Part 23

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

This joint, technically termed the =distal interphalangeal articulation= (Articulatio phalangis tertiæ), is a ginglymus formed by the junction of the second and third phalanges and the third sesamoid bone.

=Articular Surfaces.=—The surface on the distal end of the second phalanx is convex from before backward, concave transversely. The articular surface of the third phalanx slopes sharply upward and forward; its central part is prominent, and is flanked by two glenoid cavities. It is completed behind by the articular surface of the third sesamoid or navicular bone.

=Joint Capsule.=—This is attached around the margins of the articular surfaces. In front and laterally it is tight, and is blended with the extensor tendon and the lateral ligaments respectively. Posteriorly, it forms a considerable pouch which extends upward to about the middle of the second phalanx, where it is separated by a fibrous membrane from the digital synovial sheath. Laterally small pouches project outward (especially during volar flexion) against the lateral cartilages, just behind the lateral ligaments.[36]

=Ligaments.=—The =lateral ligaments=, external and internal (Ligamentum collaterale ulnare, radiale), are short strong bands which are attached above in the depressions on either side of the lower part of the second phalanx, under cover of the lateral cartilage. They widen below and end in the depressions on either side of the extensor process and on the anterior end of the lateral cartilages.

The =suspensory navicular ligaments=, external and internal (Ligamentum sesamoideum collaterale ulnare, radiale),[37] are strong, somewhat elastic bands, which form a sort of suspensory apparatus for the third sesamoid. They are attached superiorly in and above the depressions on either side of the distal end of the first phalanx and are here partly blended with the lateral ligaments of the pastern joint. They are directed obliquely downward and backward, and end chiefly on the ends and proximal border of the third sesamoid, but detach a branch to the inner surface of each lateral cartilage and wing of the third phalanx.

The =inferior navicular ligament= (Ligamentum phalangeo-sesamoideum) reinforces the capsule interiorly. It is a strong layer of fibers which extend from the distal border of the third sesamoid to the tendon surface of the third phalanx, near the posterior margin of the articular surface.

=Movements.=—The chief movements are flexion and extension. In the standing position the joint is extended. During volar flexion a very small amount of lateral movement and rotation can be produced by manipulation. Dorsal flexion is very limited.

Dorsal flexion appears to be checked mainly by the deep flexor tendon, since in cases of rupture of the latter the toe turns up. The slight mobility of the posterior part of the socket for the second phalanx (formed by the third sesamoid) diminishes concussion when the weight comes on the foot.

LIGAMENTS OF THE LATERAL CARTILAGES

In addition to the bands mentioned above, which attach the lateral cartilages to the extremities of the navicular bone, there are three ligaments on either side which attach the cartilages to the phalanges.

An ill-defined elastic band passes from the middle part of the border of the first phalanx to the upper part of the cartilage, detaching a branch to the plantar cushion.

A short strong band connects the anterior extremity of the cartilage with the rough eminence on the second phalanx in front of the attachment of the lateral ligament of the coffin joint.

The lower border of the cartilage is covered externally by fibers which attach it to the wing of the third phalanx.

THE ARTICULATIONS OF THE PELVIC LIMB

THE SACRO-ILIAC ARTICULATION

This joint (Articulatio sacro-iliaca) is a diarthrosis formed between the auricular surfaces of the sacrum and ilium. These surfaces are not smooth in the adult, but are marked by eminences and depressions, and are covered by a thin layer of cartilage. The joint cavity is a mere cleft, and is often crossed by fibrous bands.

The =capsule= is very close fitting, and is attached around the margins of the articular surfaces. It is reinforced by the =ventral sacro-iliac ligament= (Ligamentum sacro-iliacum ventrale), which surrounds the joint, and is exceedingly strong above.

The =movements= are inappreciable in the adult—stability, not mobility, being the chief desideratum.

The following ligaments may be regarded as accessory to the joint, although not directly connected with it.

The =dorsal sacro-iliac ligament= (Ligamentum sacro-iliacum dorsale breve) is a strong band which is attached to the internal angle (Tuber sacrale) of the ilium and the summits of the sacral spines.

