Part 35

_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.

_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.

_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.

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.

_Relations._—Anteriorly, the tibia and fibula, the popliteus, lateral extensor, tibialis anterior, and the tibial vessels; posteriorly, the gastrocnemius, superficial flexor, and the tibial nerve; externally, the fascia, skin, and the soleus; internally, the fascia and skin.

_Blood-supply._—Posterior tibial artery.

_Nerve-supply._—Tibial nerve.

5. =Popliteus= (Fig. 203).—This thick and triangular muscle lies on the posterior surface of the femoro-tibial articulation and the posterior surface of the tibia above the popliteal line.

_Origin._—A small depression on the external epicondyle of the femur, close to the articular surface and under the lateral ligament.

_Insertion._—A triangular area on the posterior surface of the tibia, above and internal to the popliteal line; also the proximal half of the inner border and a narrow adjacent part of the internal surface of the tibia.

_Action._—To flex the femoro-tibial joint and to rotate the leg inward.

_Structure._—The strong tendon of origin lies at first under the lateral ligament, and curves backward and inward over the external condyle of the tibia and its semilunar cartilage, being invested by a reflection of the synovial capsule of the joint (Fig. 459). The tendon is succeeded by a thick triangular belly, the fibers of which are directed obliquely downward and inward.

_Relations._—Superficially, the fascia and skin, semitendinosus, gastrocnemius, superficial flexor; deeply, the femoro-tibial joint, the tibia, the popliteal vessels and their divisions. The saphenous vessels and nerve lie along the inner border of the muscle, separated from it, however, by the deep fascia.

_Blood-supply._—Popliteal and posterior tibial arteries.

_Nerve-supply._—Tibial nerve.

MUSCLES OF THE METATARSUS AND DIGIT

=Extensor pedis brevis= (M. extensor digitalis brevis).—This small muscle lies in the angle of union of the long and lateral extensors of the digit. (Shown in Fig. 201, but not marked.)

_Origin._—The outer tendon of the peroneus tertius, the middle annular ligament, and the outer lateral ligament of the hock.

_Insertion._—The tendon of the anterior or long extensor.

_Action._—To assist the anterior or long extensor.

_Structure._—It is principally fleshy, having a superficial origin from the annular ligament, and a deep one (by a thin tendon) from the outer tendon of the peroneus tertius. The insertion is by a thin tendon.

_Relations._—Superficially, the skin and fascia and the tendons of the anterior and lateral extensors; deeply, the joint capsule, the great metatarsal artery, and the deep peroneal nerve.

_Blood-supply._—Great metatarsal artery.

_Nerve-supply._—Deep peroneal nerve.

The =interossei= and =lumbricales= are arranged like those of the thoracic limb, the only noticeable difference being the greater development of the lumbricales in the pelvic limb.

THE MUSCLES OF THE OX

MUSCLES OF THE FACE

The =panniculus= is much more developed than in the horse, presenting as a special feature the extensive =frontalis= muscle, which covers the frontal and nasal regions.

The =orbicularis oris= does not form a complete ring, the defect being in the middle of the upper lip.

The =levator nasolabialis= is extensive, thin, and not very distinct from the frontalis; it divides into two layers, between which the levator labii superioris proprius and the lateral dilator of the nostril pass. The superficial layer ends in the nostril and upper lip, the deep layer on the accessory (lateral) nasal cartilages and on the nasal process of the premaxilla.

The =levator labii superioris proprius= arises on and before the facial tuberosity and terminates by several tendons in the muzzle. It passes between the two layers of the preceding muscle, blending in part with the deep layer.

The =zygomaticus= is much stronger than in the horse. It arises on the masseteric fascia, and ends chiefly in the upper lip.

The =depressor labii superioris= does not resemble the muscle of the same name in the horse. It arises just in front of the facial tuberosity, and divides usually into two branches, which terminate in a number of tendons that form a network in the muzzle and upper lip.

The =incisivus inferior= is a small, rounded muscle, which arises on the body of the mandible below the second and third incisors, and ends in the lower lip, blending with the orbicularis.

The =depressor labii inferioris= is thin, and does not extend as far backward as in the horse; only the anterior end is distinct from the buccinator.

