Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 4 On the Structural Relationships of Bones

Not all joints move in the same way

Nature has linked each type of articulation in no simple kind of structure, as equal motion is not given to all joints. Some are flexed and extended, and adducted and abducted on this side and that, and finally they are also rotated in a circle. The femur and the arm show these motions [articulatio spheroidea]; you adduct the arm to the chest, you take it back to the dorsum, then you move it upward to the head, and downward to the hips; finally, you move it around as well when you fix your thumb to the table and make a circle as much as you can with the rest of your fingers. So also you move the femur forward and backward and inward toward the other femur, and outward away from it; likewise you turn it in a circle when you fix your heel on the ground 21 and move the toes this way and that to the sides. This is the sense in which it is convenient to understand the motion of rotation, but not when you move the arm now forward, now upward, now backward, now downward in successive motions as if in a circle [circumduction]. We shall pursue this at greater length in explaining the uses of the muscles. 22 Other joints are only bent and extended and moved to the sides, lacking all movements of rotation [ginglymus], for example the first joints of the digits, 23 and the wrist itself where it is joined to the forearm. Others are bent and extended and at the same time rotated, without at the same time claiming for themselves any motion to the sides. So the radius together with the ulna 24 are bent and extended on the humerus, and even rotated on it whether they are pronated or supinated [articulatio trochoidea]. Others are only bent and extended, like the ulna to the humerus, the tibia to the femur, the second and third joints of the fingers, and the third joint of the thumb [ginglymus]. Others are only rotated [articulatio trochoidea], as the first vertebra of the neck is rotated above the second as if on an axle with the entire head, and as the radius is moved above the ulna only in a pronating or supinating rotary motion. Since, therefore, all joints do not command the same movement, it should appear by no means strange that they are put together with a different form of construction.



Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 4 On the Structural Relationships of Bones