Part 47

1, Large metacarpal bone; 3, fetlock joint; 4, proximal sesamoid bone; 5, first phalanx; 6, pastern joint; 7, second phalanx; 8, coffin joint; 9, third phalanx; 10, distal sesamoid (navicular bone); 12, suspensory ligament; 14, deep flexor tendon; 15, superficial flexor tendon; 16, posterior annular ligament of fetlock; 20, inferior sesamoidean ligaments; 21, extensor tendon; 24, plantar cushion; 25, periople; 28, wall of hoof; 29, sole of hoof; A, navicular bursa, proximal part. (After Ellenberger-Baum, Anat. für Künstler.) ]

FIG. 149.—ARTICULAR SURFACES OF FIRST PHALANX AND SESAMOIDS AT FETLOCK, WITH INTERSESAMOID AND SUSPENSORY LIGAMENTS. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =joint capsule= is attached around the margin of the articular surfaces. It is thick and ample in front; here a bursa is interposed between it and the extensor tendons, but the tendons are also attached to the capsule. Posteriorly it forms a thin-walled pouch which extends upward between the metacarpal bone and the suspensory ligament about as high as the point of bifurcation of the latter. The capsule is reinforced by two lateral ligaments.

The =lateral ligaments=, =external= and =internal= (Ligamentum collaterale ulnare, radiale) are partially divided into two layers: the =superficial layer= arises from the eminence on the side of the distal end of the large metacarpal bone, and passes straight to the rough lateral area below the margin of the articular surface of the first phalanx; the =deep layer=, shorter and much stronger, arises in the lateral depression on the distal end of the metacarpal bone, and passes obliquely downward and backward to be inserted into the outer surface of the sesamoid and the proximal end of the first phalanx.

The capsule is further strengthened by a layer of oblique fibers which pass over the lateral ligament on either side and end on the extensor tendon and the proximal extremity of the first phalanx. It may properly be regarded as fascia rather than ligament.

FIG. 150.—LIGAMENTS AND TENDONS OF DISTAL PART OF LIMB OF HORSE.

Mc.III, Large metacarpal bone; Ph.I, first phalanx; Ph.II, second phalanx; Ph.III, third phalanx; 1, deep flexor tendon; 2, band from first phalanx to plantar cushion. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

=Movements.=—These are of the nature of flexion and extension, the axis of motion passing through the upper attachments of the lateral ligaments. In the ordinary standing position the joint is in a state of partial =dorsal flexion=, the articular angle (in front) being about 140° to 150°. (In the hind limb it is about 5° greater.) Diminution of this angle (sometimes termed “overextension”) is normally very limited on account of the resistance offered by the sesamoidean apparatus, but it varies considerably in amount in different subjects. =Volar flexion= is limited only by contact of the heels with the metacarpus. During volar flexion a small amount of lateral flexion is possible.

THE SESAMOIDEAN LIGAMENTS

Under this head will be described a number of important ligaments which are connected with the sesamoid bones and form a sort of stay apparatus or brace.

The =intersesamoidean ligament= (Ligamentum intersesamoideum) not only fills the space between and unites the sesamoid bones, but also extends above them, entering into the formation of the articular surface of the fetlock joint. Other facts in regard to it have been given above.

The =lateral sesamoidean ligaments=, outer and inner (Ligamenta sesamoidea ulnare et radiale), arise on the abaxial surface of each sesamoid bone, pass forward, and divide into two branches, one of which ends in the depression on the distal end of the large metacarpal bone, the other on the eminence on the proximal end of the first phalanx. They are partly covered by the branches of the suspensory or superior sesamoidean ligament.

FIG. 151.—DEEP DISSECTION OF DISTAL PART OF RIGHT FORE LIMB OF HORSE, SHOWING JOINTS AND LIGAMENTS, POSTERIOR VIEW.

