The superior extremity of the ulna presents anteriorly a deep cavity, ( the greater sigmoid cavity,) which is concave from above down wards and convex in the transverse direction : it is bounded behind by the olecranon and in front by the coronoid process; the surface of this cavity is smooth and covered by cartilage, with the exception of a rough transverse notch which extends from the internal side nearly the whole way across it, and the inequalities of which are effaced in the recent state by a cushion of soft adipose tissue : on the external side of the coronoid process there is a small smooth lateral surface, oval in shape, (the lesser sigmoid cavity,) which is concave from before backwards ; this depression is covered by an extension of the cartilage of the greater sigmoid cavity, and receives the internal side of the head of the radius.
The superior extremity of the radius forms a shallow circular depression which receives the lesser head of the humerus; this surface is covered by a cartilage which extends over its circumference on a circular surface applied to the lesser sigmuid cavity of the ulna internally, and embraced by the annular ligament in the rest of its extent : the articular head of the radius is supported on a cylindrical portion, called its neck, which is much smaller in its circumference, of about a finger's breadth long and curved a little outwards, its junction with the shaft of the bone being marked internally by a rough tuberosity, the tubercle of the ra dius, into the posterior side of which the tendon of the triceps is inserted.
Ligaments.—The fibrous ligaments of the elbow are four in number ; 1st, the anterior ligament consists of oblique and perpendicular fibres arising superiorly from the front of the condyles and the part of the humerus imme diately above the two anterior articular fussrc, and is inserted into the anterior edge of the coronoid process of the ulna inferiorly ; 2d, the posterior ligament is less distinct than the an terior, consisting of transverse fibres extending from one condyle to the other, which become more evident when the elbow is flexed; 3d, the external lateral ligament arises from the ante rior surface of the external condyle by a thick cord-like fasciculus of shining silvery fibres, and spreads out into a broad flat expansion, which is inserted into the whole length of the annular ligament of the radius and into the anterior and posterior margins of the lesser sig mold cavity of the ulna; the tendons of origin of the supinator brevis and extensor muscles of the hand are intimately connected to the exter nal surface of this ligament, but can be easily separated from it by careful dissection ; 4th, the internal lateral ligament arises from the an terior surface of the internal condyle of the humerus, and passing over the internal side of the synovial capsule, divides into two portions, an anterior and a posterior, the former of which is inserted into the inner side of the coronoid process, and the latter into the internal side of the olecranon : this ligament presents more of a flattened form, and is more easily separated from the tendons of the muscles which cover it than the external lateral ligament.
The synovial capsule, having covered the ar ticular surface of the humerus, ascends above this surface as high as an irregular continuous line, including the two anterior articular fossw in front, the posterior articular fossa behind, and limited by the bases of the condyles late rally ; at the level of this line the capsule is re flected from the humerus, and descends on the internal surfaces of the fibrous ligaments to be expanded over the articular surfaces of the radius and ulna, to the cartilaginous coverings of which it adheres in the same intimate man ner as to that of the articular surface of the humerus ; the portion of it corresponding to the radius descends within the annular liga ment, below which it is reflected on the neck, and thence continued over the head of that bone; while it becomes attached to the ulna at the line which circumscribes the greater arid lesser sigmoid cavities over the surfaces of which it is extended ; this capsule, which is rather tense where it lines the lateral ligaments, is flaccid and sacculated anteriorly and poste riorly, so as not to interfere with the freedom of flexion and extension of the elbow : below the margin of the annular ligament and before it is attached to the neck of the radius, it forms a cul-de-sac so loose as to permit the rotatory motions of that bone to be executed without restraint.
Several masses of adipose cellular tissue are situated around the articulation external to the synovial capsule, more especially in the articu lar fosse: at the posterior margin of the olecra non : between the radius and ulna and in the notcb on the Internal side of the greater sigmoid cavity, there always occurs a mass of this sub stance from which a production extends over the rough groove described above, dividing the sigmoid cavity transversely.