Part 30

FIG. 191.—MUSCLES OF LEFT THORACIC LIMB OF HORSE FROM ELBOW DOWNWARD; EXTERNAL VIEW.

_a_, Extensor carpi radialis; _g_, brachialis; _g′_, anterior superficial pectoral; _c_, anterior or common digital extensor; _e_, flexor carpi externus. (After Ellenberger-Baum, Anat. für Künstler.) ]

_Relations._—Superficially, the skin and fascia; deeply, the outer face of the radius and ulna; in front, the common extensor, the oblique extensor, and the interosseous artery; behind, the external flexor of the carpus and the deep flexor of the digit.

_Blood-supply._—Interosseous artery.

_Nerve-supply._—Radial nerve.

4. =Extensor carpi obliquus= (M. abductor pollicis longus et extensor pollicis brevis; oblique extensor of the metacarpus; extensor metacarpi obliquus).—This is a small muscle which curves obliquely over the distal half of the radius and the carpus.

_Origin._—The external border and adjacent part of the anterior surface of the radius (the attachment area beginning at a point above the middle of the bone and extending down to its lower fourth).

_Insertion._—The head of the inner (second) metacarpal bone.

_Action._—To extend the carpal joint.

_Structure._—The muscle is pennate and has a flat belly which curves downward, forward, and inward over the distal part of the radius. The tendon continues the direction of the muscle, crossing obliquely over the tendon of the extensor carpi radialis; it then occupies the oblique groove at the distal end of the radius, and crosses the inner face of the carpus. It is provided with a =synovial sheath=.

[Illustration:

FIG. 192.—CROSS-SECTION OF MIDDLE OF RIGHT FOREARM OF HORSE.

_a_, Accessory cephalic vein; _b_, cutaneous branch of median nerve; _c_, extensor carpi obliquus; _d_, posterior radial artery; _e_, satellite vein; _f_, median nerve; _g_, _g′_, ulnar vessels; _h_, ulnar nerve; _i_, cephalic vein; _k_, tendon of ulnar head of deep flexor; _l_, interosseous artery; _m_, extensor carpi radialis; _n_, anterior digital extensor; _o_, lateral digital extensor; _p_, flexor carpi externus; _q_, _q_, _q_, humeral heads of deep digital flexor; _r_, radial head of same; _s_, superficial digital flexor; _t_, flexor carpi medius; _u_, flexor carpi internus; _v_, radius; _w_, skin; _x_, anterior radial artery. (After Ellenberger, in Leisering’s Atlas.) ]

_Relations._—Superficially, the skin and fascia, the lateral extensor, and the common extensor; deeply, the radius, the extensor carpi radialis, the carpal capsule, and the internal lateral ligament.

_Blood-supply._—Interosseous and anterior radial arteries.

_Nerve-supply._—Radial nerve.

B. FLEXOR DIVISION

1. =Flexor carpi internus= (M. flexor carpi radialis s. radialis volaris; internal flexor of the metacarpus).—This muscle lies on the inner surface of the forearm, immediately behind the inner border of the radius.

_Origin._—The flexor (internal) epicondyle of the humerus, below and behind the lateral ligament.

_Insertion._—The proximal end of the internal (second) metacarpal bone.

_Action._—To flex the carpal joint and to extend the elbow.

_Structure._—The muscle has a short tendon of origin, which is succeeded by a somewhat flattened, fusiform belly. The tendon of insertion begins near the distal fourth of the radius. It is provided with a synovial sheath which begins two or three inches (ca. 5 to 8 cm.) above the carpus and extends almost to the insertion of the tendon.

_Relations._—Superficially, the skin and fascia, the posterior superficial pectoral and the tensor fasciæ antibrachii; deeply, the elbow joint, the radius, the deep flexor, the flexor carpi medius, the posterior radial vessels, and the median nerve. At the elbow the artery and nerve lie in front of the muscle, but below they dip beneath it.

_Blood-supply._—Posterior radial artery.

_Nerve-supply._—Median nerve.

On removing the deep fascia on the inner surface of the elbow the student may notice a small muscle lying along the lateral ligament. This is the =pronator teres=, which is usually not present or a mere vestige in the horse. It arises by a small, flat tendon from the flexor epicondyle of the humerus, and is inserted into the internal lateral ligament of the elbow. On account of its small size and the fact that the forearm is fixed in the position of pronation, the muscle can have no appreciable function. It is usually represented by the long portion of the internal lateral ligament.

2. =Flexor carpi medius= (M. flexor carpi ulnaris s. ulnaris medialis; oblique or middle flexor of the metacarpus).—This muscle lies on the inner and posterior aspect of the forearm partly under, partly behind, the preceding muscle. It arises by two heads—humeral and ulnar.

_Origin._—(1) The flexor (internal) epicondyle of the humerus just behind the preceding muscle; (2) the inner surface and posterior border of the olecranon.

_Insertion._—The upper edge of the accessory carpal (pisiform) bone.

