Part 32

_Insertion._—An oblique line on the posterior surface of the femur, near the lower part of the internal trochanter.

_Action._—To extend the hip joint, and to adduct the thigh and rotate it outward.

_Structure._—It is composed of parallel bundles of fibers directed downward, forward, and outward.

_Relations._—Internally, the adductor, semimembranosus, and the obturator vessels; externally, the obturator externus and biceps femoris, the deep femoral artery, and the great sciatic nerve.

_Blood-supply._—Deep femoral and obturator arteries.

_Nerve-supply._—Great sciatic nerve.

2. =Obturator Externus= (Fig. 456).—This is a pyramidal muscle which extends across the back of the hip joint from the obturator foramen to the trochanteric fossa.

_Origin._—The ventral surface of the pubis and ischium, and the margin of the obturator foramen.

_Insertion._—The trochanteric fossa.

_Action._—To adduct the thigh and to rotate it outward.

_Structure._—It is almost entirely fleshy, the muscle-bundles being rather loosely connected. The insertion is pointed, flattened, and partly tendinous. The origin is perforated by the obturator vessels and nerve.

_Relations._—Internally, the adductor and quadratus femoris and the deep femoral vessels; externally, the gemellus, the tendon of the obturator internus, the biceps femoris, and the great sciatic nerve; anteriorly, the hip joint, the pectineus, and the external pudic vein.

_Blood-supply._—Deep femoral and obturator arteries.

_Nerve-supply._—Obturator nerve.

3. =Obturator internus= (Fig. 451).—This arises by two heads within the pelvic cavity, the tendon emerging through the lesser sciatic foramen.

_Origin._—(1) The pelvic surface of the pubis and ischium around the obturator foramen; (2) the pelvic surface of the shaft of the ilium and the wing of the sacrum.

_Insertion._—The trochanteric fossa.

_Action._—To rotate the femur outward.

_Structure._—The ischio-pubic head lies on the pelvic floor and covers the obturator foramen. It is thin and fan-shaped. The iliac head extends along the lateral wall of the pelvis, and is pennate, with a central tendon throughout. Both terminate on a flat tendon which passes outward through the lesser sciatic foramen to be inserted into the trochanteric fossa. A synovial bursa facilitates the play of the tendon over the external border of the ischium.[69]

_Relations._—The pelvic surface is covered by the pelvic fascia and the peritoneum. The obturator vessels and nerve lie between the two heads, and the internal pudic vessels and nerve lie along the dorsal edge of the iliac head.

_Blood-supply._—Obturator and internal pudic arteries.

_Nerve-supply._—Great sciatic nerve.

4. =Gemellus=[70] (Fig. 455).—This is a thin, triangular muscle, which extends from the external border of the ischium to the trochanteric fossa.

_Origin._—The external border of the ischium near the ischiatic spine.

_Insertion._—The trochanteric fossa.

_Action._—To rotate the femur outward.

_Structure._—Fleshy, some fibers being inserted into the tendon of the obturator internus.

_Relations._—Dorsally, the tendon of the obturator internus and the gluteus profundus; ventrally, the obturator externus.

_Blood-supply._—Obturator artery.

_Nerve-supply._—Sciatic nerve.

III. ANTERIOR MUSCLES OF THE THIGH

This group consists of the sartorius, quadriceps femoris, and capsularis.

1. =Sartorius.=—This is described on p. 281.

2. =Quadriceps femoris= (Figs. 179, 186, 200) (Crural triceps).—This constitutes the large muscular mass which covers the front and sides of the femur. It has four heads, one of which, the rectus, arises from the ilium; the other three arise from the femur. All are inserted into the patella.

(1) =Rectus femoris.=—This is fusiform and rounded. It arises by two tendons.

_Origin._—Two depressions on the shaft of the ilium above and in front of the acetabulum.

_Insertion._—The base and anterior surface of the patella.

_Action._—To extend the stifle joint and to flex the hip joint.

_Structure._—It has two short strong tendons of origin; beneath the outer one is a =bursa=. The belly is rounded and rests in a groove formed by the other portions of the quadriceps. Its sides are covered by a strong tendinous layer which furnishes insertion to fibers of the vasti. The tendon of insertion is formed by the fusion of these tendinous layers on the lower part of the muscle. The lower portion of the muscle is pennate, the fibers on either side converging on the tendon at an acute angle.

