Whatever may be the point of view in which we consider the force of the contractions of the muscular system of animal life, it is always very great in proportion to the effect which results from these contractions. Nature in the economy follows a law the reverse of that of the motion of our common machines, the great advantage of which is to increase the moving powers, to produce a great effect with a small force. Here there is always a great expenditure of force for a small effect, which is owing to the numerous causes that tend to destroy the effect of this force. 1st. The muscles act almost always upon a very unfavourable lever, upon that in which the power they represent is nearer the point of support than the resistance. 2d. All in contracting have to overcome the resistance of the antagonists. 3d. As in each motion there is always a fixed point, the effort which, after contraction, is carried upon this fixed point, is entirely lost. 4th. Various frictions injure also the motion. 5th. The obliquity of the insertion of the muscles upon the bones, an obliquity that approaches nearer a horizontal than a perpendicular direction, an obliquity not less remarkable for the fleshy attachments upon the tendon or aponeuroses, offers a double cause of weakness. All these and many other reasons which we might with Borelli, who was the first to make these important remarks upon muscular motion, add to them, prove that the absolute or real force of the muscles is infinitely superior to their effective force. Yet all are not so unfavourably arranged; in some, as the solæus, the insertion is perpendicular to the bone; in others, as the muscles which act upon the head, we observe that they are powers of a lever of the first kind. In general, in order to estimate the force of a separate muscle, the deltoid, for example, it is necessary especially to have regard to the distance of its insertion at the point of support, to the degree of opening of the angles formed by the fleshy fibres upon the tendon, and afterwards by the tendon upon the bone, and to the division of the forces between the fixed and moveable points. Some advantages seem to compensate in a slight degree in certain muscles for their bad arrangement as to the power of motion; such are, 1st, the sesamoids, the patella, the different eminences of insertion, the enlargement of the large bones at their extremities, &c. which remove the fibres to a distance from the moveable points; 2d, the intermuscular fat, that which is in the neighbourhood of the muscles, the fluid of synovial sheaths, which facilitate motions by lubricating the surfaces that execute them; 3d, the aponeurotic expansions that confine down the motions on the extremities; 4th, these motions themselves, those of flexion for example, which, as they take place, diminish the obliquity of the insertion of the flexors, and render it even perpendicular, as has been well observed by a modern author.