The =lateral sacro-iliac ligament= (Ligamentum sacro-iliacum dorsale longum) is a triangular, thick sheet which is attached in front to the internal angle and border of the ilium above the great sciatic notch, and below to the lateral border of the sacrum. It blends above with the dorsal sacro-iliac ligament, below with the sacro-sciatic ligament, and behind with the coccygeal fascia.

The =sacro-sciatic ligament= (Ligamentum sacrospinosum et tuberosum) is a quadrilateral sheet which completes the lateral pelvic wall. Its upper border is attached to the border of the sacrum and the transverse processes of the first and second coccygeal vertebræ. Its lower border is attached to the superior ischiatic spine and tuber ischii. Between these it bridges over the external border of the ischium and completes the lesser sciatic foramen. The anterior border is concave, and completes the greater sciatic foramen. The posterior border is fused with the vertebral head of the semimembranosus muscle.

The =ilio-lumbar ligament= (Ligamentum ilio-lumbale) is a triangular sheet which attaches the ends of the lumbar transverse processes to the ventral surface of the ilium below the attachment of the longissimus muscle (Fig. 184).

SYMPHYSIS PELVIS

The =symphysis pelvis= is formed by the junction of the two ossa coxarum at the ventral median line. In the young subject the bones are united by a layer of cartilage; in the adult the latter is gradually replaced by bone, the process beginning in the pubic portion and extending backward. The union is strengthened by white fibrous tissue above and below, and a transverse band also covers the anterior border of the pubis (pecten). No appreciable movement occurs even before synostosis takes place.

OBTURATOR MEMBRANE

This (Membrana obturatoria) is a thin layer of fibrous tissue which covers the obturator foramen, leaving, however, a passage (Canalis obturatorius) for the obturator vessels and nerve.

THE HIP JOINT

This joint (Articulatio coxæ) is an enarthrosis formed by the proximal end of the femur and the acetabulum.

=Articular Surfaces.=—The head of the femur presents an almost hemispherical articular surface, which is continued a short distance on the upper surface of the neck. It is more extensive than the socket which receives it. Internally it is cut into by a deep notch for the attachment of the round and pubo-femoral ligaments. The acetabulum is a typical cotyloid cavity. Its articular surface is somewhat crescentic, being deeply cut into internally by the acetabular notch and fossa. It is increased and deepened by a ring of fibro-cartilage, the =cotyloid ligament= (Labrum glenoidale), which is attached to the bony margin; that part of the ligament which crosses the notch is called the =transverse ligament= (Fig. 456).

The =joint capsule= is roomy. It is attached around the margin of the acetabulum and the neck of the femur. It is thickest externally.

The attachment on the femur is about 1 cm. from the margin of the articular surface, except above, where 2 to 3 cm. of the neck is intracapsular. A thin oblique band corresponding in direction with the rectus parvus muscle reinforces the antero-external part of the capsule; this appears to be the feeble homologue of the very strong ilio-femoral ligament of man. The capsule is very thin under the ilio-psoas, and is adherent to the muscle. Internally, its fibrous part is perforated by the pubo-femoral and round ligaments and the articular vessels.

The =round ligament= (Ligamentum teres) is a strong hand which is attached in the subpubic groove close to the acetabular notch, passes outward, and ends in the notch on the head of the femur (Fig. 456).

The =pubo-femoral ligament= (Ligamentum accessorium) does not occur in the domestic animals other than the equidæ. It is a strong band detached from the prepubic tendon of the abdominal muscles (Fig. 456). It is directed outward, backward, and upward, passes through the acetabular notch above the transverse ligament, and ends behind the round ligament in the notch on the head of the femur. The origin of the pectineus muscle is perforated by the ligament, which furnishes attachment to the greater part of the fibers of the muscle.

The =synovial membrane= is reflected over the intracapsular parts of these ligaments and covers the fossa acetabuli. A pouch also extends from the acetabular notch for a variable distance along the subpubic groove above the pubo-femoral ligament.

=Movements.=—This joint is capable of all the movements of a ball-and-socket joint, viz., flexion, extension, abduction, adduction, rotation, and circumduction. The greatest range of movement is displayed in flexion and extension. When standing at rest, the joint is partially flexed, the articular angle (in front) being about 100° to 115°. The other movements occur to a very limited extent in normal action.

Abduction appears to be checked by tension of the round ligament. The pubo-femoral ligament is tensed most promptly by inward rotation of the thigh.