The =buccinator= shows no marked variation, but its superficial layer is well developed.

The =dilatator naris lateralis= arises in front of the facial tuberosity, passes forward between the branches of the levator nasolabialis, and terminates in the outer wing of the nostril.

The dilatator naris transversus is replaced by the =dilatator naris apicalis=, which is situated in the muzzle and joins its fellow at a median raphé. It arises on the border and upper surface of the body of the premaxilla, the fibers passing obliquely upward and outward to the inner wing of the nostril.

The =dilatator naris superior= arises from the alar cartilage of the nostril and ends in the inner wing of the nostril.

The =dilatator naris inferior= consists of two layers which arise on the nasal process of the premaxilla and the lateral nasal cartilage and end in the outer wing of the nostril.

The =orbicularis oculi= is well developed.

The corrugator supercilii is not present as a separate muscle, its place and function being taken by the frontalis.

The =malaris= is broad, and spreads out below on the fascia over the buccinator and masseter; it is divided into two parts.

MUSCLES OF MASTICATION

The =masseter= is not so large as in the horse; a considerable part of it arises on the facial tuberosity and is directed obliquely backward and downward, so that it would draw the lower jaw forward as well as upward.

The =temporalis= conforms to the temporal fossa, and is therefore longer and entirely lateral in position.

The =pterygoidei= are not quite clearly separated; their direction is more oblique, and the origin of the pterygoideus internus is nearer the median plane than in the horse.

The =stylo-mandibularis= is absent.

The =digastricus= has a tendinous origin on the paramastoid or styloid process of the occipital bone; its bellies are short and thick. It does not perforate the stylo-hyoideus. The two digastrici are connected beneath the root of the tongue by a layer of transverse muscle-fibers (Transversus mandibulæ).

HYOID MUSCLES

The =mylo-hyoideus= is thicker and more extensive than in the horse.

The =stylo-hyoideus= has a long thin tendinous origin and is not perforated by the digastricus.

The =genio-hyoideus= is more developed.

The =kerato-hyoideus= has an additional attachment on the middle cornu of the hyoid bone.

The =hyoideus transversus= is bifid.

The =sterno-thyro-hyoideus= has no intermediate tendon and is thicker.

The =omo-hyoideus= arises as a thin band from the fascia over the third and fourth cervical vertebræ. It blends here with the rectus capitis anterior major. The =occipito-hyoideus= is thick.

MUSCLES OF THE NECK

A. VENTRAL GROUP

The =sterno-cephalicus= consists of two muscles. They arise from the manubrium sterni and first rib. The superficial muscle (Sterno-mandibularis) is inserted on the anterior border of the masseter, the ramus of the mandible, and the buccal fascia. The deep muscle (Sterno-mastoideus) crosses under the preceding and ends on the mastoid process, the mandible, and, in common with the rectus capitis anterior major, on the basilar process of the occipital bone.

There are two scaleni. The =scalenus ventralis= (s. primæ costæ) arises on the first rib and ends on the transverse processes of the third to the seventh cervical vertebræ. It is traversed by the roots of the brachial plexus, which partially divide it into a small dorsal and a large ventral part. The brachial vessels lie below the latter. The =scalenus dorsalis= (s. supracostalis) arises usually on the second, third, and fourth ribs, and ends on the transverse processes of the third to the sixth cervical vertebræ.

The =rectus capitis anterior major= arises on the third to the sixth cervical transverse processes, and blends at its insertion with the sterno-mastoideus and the mastoid portion of the mastoido-humeralis.

The =rectus capitis anterior minor= is larger than in the horse.

The =rectus capitis lateralis= and =longus colli= resemble those of the horse.

The =intertransversales= are large. From the sixth cervical vertebra forward they form a muscular mass (M. intertransversarius longus) which is inserted into the wing of the atlas.

B. LATERAL GROUP

The =splenius= is thin. It arises directly from the first three or four thoracic spines, and ends by a thin tendon on the occipital bone, the wing of the atlas, and the transverse process of the axis, blending with the mastoido-humeralis, trachelo-mastoideus, and omo-transversarius. The remaining muscles present no very marked differential features.

MUSCLES OF THE THORAX

The =levatores costarum= number ten or eleven pairs.