d. Am phiarthrosis (ap96, utrinque, cteeov, articulus, i. e. a mixed form of- articulation. Articulatio dubia, Bartholin. Synartkosis diar throdica).—This is a form of articulation where two plane or mutually adapted surfaces are held together by a cartilaginous or fibro-cartilaginous lamina of considerable thickness, as well as by external ligaments. In virtue of the elasti city of the interposed cartilaginous or fibro cartilaginous lamina, the amphiarthrosis pos sesses a manifest, although certainly a very limited degree of motion, and hence most systematic writers class it with the diarthrodial articulations. To me it appears much more consistent to place it among the synarthrodial joints, for, 1. its anatomical characters agree precisely with those of synarthrosis ; 2. the surfaces in amphiarthrosis being continuous, it would make an exception in diarthrosis were we to place it there ; and, 3. its degree of motion is greater than that of suture, only because of the greater development of the in terosseous substance. These points of similarity led some anatomists to call it Diarthrosis syn arthrodica ; for the reasons above stated, as well as because it has one point of resemblance to diarthrosis in its greater latitude of motion, I propose the appellation Sunarthrosis diar throdica.
The examples of this form of joint in the human body are the articulation between the bodies of the vertebrae, that between the two ossa pubis at what is called the symphysis, and that between the ilium and sacrum. We may also, .I think, place here the articulation of the ribs with the sternum by means of the costal cartilages.* The bodies of the vertebra in most of the mammalia are articulated in the same way ; so are they in fishes also ; but in these last there is a peculiarity already re ferred to, which increases the degree of motion of which the joint is susceptible.t Like the sutures, the amphiarthrosis is liable to become obliterated by age, and from the same cause, namely, the ossification of the interosseous la mina. This is very common in the costo-sternal joints, less so in the interpubic, and still more rare in the inter-vertebral and sacro-iliac.
Diarthrosis.—Evident mobility is the dis tinguishing characteristic of this class of joints; the articular surfaces are contiguous, each co vered by a lamina of cartilage (diarthrodial cartilage), having a synovial sac, and in some cases two synovial sacs interposed, which are separated by a meniscus. The in tegrity of the articulation is maintained by liga ments which pass from the one bone to the other. Their mechanism is much more com plicated than that of synarthrodial joints, being intended not only for security, but also to give a certain direction to the motions of which they are the centre.
Before proceeding to the enumeration of the varieties of joints that come under this head, it will not be amiss to describe briefly the various motions which may take place between any two segments of a limb, and which it is the object of these joints to admit of. It is obvious that the most simple kind of motion which can exist between two plane or contiguous surfaces,is that of gliding : one surface glides over the other, limited by the ligaments which extend be tween the bones. This motion, however, is not confined to plane surfaces, it may exist evidently between contiguous surfaces whatever their form.
When two segments of a limb, placed in a direct line or nearly so, can be brought to form an angle with each other, the motion is that of flexion, the restoration to the direct line is ex tension. These two motions belong to what Bichat calls limited opposition ; the flexion and extension of the fore-arm on the arm illustrate it. Sometimes a motion of this kind takes place in four directions, indicated by two lines which cut at right angles. This is best under stood by a reference to the motions which take place at the hip joint : there it will be seen that the thigh-bone may be brought forward so as to form an angle with the trunk, flexion—or it may be restored, extension ; it may be sepa rated from the middle line of the body so as to form an angle with the lateral surface of the trunk, abduction—or it may be restored and made to approximate the middle line, adduc tion. Bichat terms this "opposition vague." It is evident that a joint, which is suscepti ble of these four motions, may also move in directions intermediate to them. When these motions are performed rapidly, one after the other, it appears as one continuous motion, in which the distal extremity of the bone describes a circle indicating the base of a cone whose apex is the articular extremity moving in the joint ; this motion is called circumduction.
Rotation is simply the revolving of a bone round its axis. It is important to bear this definition in mind : through losing sight of it many anatomists have attributed rotation to a joint which really does not possess it.
The varieties of the diarthrodial joint are as follows : a. Arthrodia (articulatio Plana or plani formis.)—In this species the surfaces are plane or one is slightly concave, and the other slightly convex: the motion is that of gliding, limited in extent and direction only by the ligaments of the joint or by some process or processes con nected with the bones. The examples in man are, the articular processes of the the radio-carpal, carpal, carpo-metacarpal, infe rior radio-ulnar, superior tibio-fibular, tarsal and tarso-metatarsal, temporo-maxillary, acro mio-clavicular and sterno-clavicular joints. This last articulation and the wrist-joint possess a greater latitude of motion than the others ; the former, in consequence of the shape of its articular surfaces : each surface is convex in one diameter and concave in the other, so that the gliding that takes place in this joint is in the direction of the long and short diameters, which intersect each other at right angles. It is capable, therefore, of vague opposition in those lines, but certainly not in the interme diate directions, the nature of the surfaces being calculated to prevent this. The wrist owes its mobility to the laxity of its ligaments, permit it to move as well in its transverse as in its antero-posterior diameters, as also in the in termediate directions; it consequently admits of vague opposition and circumduction. The articulation of the metacarpal bone of the thumb with the trapezium, is also an arthrodia very similar to the sterno-clavicular, but with a greater degree of motion. Arthrodial joints 'are generally provided with ligaments, placed at the extremities of the lines in the direction of which the gliding motion takes place.