THE STIFLE JOINT

This joint (Articulatio genu), which corresponds to the knee-joint of man, is the largest and most elaborate of all the articulations. Taken as a whole, it may be classed as a ginglymus, although it is not a typical example of the group. In reality it consists of two joints—the =femoro-patellar= and the =femoro-tibial=.

The =femoro-patellar articulation= (Articulatio femoro-patellaris) is formed between the trochlea of the femur and the articular surface of the patella.

=Articular Surfaces.=—The trochlea consists of two slightly oblique ridges, with a wide and deep groove between them. The inner ridge is much the larger of the two, especially at its upper part, which is wide and rounded. The outer ridge is much narrower, and is more regularly curved; its upper part lies about an inch behind a frontal plane tangent to the inner ridge. The articular surface of the patella is much smaller than that of the trochlea. It is completed internally by a supplementary plate of fibro-cartilage (Fibrocartilago patellæ), which curves over the internal surface of the inner lip of the trochlea. A narrow strip of cartilage is found along the outer border also. The articular cartilage on the trochlea completely covers both surfaces of the inner ridge, but only a narrow marginal area on the external surface of the outer ridge.

=Joint Capsule.=—This is thin and is very capacious. On the patella it is attached around the margin of the articular surface, but on the femur the line of attachment is at a varying distance from the articular surface. On the inner side it is an inch or more from the articular cartilage; on the outer side and above, about half an inch. It pouches upward under the quadriceps femoris for a distance of two or three inches, a pad of fat separating the capsule from the muscle. Below the patella it is separated from the straight ligaments by a thick pad of fat, but inferiorly it is in contact with the femoro-patellar capsules. The joint cavity is the most extensive in the body. It usually communicates with the inner femoro-tibial joint cavity by a slit-like opening situated at the lowest part of the inner ridge of the trochlea. A similar, but smaller, communication with the outer femoro-patellar capsule is often found at the lowest part of the outer ridge.

The inner communication appears to be constant in adult horses, but is liable to be overlooked on account of the fact that it is covered by a valvular fold of the synovial membrane. It is about half an inch wide, and lies under the narrow articular area which connects the trochlea and internal condyle. The outer communication occurs in 18 to 25 per cent. of cases, according to Baum. It is instructive to distend this capsule and thus obtain an idea of its potential capacity and relations ([Fig. 156]).

=Ligaments.=—The =lateral femoro-patellar ligaments=, external and internal (Ligamentum femoro-patellare fibulare, tibiale), are two thin bands which reinforce the capsule on either side. The external ligament is fairly distinct; it arises from the external epicondyle of the femur just above the lateral femoro-tibial ligament, and ends on the external border of the patella. The internal ligament is thinner and is not distinct from the capsule; it arises above the internal epicondyle, and ends on the patellar fibro-cartilage.

The =patellar ligaments= (Ligamenta patellæ), also called the straight ligaments of the patella, are three very strong bands which attach the patella to the tuberosity of the tibia. The =external patellar ligament= is attached above to the outer part of the anterior surface of the patella, and below to the outer part of the tuberosity. It receives a strong tendon from the biceps femoris muscle. The =middle patellar ligament= extends from the front of the apex of the patella to the lower part of the groove on the tuberosity of the tibia, a bursa being interposed between the ligament and the upper part of the groove. The =internal patellar ligament= is attached above to the patellar fibro-cartilage, and ends on the tuberosity of the tibia, internally to the groove. It is joined by the common aponeurosis of the gracilis and sartorius. These so-called ligaments are, in reality, the tendons of insertion of the quadriceps femoris muscle, and transmit the action of the latter to the tibia; they also function similarly for the other muscles attached to them as noted above.

It will be noticed that the upper attachments are further apart than the lower ones, so that the ligaments converge below. The inner ligament is especially oblique. The middle ligament is more deeply placed than the others, and therefore cannot usually be felt distinctly in the living animal.

The =femoro-tibial articulation= (Articulatio femoro-tibialis) is formed between the condyles of the femur, the proximal end of the tibia, and the interposed semilunar cartilages.