_Relations._—Internally, the iliacus, sartorius, and vastus internus; externally, the tensor fasciæ latæ, glutei, and vastus externus; posteriorly, the hip joint and the vastus intermedius; anteriorly, the fascia lata and the skin. The anterior femoral artery and branches of the femoral nerve descend into the interspace between the upper part of the rectus and the vastus internus; similarly, the iliaco-femoral artery dips in between the rectus femoris and vastus externus.

_Blood-supply._—Femoral and iliaco-femoral arteries.

_Nerve-supply._—Femoral nerve.

(2) =Vastus externus= (s. lateralis).—This lies on the outer surface of the thigh, extending from the great trochanter to the patella. It is thick and wide in its upper part, and becomes much thinner and narrow below.

[Illustration:

FIG. 200.—CROSS-SECTION OF MIDDLE OF RIGHT THIGH OF HORSE. ]

_Origin._—The external border and surface of the femur, from the great trochanter to the supracondyloid fossa.

_Insertion._—(1) The outer part of the anterior surface of the patella; (2) the tendon of the rectus femoris.

_Action._—To extend the stifle joint.

_Structure._—The fibers are directed downward and forward, many being inserted into the tendinous sheet which covers the side of the rectus. A =bursa= is often found between the distal end and the patella.

_Relations._—Externally, the fascia lata and skin, tensor fasciæ latæ, superficial gluteus, and biceps femoris; internally, the femur and femoro-patellar joint capsule, the rectus femoris, vastus intermedius, and the iliaco-femoral artery.

_Blood-supply._—Iliaco-femoral artery.

_Nerve-supply._—Femoral nerve.

(3) =Vastus internus= (s. medialis).—This is smaller than the preceding muscle, and lies in a similar position on the inner side of the thigh.

_Origin._—The internal surface of the femur, from the neck to the distal third.

_Insertion._—(1) The upper part of the inner border of the patella and its cartilage; (2) the tendon of the rectus femoris.

_Action._—To extend the stifle joint.

_Structure._—This is very similar to that of the vastus externus. It is, however, more difficult to separate from the intermedius, because many fibers of the latter arise on the tendinous sheet which covers the contact surface of the inner vastus. Its insertion into the patella is chiefly by means of a broad strong tendon. From the deep face fleshy fibers are inserted also into the femoro-patellar capsule.

_Relations._—Internally, the skin and fascia lata, the iliacus, sartorius, pectineus, and adductor, the femoral vessels and saphenous nerve; externally, the femur, femoro-patellar joint capsule, rectus femoris, and vastus intermedius, the anterior femoral artery, and branches of the femoral nerve.

_Blood-supply._—Femoral and anterior femoral arteries.

_Nerve-supply._—Femoral nerve.

(4) =Vastus intermedius= (Crureus).—This muscle is deeply situated on the anterior face of the femur, and is entirely covered by the preceding heads.

_Origin._—(1) The anterior and external surfaces of the femur, from the proximal to the distal fourth; (2) the aponeurosis of the vastus internus.

_Insertion._—(1) The base of the patella; (2) the femoro-patellar joint capsule.

_Action._—(1) To extend the stifle joint; (2) to tense (raise) the femoro-patellar capsule during extension of the joint.

_Structure._—The muscle is usually quite difficult to isolate from the other vasti, so that many since Günther have declared it an artefact.[71] It is entirely fleshy, and is small at its proximal end, but when traced downward increases in bulk by the accession of fibers arising on the femur and the tendinous covering of the vastus internus. The terminal part is intimately adherent to the femoro-patellar joint capsule, where the latter bulges upward above the level of the patella.

_Relations._—Internally, the vastus internus; externally, the vastus externus; anteriorly, the rectus; posteriorly, the femur and femoro-patellar capsule.

_Blood-supply._—Iliaco-femoral and anterior femoral arteries.

_Nerve-supply._—Femoral nerve.

The straight ligaments of the patella are to be regarded as tendons of the quadriceps which communicate the action of the latter to the tibia, the patella being intercalated as a sesamoid bone.

