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

Appendix: Ginglymus (the hinge joint, 1555 version)

Gi/gglumoj is made with obvious depressions and heads, but they are in no way simple, nor do they consist of a single convex, concave, or flat surface. Ginglymus occurs when bones are joined by entry into each other in such a way that the prominences of one bone enter depressions of the other, 68 and the depressions of one bone in turn admit the prominences of the other, just as the hinges of doors and windows, from which this third type of joint takes its name, are built. Just as two principal types of hinge are seen, so there is above all a double system for the mutual attachment of bones. Hinges, //1555 p. 17// which generally have two connected beams, are protuberant and hollow in such a way that the protruding parts of one iron piece are received by the hollow parts of the other and the hollow parts of one admit the protruding parts of the other, and the pin then runs through all the cavities at once. 69 The hinges by which doors and windows are attached to walls are so arranged that the iron piece attached to the right wall has a smoothly rounded protrusion running perpendicularly upward which fits up into the hollow circle of the iron piece attached by nails to the door or window, so that the hollow circle, ring, or vertebra 70 receives the rounded protuberance, and the iron piece from which the latter is produced takes on the vertebra by mutual entry. 71 In the likeness of these hinges (particularly if one member has just one protrusion [eminentia] in its middle and is cut away above and below, while the other shows two protrusions and is hollowed out [fossa] between them) the second and third joints of the digits of the hand and foot are joined and, in addition to very many others, the joint [articulatio tibiofibularis] of the femur to the tibia. In these joints, one bone has two heads separated by a depression between [fossa intercondylaris]; the other has two depressions [condylus medialis, c. lateralis] set off by a prominence [tuberculum intercondylare] between. In this way the two heads of one bone enter the twin depressions of the other, and the depression of one bone receives the protuberance of the other. The ulna is seen to be joined to the humerus in the manner of the second hinge, when the round depression [incisura trochlearis] of the ulna is flexed and extended on the orbit or wheel of the humerus [trochlea humeri], or otherwise swiveled upon it — though in this joint too [articulatio humero-ulnaris] many features besides resemble the first hinge. Nature contrives such joints when she particularly wishes to promote the interests of their strength and it suffices that they perform only a single motion, as the joints just mentioned attest, which we can only flex and extend, as they require no other motion.

The upper figure illustrates a hinge by which we write here that two beams join. A marks one iron, B the other; by means of the pin, marked C, they are secured to their point of mutual entry. In the lower figure another hinge is drawn in which D identifies the iron attached to the wall, E the iron by which it is attacked to the door or window by nails.

Why Nature sometimes joined two bones with several joints

It is fitting to understand the purposes of Nature so far mentioned in forming three types of joint in cases where they are simple and single in the attachment of only two bones. For on occasion when constrained by an unusual situation, she was unable to join two bones with a simple joint, but fashioned two and sometimes more connections or joints at a distance from each other and not immediately contiguous. To prevent the radius from separating too far from the ulna midway along the forearm without good cause, she joined the radius to the ulna by a double joint 72 rather than a simple tight joint through the entire length of the forearm: next to the bend of the elbow, where the capitulum of the radius goes up into the depression of the ulna and next to the brachium, where the capitulum of the ulna articulates into the depression of the radius, though at those two joints the radius only rotates or is pronated and supinated. Next, because of the passage of the dorsal medulla [medulla spinalis], the occipital bone is connected by a double joint [articulatio atlanto-occipitalis] to the first vertebra, each constructed more or less in the style of enarthrosis; but by virtue of that joint, the head moves only backward and forward above the first vertebra, rightly because of the structural principle in joints separated from each other. In addition, the first vertebra is joined above the sides of the second vertebral body by two joints resembling arthrodia or flat joints; and the dens of the second vertebra is joined to the first by a kind of enarthrosis. 73 Yet in those three joints the first vertebra and the head is only rotated above the second [articulatio trochoidea] (contrary to what Galen believed, as we shall later explain). Moreover, because also for the sake of the dorsal medulla the other vertebrae of the dorsum had to be perforated and at the same time made in a complex way, Nature employed no simple connections and joints in the juxtaposition of // 1555 p. 18 // two vertebrae. In addition to a rare and peculiar articulation by which the vertebral bodies are attached by no smooth and slippery point of contact, as are all the other joints, there is a kind of arthrodia [articulationes zygapophysiales] from the second cervical vertebra to the twelfth thoracic by which the processes of each lower veretebra that we will call the ascending are placed beneath the descending processes of the incumbent vertebra placed above it. At the same time, these vertebrae experience lateral motion in addition to their obvious flexion and extension; similarly the lumbar vertebrae, which like the twelfth thoracic are articulated by enarthrosis of the descending processes into the ascending processes of the vertebra beneath. I do not know what came into Galen’s head when he classified vertebral articulation as ginglymus. 74 For though besides the first cervical and the twelfth thoracic (the latter of which is received above and below via its own ascending and descending processes by the vertebrae next to it, like the first, as if admitting the bones coterminous to it above and below) all the vertebrae are on one side received and on the other, as I have said, receive; nevertheless this must not be classified ginglymus, as he thought. For Galen did not consider that he had assigned to ginglymus three bones, as follows: the first, one which is received on its upper surface; the second, one that admits this upper surface; and the third, one that is received by the lower surface of the first bone. But now it would be timely to go into joints which are lacking in any motion, of which the first is called gomphosis. 75

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