Chap. XLVIII.—Of a Man’s complex Strength, but first of the Arm.
The muscles which serve either to straighten or bend the arm, arise from the different processes of the Scapula; some of them from the protuberances of the Humerus, and others about the middle of the Os humeri. The extensors of the arm arise from behind, and the flexors from before.
That a man has more power in pulling than in pushing, has been proved by the ninth proposition De Ponderibus[1], where it is said, that of two equal weights, that will have the greatest power which is farthest removed from the pole or centre of its balance. It follows then of course, that the muscle N B, Plate II. and the muscle N C, being of equal power, the inner muscle N C, will nevertheless be stronger than the outward one N B, because it is inserted into the arm at C, a point farther removed from the centre of the elbow A, than B, which is on the other side of such centre, so that that question is determined. But this is a simple power, and I thought it best to explain it before I mentioned the complex power of the muscles, of which I must now take notice. The complex power, or strength, is, for instance, this, when the arm is going to act, a second power is added to it (such as the weight of the body and the strength of the legs, in pulling or pushing), consisting in the extension of the parts, as when two men attempt to throw down a column; the one by pushing, and the other by pulling[2].