of the muscles, the muscles of the calf, for instance, preventing the falling forward of the leg. In the erect posture, too, the body fulfils the neces sary condition of having its centre of gravity within its base of support. Thus, a line dropped from the centre of gravity, which is in front of the sacrum, would fall between the feet a little way in front of the ankle-joints.

Walking.—The weight of the body is sup ported alternately by one limb and then the other, being hardly balanced on one limb before it is thrown to the other. At the moment when the advanced foot has reached the ground the muscles of the calf of the leg behind contract and raise the body on the ball of the toes (Fig. 77), thrusting it forwards and to the side,

The weight of the body is thus thrown on the advanced leg, and, owing to the forward move ment, the leg that is behind swings forwards like a pendulum, and now becomes the advanced leg, and so the process goes on.

Running.—For a short period both feet are oil the ground. Running differs from walking also in the quick violent contractions of the muscles, and in the fact that the advance is due to muscular effort alone, and therefore there is proportionately a much greater expenditure of force than in walking.

Thus motion and locomotion are dependent upon mechanical relations subsisting between muscles, bones, and joints. When these rela tions are interfered with there is difficulty of movement greater or less according to the amount of the interference, as will be seen by considering the diseases of movement in the , following section.