DIRECTOR   Lawrence Ecnerwal


If we admit the idea of symmetry in structure between arms and legs, and would compare the movements of the two in man and animals, we must change in some respects the terms flexion and extension, from those ordinarily used in the description of the human body. We will suppose the human skeleton suspended with the vertebral column horizontal, the limbs slightly flexed, the toes and fingers pointing downwards, the palms facing forwards and the soles backwards. Flexion of the humerus would be backwards, of the femur forwards ; of the fore arm forwards, of the leg backwards ; of the hand backwards, that is by carrying the back of it towards the back of the fore arm, and the foot forwards. Thus the movements would be symmetrical throughout in the two limbs. Supposing the limbs to be of equal strength, somewhat flexed, the soles and palms resting on the ground, they would antagonize each other in their action; the fore limbs, if extended, would, in consequence of their obliquity, tend to push the body upwards and backwards, and the hind limbs under the same conditions upwards and forwards. The two acting together would give rise to a resultant motion upwards. By the rotation of the fore arm in the embryo the action of the fore limb is reversed, and thus in the forward movement of the body cooperates with, instead of antagonizing, the hind limb. When fully developed, therefore, the fore and hind limbs of animals have a general symmetry except in the following respects; the bones of the fore arm cross each other, while those of the legs do not, and the toes and fingers are both directed forwards. If, however, the fore arm be rotated outwards through half a circle, so as to make the bones parallel, as in the leg, thus counteracting the change which took place during development, the general symmetry would be restored, and be complete. There would, however, exist a certain amount of special asymmetry in the position of the thumb and great toe, for these would be on opposite sides of the two limbs. Of this difficulty we shall speak again further on.