1, Lateral cartilage; 2, tendon surface of navicular bone; 3, inferior navicular or interosseous ligament; 4, insertion of deep flexor tendon. Small arrows point to openings made in capsules of pastern and coffin joints. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =suspensory= or =superior sesamoidean ligament= (Musculus interosseus medius) lies for the greater part in the metacarpal groove, where it has the form of a wide, thick band. It is attached above to the upper part of the posterior surface of the large metacarpal bone and to the distal row of carpal bones. At the lower fourth of the metacarpus it divides into two diverging branches. Each branch passes to the abaxial face of the corresponding sesamoid, on which a considerable part, is attached. The remainder passes obliquely downward and forward to the anterior surface of the first phalanx, where it joins the extensor tendon. This ligament possesses considerable elasticity, and is the highly modified interosseous medius muscle. It consists mainly of tendinous tissue, but contains a variable amount of striped muscular tissue, especially in its deep part and in young subjects. Its principal function is to support the fetlock, i. e., to prevent excessive dorsal flexion of the joint when the weight is put on the limb. The branches which join the common extensor tendon limit volar flexion of the interphalangeal joints in certain phases of movement.

The =inferior sesamoidean ligaments= are three in number—superficial, middle, and deep. The =superficial= or =straight ligament= (Ligamentum sesamoideum rectum) is a flat band and is somewhat wider above than below. It is attached above to the bases of the sesamoid bones and the intersesamoid ligament, below to the complementary fibro-cartilage of the proximal end of the second phalanx. The =middle ligament= is triangular, with thick, rounded margins (Ligamenta obliqua) and a thin central portion. Its base is attached to the sesamoid bones and intersesamoid ligament, and its deep face to the triangular rough area on the posterior surface of the first phalanx. The =deep= or =cruciate ligament= (Ligamenta sesamoidea cruciata) consists of two thin layers of fibers which arise on the base of the sesamoid bones, cross each other, and end on the opposite eminence on the proximal end of the first phalanx.

The =short sesamoidean ligaments= (Ligamenta sesamoidea brevia) are best seen by opening the joint in front and pushing the sesamoid bones backward; they are covered by the synovial membrane. Each is a short band which extends from the anterior part of the base of the sesamoid bone outward to the posterior margin of the articular surface of the first phalanx.

The inferior sesamoidean ligaments may be regarded as digital continuations of the suspensory ligament, the sesamoid bones being intercalated in this remarkable stay apparatus by which the fetlock is supported and concussion diminished.

THE PASTERN JOINT

This, the =proximal interphalangeal articulation= (Articulatio phalangis secundæ), is a ginglymus formed by the junction of the distal end of the first phalanx and the proximal end of the second phalanx.

The =articular surfaces= are: (1) On the first phalanx, two slightly unequal convex areas with an intermediate shallow groove; (2) on the second phalanx, a corresponding surface, completed behind by a plate of fibro-cartilage.

The =joint capsule= is close fitting in front and laterally, where it blends with the extensor tendon and the lateral ligaments respectively. Behind it pouches upward a little and is reinforced by the straight sesamoidean ligament and the branches of the superficial flexor tendon.

There are two lateral and four volar ligaments.

The =lateral ligaments=, internal and external (Ligamentum collaterale radiale, ulnare) are very short and strong bands which are attached above on the eminence and depression on each side of the distal end of the first phalanx, and below on the eminence on either side of the proximal end of the second phalanx. The direction of the ligaments is about vertical and, therefore, does not correspond to the digital axis.

The =volar= or =posterior ligaments= consist of central and lateral pairs of bands which are attached below to the posterior margin of the proximal end of the second phalanx and its complementary fibro-cartilage. The lateral pair is attached above to the middle of the borders of the first phalanx, the central pair lower down and on the margin of the triangular rough area.

These ligaments are very commonly thickened as a result of chronic inflammation, and then are not well defined. The central ones blend below with the branches of the superficial flexor tendon and with the straight sesamoidean ligament.

=Movements.=—These are very limited, and consist of flexion and extension. The axis of motion passes transversely through the distal end of the first phalanx. In the standing position the joint is extended. A small amount of volar flexion is possible, and in this position slight lateral flexion and rotation can be produced by manipulation. Dorsal flexion is prevented by the lateral, volar, and straight sesamoidean ligaments.

THE COFFIN JOINT