3. =Capsularis= (Fig. 154) (Rectus parvus).—This is a small muscle (scarcely as large as one’s finger), which arises on the ilium immediately above the outer tendon of the rectus femoris, and passes down between the vastus internus and externus to be inserted into the anterior surface of the femur. It passes over the front of the hip joint, to the capsule of which some fibers are attached. Sometimes the muscle has two distinct heads, in which case the additional head arises between the two tendons of origin of the rectus femoris. Its action may be to raise the capsule during flexion of the joint.

IV. THE MUSCLES OF THE LEG AND FOOT

[Illustration:

FIG. 201.—MUSCLES OF LOWER PART OF THIGH, LEG, AND FOOT OF HORSE, EXTERNAL VIEW.

_o′_, Fascia lata; _q_, _q′_, _q″_, biceps femoris; _r_, semitendinosus; _21′_, external condyle of tibia. (After Ellenberger-Baum, Anat. für Künstler.) ]

The muscles of this region cover almost all of the tibia except its internal face, which is largely subcutaneous. As in the forearm, the muscles fall into two groups, an anterior or dorso-lateral, and a posterior or plantar. The muscles of the first group are extensors of the digit and flexors of the hock, those of the second have the opposite action.

A. ANTERIOR GROUP

1. =Anterior= or =long digital extensor= (M. extensor digitalis longus; extensor pedis; anterior extensor of the phalanges).—This muscle is situated superficially on the antero-external aspect of the leg, and is provided with a long tendon which passes down over the front of the tarsus, metatarsus, and digit.

_Origin._—The small fossa (Fossa extensoria) between the external condyle and the trochlea of the femur.

_Insertion._—(1) The extensor process of the third phalanx; (2) the anterior surface of the proximal extremities of the first and second phalanges.

_Action._—To extend the digit and flex the hock. It also assists in fixing the stifle joint.

_Structure._—The origin is by means of a strong tendon in common with the peroneus tertius, on which also many fibers arise. The common tendon passes downward in the groove between the outer condyle and the tuberosity of the tibia, where a pouch from the femoro-tibial capsule descends four or five inches (ca. 10 to 12 cm.) beneath the tendon. The belly is fusiform and somewhat flattened. The long tendon of insertion begins in the belly about its middle, and is clear of the fleshy part near the tarsus. It passes downward over the front of the hock, bound down by the three annular ligaments already described (see crural fascia), and enveloped by a =synovial sheath= which begins a little above the level of the external malleolus, and extends nearly to the junction with the lateral extensor tendon. This union occurs usually about a hand’s breadth below the tarsus. In the angle of union the extensor brevis also joins the principal tendon. Beyond this point the arrangement is the same as in the fore limb.

_Relations._—Superficially, the skin and fascia; deeply, the femoro-tibial capsule, peroneus tertius, and tibialis anterior; behind, the lateral extensor and the superficial and deep peroneal nerves. In front of the tarsus the anterior tibial artery crosses the deep face of the tendon (Fig. 460).

_Blood-supply._—Anterior tibial artery.

_Nerve-supply._—Peroneal nerve.

2. =Lateral digital extensor= (M. extensor digitalis lateralis; peroneus; lateral extensor of the phalanges).—This muscle lies on the outer surface of the leg, behind the preceding one.

_Origin._—The external lateral ligament of the stifle joint, the fibula, the external border of the tibia, and the interosseus ligament.

_Insertion._—The tendon of the anterior extensor, about a third of the way down the metatarsus.

_Action._—To assist the anterior extensor.

_Structure._—The belly is fusiform, flattened, and pennate. The tendon runs through the entire length of the belly and becomes free from it at the lower fourth of the tibia. It passes downward through the groove on the external malleolus, bound down by an annular ligament, and, inclining forward, blends (usually) with the tendon of the anterior extensor. It is provided with a =synovial sheath=, which begins about one inch (ca. 2 to 3 cm.) above the external malleolus and ends about one and one-half inches (ca. 3 to 4 cm.) above the junction. Sometimes the fusion does not occur, and the tendon then passes down the metatarsus, alongside of that of the long extensor, to lie inserted into the first phalanx like the corresponding muscle of the thoracic limb.

_Relations._—Externally, the skin and fascia and the superficial peroneal nerve; internally, the tibia and fibula; anteriorly, the intermuscular septum, the long extensor, and the tibialis anterior; posteriorly, the deep flexor and the soleus.

_Blood-supply._—Anterior tibial artery.

