b. Enarthrosis( diarthrosis and-socket joint.)—This is a highly developed arthrodia. The convex surface assumes a glo bular shape, and the concavity is so much deepened as to be cup-like, hence the appella tion ball and socket. The ball is kept in appo sition with the socket by means of a capsular ligament, which is sometimes strengthened by accessory fibres at certain parts that are likely to be much pressed upon. The best example of enarthrosis is the hip-joint, and next to it the shoulder : in the latter the cavity is but imper fectly developed. All the quadrupeds have their shoulder and hip joints on this construc tion, and the same common plan is observed in the vertebrate generally whose extremities are developed. In birds and reptiles the bodies of the vertebra are articulated by enarthrosis, and the solid calcareous spines on the external surface of the shells of echinida are adapted to round tubercles on which they move, thus ex hibiting a very complete form of enarthrosis.* This species of joint is capable of motion of all kinds, opposition and circumduction being the most perfect, but rotation limited. Indeed what is called rotation at the hip-joint, is effected by a gliding of the head of the femur from before backwards, and vice versa in the acetabulum ; it is not a rotation of the head and neck, but of the shaft of the femur.
c. Ginglymus (ybiryxvihoc, cardo, articulatio cardinybrmis, articulation en charniere, en ge nou, hinge-joint.)—The articular surfaces in the hinge-joint are marked with elevations and depressions which exactly fit into each other, so as to restrict motion in all but one line of direction. They are always provided with strong lateral ligaments, which are the chief bonds of union of the articular surfaces.
The elbow and ankle joints in man are per fect ginglymi ; the knee also belongs to this class, but is by no means a perfect specimen, for in a certain position of the bones of this joint, the ligaments are so relaxed as to allow a slight rotation to take place. The phalangeal articulations, both of the fingers and toes, are ginglymi. This form of joint is most exten sively employed among the lower animals. In quadrupeds, most of the joints of the extremi ties come under this head. In amphibia and reptiles, too, there are many examples of the hinge-joint. The bivalve shells of conchiferous
mollusca are united by a very perfect hinge, and a great number of the joints of crustacea and insects are of this form.
The true ginglymus is only susceptible of limited opposition : hence the knee-joint can not be regarded as a perfect example ; in fact, in the perfect ginglymus there' is every possible provision against lateral motion.
d. Diarthrosis rotatorius (commissura tro choides.)—A pivot and a ring constitute the mechanism of this form of joint. The ring is generally formed partly of bone and partly of ligament, and sometimes moves on the pivot, sometimes the pivot moves in it. The motion is evidently confined to rotation, the axis of which is the axis of the pivot.
In the human subject the best example of this articulation is that between the atlas and odontoid process of the axis or vertebra dentata. The ring is formed by a portion of the anterior arch of the atlas, completed behind by a trans verse ligament. Here the atlas rotates round the odontoid process, which is the axis of mo tion. Another example is the superior radio ulnar articulation : here the ring is formed one fourth by bone, namely the lesser sigmoid cavity of the ulna, and the remaining three-fourths by the round ligament called the coronary ligament of the radius. In this case there is rotation as perfect as in that just mentioned, but the head of the radius rolls in the ring, sand the axis of motion is the axis of the head and neck of the bone. Some anatomists consider this joint a species of ginglymus, which they designate lateral.
The terms Symphysis, Synchondrosis, Syn neurosis, Syssarcosis, Meningosis, have been employed by anatomists to designate certain kinds of articulation, chiefly in reference to the nature of the connecting media. Symphysis, although originally employed with great extent of meaning, seems to have been in later days applied exclusively to denote the articulations of the pelvis, which we have classed under Amphiarthrosis. I pass over the other terms, because they ought to be discarded from use, as only tending to encumber a vocabulary already too much crowded with difficult and unnecessary terms.
The descriptive anatomy of the several joints will be found under the heads—ANKLE, CRA