=Articular Surfaces.=—The condyles of the femur are slightly oblique in direction. The articular surface of the outer one is more strongly curved than that of the inner one; the latter is confluent below with the inner ridge of the trochlea, while the narrow ridge which connects the external condyle with the trochlea is usually non-articular. The saddle-shaped surfaces of the condyles of the tibia are not adapted to the femoral condyles, and are in contact with only a small part of them.

The =semilunar cartilages= (Meniscus lateralis, medialis) are two C-shaped or crescentic discs of fibro-cartilage which produce congruence in the articular surfaces. Each has an upper concave surface adapted to the condyle of the femur, and a lower surface which fits the corresponding condyle of the tibia. The external cartilage does not cover the outer and posterior part of the condyle, over which the tendon of origin of the popliteus muscle plays. The peripheral border is thick and convex, the central one very thin and concave. The fibrous ends or =cornua= are attached to the tibia in front of and behind the spine. The external cartilage has a third attachment by means of an oblique band (Ligamentum femorale menisci lateralis) which passes from the posterior cornu to the posterior part of the intercondyloid fossa of the femur.

The cornua of the internal cartilage (Ligamenta tibiæ anterius et posterius menisci medialis) are attached in front of and behind the inner eminence of the spine. The anterior cornu of the external cartilage (Ligamentum tibiæ anterius menisci lateralis) is attached in front of the outer eminence of the spine. The posterior cornu bifurcates; the lower branch (Ligamentum tibiæ posterius menisci lateralis) is inserted at the popliteal notch, the upper (Ligamentum femorale menisci lateralis) in a small fossa in the extreme posterior part of the intercondyloid fossa.

The =joint capsule= is attached to the margin of the tibial articular surface, but on the femur the line of attachment is for the greater part about half an inch from the articular margin. It is also attached to the convex borders of the semilunar cartilages and to the crucial ligaments. It is strong posteriorly, but in front it practically consists only of the synovial layer. There are two synovial sacs, corresponding to the double nature of the articular surfaces; they do not usually communicate, and each is partially divided into an upper and a lower compartment by the semilunar cartilage. The inner sac pouches upward about half an inch over the condyle of the femur. The external sac invests the tendon of origin of the popliteus muscle, and also pouches downward about three or four inches (ca. 8 to 10 cm.) beneath the peroneus tertius and long extensor muscles (Fig. 156). As stated above, the outer sac sometimes communicates with the femoro-patellar joint cavity, and the inner sac usually, if not always, does so in the adult.

_18_, Femur; _20_, patella (base); _21_, tibia; _d_, internal semilunar cartilage. (After Ellenberger-Baum, Anat. für Künstler.) ]

=Ligaments.=—There are four of these—two lateral and two crucial.

The =internal lateral ligament= (Ligamentum collaterale tibiale) is attached above to the prominent internal epicondyle of the femur, and below to a rough area below the margin of the internal condyle of the tibia.

The =external lateral ligament= (Ligamentum collaterale fibulare) is somewhat thicker; it arises from the upper depression on the external epicondyle, and ends on the head of the fibula. It covers the tendon of the origin of the popliteus muscle, and a bursa is interposed between the lower part of the ligament and the margin of the external condyle of the tibia.

The =crucial ligaments= are two strong rounded bands situated mainly in the intercondyloid fossa of the femur, between the two synovial sacs. They cross each other somewhat in the form of an X, and are named according to their tibial attachments. The =anterior crucial ligament= (Ligamentum cruciatum anterius) arises in the central fossa on the tibial spine, extends upward and backward, and ends on the upper part of the inner surface of the external condyle of the femur. The =posterior crucial ligament= (Ligamentum cruciatum posterius) is internal to the preceding, and is somewhat larger. It is attached to an eminence at the popliteal notch of the tibia, is directed upward and forward, and ends in the anterior part of the intercondyloid fossa of the femur.

It may be added that these ligaments do not lie in a sagittal plane, but are somewhat twisted across each other; outward rotation of the leg untwists and slackens them.

=Movements.=—The principal movements of the stifle joint as a whole are flexion and extension. In the ordinary standing position the articular angle (behind) is about 140° to 150°. Flexion is limited only by contact of the leg with the thigh if the hock is also flexed. Extension is incomplete, _i. e._, the femur and tibia cannot be brought into the same straight line. Rotation is limited, and is freest during semiflexion. The patella glides on the femoral trochlea upward in extension, downward in flexion.