_Nerve-supply._—Peroneal nerve.

3. =Peroneus tertius= (Tendinous part of the flexor metatarsi; tendo femorotarseus [Schmaltz]).—This consists in the horse of a strong tendon which lies between the anterior extensor and the tibialis anterior.[72]

[Illustration:

FIG. 202.—MUSCLES OF PELVIC LIMB OF HORSE, ANTERIOR VIEW. _y_, Rectus femoris; _20_, patella. (After Ellenberger-Baum, Anat. für Künstler.) ]

_Origin._—The fossa extensoria (between the external condyle and the trochlea of the femur), in common with the anterior extensor.

_Insertion._—(1) The proximal extremities of the large (third) and external small (fourth) metatarsal bones, and the third tarsal bone; (2) the fibular and fourth tarsal bones.

_Action._—Mechanically to flex the hock when the stifle joint is flexed.

_Structure._—This is entirely tendinous. The proximal end and the underlying prolongation of the synovial membrane of the femoro-tibial joint have been mentioned in the description of the anterior extensor. Fibers of the latter muscle and of the tibialis anterior arise on the tendon as it passes downward to the front of the hock. At the distal end of the tibia the tendon divides into two branches, between which the tendon of the tibialis anterior emerges. The anterior branch is attached to the third tarsal and third and fourth metatarsal bones, while the outer one curves outward, bifurcates, and is inserted into the fibular and fourth tarsal bones (Fig. 460).

_Relations._—Superficially, the anterior extensor; deeply, the tibialis anterior. The anterior tibial vessels cross the deep face of the outer branch.

4. =Tibialis anterior= (Muscular portion of the flexor metatarsi).—This lies on the antero-external face of the tibia; it is wide and flattened above, pointed below.

_Origin._—The external condyle and border of the tibia and a small area on the external surface of the tuberosity.

_Insertion._—(1) The metatarsal tuberosity; (2) the first and second tarsal bones.

_Action._—To flex the hock joint.

_Structure._—The origin is fleshy, and is divided by the groove in which lie the common tendon of the long extensor and peroneus tertius and a synovial pouch. Passing downward on the tibia, the belly is united by tendinous and fleshy fibers with the peroneus tertius, and terminates close to the tarsus in a point on the tendon of insertion. The latter emerges between the branches of the peroneus tertius and bifurcates, the anterior branch being inserted into the metatarsal tuberosity, the inner one (cunean tendon) into the fused first and second tarsal bones (cuneiform parvum). The tendon is provided with a =synovial sheath= at its emergence, and a =bursa= (cunean bursa) is interposed between the inner branch and the internal lateral ligament.

_Relations._—Superficially, the anterior and lateral extensors, the peroneus tertius, and the deep peroneal nerve; deeply, the tibia, the deep flexor, and the anterior tibial vessels.

_Blood-supply._—Anterior tibial artery.

_Nerve-supply._—Peroneal nerve.

B. POSTERIOR GROUP

1. =Gastrocnemius= (Figs. 179, 201, 203).—This muscle extends from the lower third of the femur to the point of the hock. It arises by two heads.

_Origin._—(1) Outer head, from the external rough margin of the supracondyloid fossa (Fossa plantaris); (2) inner head, from the supracondyloid crest.

_Insertion._—The posterior part of the tuber calcis.

_Action._—To extend the hock.

_Structure._—The two bellies are thick, fusiform, and somewhat flattened. They are covered by a strong aponeurosis and contain tendinous intersections. They terminate toward the middle of the leg on a common tendon (Tendo calcaneus s. Achillis), which at first lies posterior to that of the superficial flexor, but, by a twist in both, comes to lie in front of the latter. The deep fascia blends with the tendon throughout its length, and the soleus muscle is inserted into its anterior edge. A small =bursa= (Bursa tendinis calcanei) lies in front of the insertion on the tuber calcis, and a large =bursa= is interposed between the two tendons from the twist downward. The superficial flexor lies between the two heads and is adherent to the outer one (Fig. 459).

_Relations._—Anteriorly, the stifle joint, the superficial flexor, popliteus, deep flexor, popliteal vessels, and tibial nerve; internally (above), the semitendinosus, semimembranosus, and adductor, (below) the fascia and skin; externally (above), the biceps femoris and peroneal (anterior tibial) nerve, (below) the fascia and skin.

