Undue extension, (by extension is meant such movement as will cause the long axis of two articulating bones to assume a position which approaches or forms a straight line—opposite to flexion), of the scapulohumeral joint is impossible while weight is borne, because of the normally flexed position of the humerus on the scapula; whereas flexion, beyond desirable limits, is inhibited by the biceps brachii (flexor brachii or coracoradialis) muscle.
The distal end of the humerus, however, articulating with the radius and ulna in a fashion that no support is lent by any sort of contact with the body, is a ginglymus (hinge) joint and lateral motion, because of the long transverse diameter of its articular portions, is easily prevented by the medial and lateral ligaments (internal and external ligaments). Flexion of this, the humeroradioulnar joint (elbow), is restrained by the triceps brachii and extension is checked by the biceps brachii (flexor brachii).
The carpal joint (erroneously called the knee joint), is composed of the several carpal bones which interarticulate and, when taken as a group, serve as a means of attachment and articulation for the radius and metacarpal bones.
The transverse diameter of this joint is long, thus giving it contacting surfaces that are sufficiently extensive to minimize the strain upon the mesial and lateral ligaments (internal and external lateral common ligaments). Motion is that of flexion and extension; slight rotation is possible when the position is that of flexion. While supporting weight the carpus is fixed in position by a slight dorsal flexion, but undue dorsal flexion is prevented by the flexor muscles and tendons and volar-carpal or annular ligament, together with the superior check ligament.
The metacarpophalangeal articulation (fetlock joint), is a hinge joint and its articular surfaces contact one another, with respect to their having a long bearing surface from side to side, as do all ginglymus (hinge) joints. Two common lateral ligaments bind the bones together. While bearing weight, there is assumed a position of slight dorsal flexion, undue flexion being checked by the inhibitory apparatus of the joint—check ligaments, and their tendons and the suspensory ligament. The inhibitory apparatus of the fetlock joint is materially reinforced by the proximal sesamoid bones. Situated as they are, between the bifurcating portions of the suspensory ligament and the posterior part of the distal end of the metacarpus—with which they articulate—the sesamoid bones serve to change the course of the branches of the suspensory ligament in a manner that they give firm support to this joint. Volar flexion is limited by the extensors of the phalanges.
Fig. 4—Sagital Section of Digit and Distal Part of Metacarpus.
A, Metacarpal bone; B, first phalanx; C, second phalanx, D, third phalanx; E, distal sesamoid bone; 1, volar pouch of capsule of fetlock joint; 2, inter-sesamoidean ligament; 3, 4, proximal end of digital synovial sheath; 5, ring formed by superficial flexor tendon; 6, fibrous tissue underlying ergot; 7, ergot; 8, 9, 9', branches of digital vessels; 10, distal ligament of distal sesamoid bone; 11, suspensory ligament of distal sesamoid bone; 12, 12', proximal and distal ends of bursa podotrochlearis. (From Sisson's "Anatomy of the Domestic Animals").
The first phalanx (os suffraginis) normally sets at an angle of about 50 to 55 degrees from a horizontal plane while weight is being supported. Its distal end articulates with the second or median phalanx (os corona) and forms the proximal interphalangeal (pastern or suffraginocoronary) joint. This also, is a ginglymus joint, having but slight lateral motion, and that only when it is in a state of flexion. A rather broad articular surface—from side to side—exists here, lessening the strain on the collateral ligaments somewhat. Dorsal flexion is checked by the flexor tendons and dorsal ligaments. Volar flexion is restrained by the extensor tendons.
The distal end of the second phalanx (os corona) has but slight lateral motion and this is manifested principally when it is in a state of volar flexion. Undue dorsal flexion is prevented by the deep flexor tendon (perforans) and volar flexion is inhibited by the extensor of the digit (extensor pedis). Thus it is seen, that when the leg is a weight-bearing member, weight is supported by the bony framework whose constituent parts are joined together by ligaments and tendons and each one of the several bones articulates in such manner that the joint is locked. The articular parts of bones rest upon or against an inhibitory apparatus, and are slightly flexed, as in the carpus, or considerably flexed such as in the fetlock joint when weight is being supported. In the first instance, for example, the flexors of the carpus and the superior check ligament assisted by the flexors of the phalanges constitute the inhibitory apparatus.