The synovial capsule adheres closely to the fibrous ligaments, except where masses of adi pose tissue are interposed, to which it is but loosely connected.
Notions.—The elbow is a joint remarkable for possessing great solidity, which is partly owing to the extent of its osseous surfaces and the manner in which they are locked into each other, and partly to the strong lateral ligaments and the muscles which surround it.
The motions enjoyed by the elbow-joint arc flexion and extension.
Flexion may vary in degree so as to be com plete or incomplete : in complete flexion the fore-arm is carried forwards and inwards in an oblique direction across the front of the thorax, so as to bring the hand towards the mouth ; the direction of the fore-arm is determined in this movement by the obliquity of the trochlea of the humerus from behind forwards and in wards, as described above, and influenced by the clavicle preventing the falling inwards of the shoulder; were it riot for the support of the clavicle, the hand in this movement, instead of being carried to the mouth, would be direct ed to the shoulder of the opposite side : when flexion of the elbow is carried to its greatest extent, the coronoid process and the head of the radius are received into the anterior articu lar fossa of the humerus, displacing the adipose masses from these cavities, the olecranon is brought downwards on the trochlea so as to be placed below the level of the condyles of the humerus ; the posterior part of the synovial capsule, the posterior ligament, and the triceps and anconwus muscles are made tense, and applied to the adipose mass in the posterior articular fossa and to the posterior part of the troehlea: the anterior part of the capsule and the anterior ligament are relaxed, as are also the lateral ligaments. A dislocation is rendered impossible in this state of the articulation, being effectually opposed by the hold which the coronoid process has on the front of the trochlea of the humerus.
In partial flexion or semiflexion, the several parts of the articulation are differently circum stanced ; the coronoid process being carried down is no longer applied to the front of the humerus, the olecranon is on a plane with the condyles, and the lateral ligaments are on the stretch : in this state of the parts a powerful force applied to the olecranon from behind might have the effect of displacing the ulna forwards, were it not for the great mobility of the limb, owing to which a force thus applied is moderated or altogether expended in increa sing the degree of flexion ; hence a dislocation of the ulna forwards on the humerus is an acci dent which never happens.
In extension, the olecranon, ascending above the level of the condyles, is received into the posterior articular fossa, displacing the adipose substance which previously occupied that fossa, the radius Is brought back on the lesser timid of the humerus, over the anterior part of which and of the trochlea the capsule and the anterior ligament are stretched ; the lateral ligaments, the tendon of the triceps, and the brachia us anticus are also in a state of tension : the pos terior part of the capsule and the posterior ligament are necessarily relaxed. It is when the elbow is in such a state of extension as here described that a dislocation of the fore-arm backwards usually occurs in consequence of a fall on the hand ; the force producing the dis location in this case operates in the following way, the fore-arm serving as a fixed point, the humerus becomes a lever of the first order, the fulcrum of which is the point of the olecranon applied to the posterior side of its lower extre mity, the power is represented by the weight of the trunk of the body applied to its superior extremity in front, and acting with a force pro portioned to its remoteness from the point of resistance formed by the ligaments and muscles which are found in a state of tension in front ; when this force is such as to overcome the re sistance, the ligaments in front are ruptured, the lower extremity of the humerus is then driven downwards in front of the hones of the fore-arm, the upper extremities of which are forced upwards behind the humerus, so that the coronoid process comes to occupy the nor mal situation of the olecranon in the posterior articular fossa.
Lateral motion. — Anatomists have been divided in opinion as to the possibility of any lateral motion being performed by the ulna on the humerus. Albinos, Boyer, Bedard, Cru veilliier, and others, have denied the occurrence of it; Monro and Bichat, however, have dis tinctly noticed it : they consider that this mo tion is possible only in the semiflxed state of the elbow, when the lateral ligaments are most relaxed : in complete flexion, as well as in ex tension, the tense state of these ligaments effec tually opposes any such movement. In my opinion it is easy to satisfy one's self as to the occurrence of this motion ; it consists of a slight degree of rolling of the middle prominent part of the greater sigmoid cavity in the fossa of the trochlea, produced by those fibres of the lower part of the triceps which extend from the con dyle on each side to the olecranon, and by the action of the anconwus externally, (J. Hart.)