[Illustration:

FIG. 203.—MUSCLES OF LEG AND FOOT OF HORSE, INTERNAL VIEW.

_r′_, Tendon of semitendinosus; _w_, gracilis; _x_, sartorius; _y_, vastus internus. (After Ellenberger-Baum, Anat. für Künstler.) ]

_Blood-supply._—Popliteal artery.

_Nerve-supply._—Tibial nerve.

2. =Soleus= (Fig. 201).—This muscle is very small in the horse. It lies immediately under the deep fascia, on the proximal half of the outer surface of the leg, and is directed obliquely downward and backward.

_Origin._—The head of the fibula.

_Insertion._—The tendon of the gastrocnemius, about the middle of the leg.

_Action._—To assist the gastrocnemius.

_Structure._—It is a thin, fleshy band, about an inch (ca. 2 to 3 cm.) in width, terminating on a thin tendon which fuses with that of the gastrocnemius.[73]

_Relations._—Superficially, the skin, fascia, and peroneal nerve; deeply, the lateral extensor and deep flexor.

_Blood-supply._—Posterior tibial artery.

_Nerve-supply._—Tibial nerve.

3. =Superficial digital flexor= or =flexor pedis perforatus= (Figs. 199, 201, 203) (M. flexor digitalis pedis sublimis; superficial flexor of the phalanges).—The proximal part of this muscle lies between and under cover of the two heads of the gastrocnemius (Fig. 459). It consists almost entirely of a strong tendon, the belly being very little developed.

[Illustration:

FIG. 204.—CROSS-SECTION OF LEFT LEG OF HORSE; SECTION IS CUT A LITTLE ABOVE MIDDLE OF REGION.

_a_, Tibia; _b_, fibula; _c_, tibialis anterior; _d_, peroneus tertius; _e_, anterior or long digital extensor; _f_, lateral extensor; _g_, deep head of deep flexor (flexor hallucis longus); _h_, inner head of deep flexor (flexor accessorius _s_, digitorum longus); _i_, superficial head of deep flexor (tibialis posterior); _k_, popliteus; _l_, superficial flexor tendon; _m_, gastrocnemius tendon; _n_, soleus; _o_, skin; _p_, anterior tibial artery; _q_, superficial peroneal nerve; _q′_, deep peroneal nerve; _r_, branch of _q_; _s_, posterior tibial artery; _t_, cutaneous nerve; _u_, tibial nerve; _v_, recurrent tibial vein; _w_, saphenous artery; _x_, saphenous vein; _y_, branches of saphenous nerve. (After Ellenberger, inl Leisering’s Atlas.) ]

_Origin._—The supracondyloid fossa (Fossa plantaris) of the femur.

_Insertion._—(1) The front and sides of the tuber calcis; (2) the eminences on the proximal extremity of the second phalanx, and the distal extremity of the first phalanx behind the lateral ligament.

_Action._—To flex the digit and extend the hock joint. On account of the exceedingly small amount of muscular tissue the action is to be regarded chiefly as a mechanical effect, resulting from the action of other muscles on the stifle joint.

_Structure._—The origin is by means of a strong round tendon which is incompletely covered with fleshy fibers as far as the upper third of the leg. It is pretty intimately attached to the gastrocnemius, especially to the outer head. At the distal third of the tibia it winds around the inner surface of the gastrocnemius tendon, and then occupies a position behind the latter. At the point of the hock it widens out, forming a sort of cap over the tuber calcis, detaching on either side a strong band which is inserted into the tuber calcis with the tarsal tendon of the biceps and semitendinosus. It then passes downward over the plantar (Calcaneo-metatarsal) ligament, becomes narrower, and is arranged below as in the thoracic limb. A large synovial bursa lies under the tendon from the distal fourth of the tibia to the middle of the tarsus. A =subcutaneous bursa= is sometimes found on the wide part of the tendon at the point of the hook. (Either or both of these bursæ may be involved in so-called “capped hock.”)

_Relations._—Posteriorly, the gastrocnemius, fascia, and skin; anteriorly, the femoro-patellar capsule, the popliteus, the deep flexor, and the popliteal vessels; internally, the tibial nerve.

_Blood-supply._—Femoro-popliteal artery.

_Nerve-supply._—Tibial nerve.

