Muscles, with their tendons, are the infrtuments by which animal motion is performed. It will be our bufinefs to point out inftances in which, and properties with refpect to which, the difpofition of thefe mufcles is as ftrictly mechanical, as that of the wires and ftrings of a puppet.

I. We may obferve, what I believe is univerfal, an exact relation between the joint and the mufcles which move it. Whatever motion, the joint, by its mechanical conftruction, is capable of performing, that motion, the annexed mufcles, by their pofition, are capable of producing. For example; if there be, as at the knee and elbow, a hinge joint, capable of motion only in the fame plane, the leaders, as they are called, i. e. the mufcular tendons, are placed in directions parallel to the bone, fo as, by the contraction or relaxation of the mufcles to which they belong, to produce that motion and no other. If thefe joints were capable of a freer motion, there are no mufcles to produce it. Whereas at the fhoulder and the hip, where the ball and focket joint allows by its conftruction of a rotatory or fweeping motion, tendons are placed in fuch a pofition, and pull in fuch a direction, as to produce the motion of which the joint admits.