[Illustration:

FIG. 205.—INJECTED SYNOVIAL SHEATHS AND BURSÆ OF TARSAL REGION OF HORSE, INNER VIEW.

_a_, Synovial sheath of peroneus tertius and tibialis anterior; _b_, bursa under inner (cunean) tendon of tibialis anterior; _c_, synovial sheath of flexor longus s. accessorius; _d_, tarsal sheath of deep flexor; _e_, _e′_, bursa under superficial flexor tendon; _f_, _f′_, tibio-tarsal joint capsule; _1_, anterior extensor; _2_, tibialis anterior; _2′_, inner (cunean) tendon of _2_; _3_, flexor longus; _4_, deep digital flexor; _5_, superficial flexor tendon; _6_, gastrocnemius tendon; _7_, tibia; _8_, tarsus; _9_, tuber calcis; _10_, large metatarsal bone; _11_, inner small metatarsal bone; _12_, _12′_, fascial bands. (After Ellenberger, in Leisering’s Atlas.) ]

[Illustration:

FIG. 206.—INJECTED SYNOVIAL SHEATHS AND BURSÆ OF TARSAL REGION OF HORSE, EXTERNAL VIEW.

_a_, Synovial sheath of anterior or long digital extensor; _b_, synovial sheath of lateral digital extensor; _c_, _c′_, bursa under superficial flexor tendon; _d_, capsule of hock joint; _1_, anterior or long extensor; _2_, lateral extensor; _3_, _3_, _3_, annular ligaments; _4_, deep digital flexor; _5_, tendon of gastrocnemius; _6_, superficial flexor tendon; _7_, tibia; _8_, tarsus; _9_, tuber calcis; _10_, metatarsus. (After Ellenberger, in Leisering’s Atlas.) ]

4. =Deep digital flexor= or =flexor pedis perforans= (M. flexor digitalis pedis profundus; deep and oblique flexors of the phalanges).—The belly of this muscle lies on the posterior surface of the tibia, and is divisible into three parts or heads, which, however, finally unite on a common tendon of insertion.

_Origin._—(1) The posterior edge of the external condyle of the tibia; (2) the external border of the external condyle of the tibia, just behind the facet for the fibula; (3) the middle third of the posterior surface and the upper part of the external border of the tibia, the posterior border of the fibula, and the interosseous ligament.[74]

_Insertion._—The semilunar crest of the third phalanx and the adjacent surface of the lateral cartilage.

_Action._—To flex the digit and to extend the hock joint.

_Structure._—(1) The inner head (M. flexor digitalis longus s. flexor accessorius) is easily isolated (Figs. 203, 459). It has a fusiform belly, which crosses the leg obliquely and lies in a groove formed by the other heads and the popliteus. This terminates near the lower third of the tibia on a round tendon which passes downward, partly embedded in the internal lateral ligament of the hock, and joins the common tendon about a third of the way down the metatarsus. In its course over the inner surface of the hock the tendon lies in a canal formed by the strong tarsal fascia, the tibia, and the lateral ligament, and is provided with a =synovial sheath= which extends from the distal fourth of the tibia to the junction with the principal tendon. (2) The superficial head (M. tibialis posterior) is only partially separable. It has a flattened belly, terminating near the lower third of the tibia on a flat tendon which soon fuses with the principal tendon. (3) The deep head (M. flexor hallucis longus) is much the largest. It lies on the posterior surface of the tibia, from the popliteal line outward and downward. The belly contains much tendinous tissue, and terminates behind the distal end of the tibia on a strong round tendon. The latter receives the tendon of the tibialis posterior, passes downward in the tarsal groove, bound down by the strong tarsal fascia (Ligamentum laciniatum) and enveloped in a synovial sheath, receives the tendon of the inner head below the hock, and, a little further down, the so-called check ligament (Caput tendineum [Schmaltz]). The =tarsal sheath= (Vagina tarsea) begins about two to three inches (ca. 5 to 7.5 cm.) above the level of the internal malleolus, and extends about one-fourth of the way down the metatarsus. (Distention of the sheath, as in “thoroughpin,” affects chiefly its proximal end.) The check ligament resembles that of the fore limb, except that it is longer and very much weaker; it may be absent. The remainder of the tendon is arranged like that of the thoracic limb.