_Action._—To flex the carpal joint, and to extend the elbow.

_Structure._—The humeral head is much the larger, constituting, in fact, the bulk of the muscle. It is flattened, curved, and tapers at both ends. The ulnar head, much smaller and very thin, is covered by an aponeurosis from which many of its fibers arise. It joins the large head a little above the middle of the forearm. The tendon of insertion is short and strong; it blends with the posterior annular ligament of the carpus.

_Relations._—Superficially, the tensor fasciæ antibrachii, superficial pectoral, and flexor carpi internus, the skin and fascia, and cutaneous branches of the ulnar nerve; deeply, the superficial and deep flexors of the digit. In the distal half of the forearm the ulnar vessels and nerve lie between the outer edge of this muscle and the external flexor of the carpus.

_Blood-supply._—Ulnar and posterior radial arteries.

_Nerve-supply._—Ulnar and median nerves.

[Illustration:

FIG. 193.—DEEPER MUSCLES OF FOREARM OF HORSE, INTERNAL VIEW.

_23_, Internal head of triceps; _30_, flexor carpi medius; _31_, flexor carpi internus; _31′_, tendon of _31_; _32_, superficial digital flexor; _32′_, radial check ligament; _32″_, tendon of superficial flexor; _33a_, _33b_, _33c_, humeral heads of deep flexor; _33′_, deep flexor tendon; _33″_, subcarpal check ligament; _34_, ulnar head of deep flexor; _35_, radial head of deep flexor; _36_, suspensory ligament. (After Ellenberger, in Leisering’s Atlas.) ]

3. =Flexor carpi externus= (M. extensor carpi ulnaris[61] s. ulnaris lateralis; external flexor of the metacarpus).—This muscle lies on the outer face of the forearm, behind the lateral extensor of the digit.

_Origin._—The extensor (external) epicondyle of the humerus, behind and below the lateral ligament.

_Insertion._—(1) The outer surface and upper edge of the accessory carpal bone; (2) the proximal extremity of the outer (fourth) metacarpal bone.

_Action._—To flex the carpal joint and to extend the elbow.

_Structure._—The belly of the muscle is flattened and is intersected by a good deal of tendinous tissue. There are two tendons of insertion. The short tendon is inserted into the accessory carpal bone. The long tendon is detached just above the carpus; it is smaller and rounded; it passes downward and a little forward through a groove on the outer surface of the accessory carpal bone, enveloped by a =synovial sheath=, to reach its insertion on the outer metacarpal bone. A synovial pouch lies under the origin of the muscle at the elbow joint, with the cavity of which it communicates.

[Illustration:

FIG. 194.—CROSS-SECTION OF PROXIMAL PART OF LEFT CARPUS OF HORSE.

The tendons have been removed to show the canals in which they lie. The joint cavity is black. By an oversight the carpal canal for the two flexor tendons behind the posterior ligament of the carpus is not marked. _Cr_, Radial, _Ci_, intermediate, _Cu_, ulnar, _Ca_, accessory carpal bones. ]

_Relations._—Superficially, the skin, fascia, and cutaneous branches of the ulnar nerve; deeply, the elbow joint, the ulna, and the flexors of the digit; in front, the lateral extensor of the digit; behind, the middle flexor of the carpus, the ulnar head of the deep flexor, and the ulnar vessels and nerve.

_Blood-supply_.—Interosseous, ulnar, and median arteries.

_Nerve-supply._—Radial nerve.

4. =Superficial digital flexor= or =flexor perforatus= (M. flexor digitalis sublimis; superficial flexor of the phalanges).—This muscle is situated in the middle of the flexor group, chiefly under cover of the middle flexor of the carpus.

_Origin._—(1) The flexor epicondyle of the humerus, between the flexor carpi internus and the deep flexor of the digit; (2) a ridge on the posterior surface of the radius, below its middle and near the internal border.

_Insertion._—(1) The eminences on the proximal extremity of the second phalanx behind the lateral ligaments; (2) the distal extremity of the first phalanx, also behind the lateral ligaments.

_Action._—To flex the digit and carpus, and to extend the elbow.

[Illustration:

FIG. 195.—SYNOVIAL SHEATHS AND BURSÆ OF LOWER PART OF RIGHT FORE LIMB OF HORSE, INTERNAL VIEW.

_a_, Sheath of extensor carpi obliquus; _b_, sheath of flexor carpi internus; _c_, carpal sheath; _d_, _d′_ _d″_, _d‴_, digital sheath; _e_, bursa under anterior extensor tendon; _f_, capsule of fetlock joint; _1_, extensor carpi radialis; _2_, tendon of extensor carpi obliquus; _3_, flexor carpi internus; _4_, flexor carpi medius; _5_, superficial flexor tendon; _6_, deep flexor tendon; _7_, suspensory ligament; _8_, small metacarpal bone; _9_, large metacarpal bone; _10_, posterior annular ligament of fetlock; _11_, proximal digital annular ligament; _12_, radius; _13_, radio-carpal joint; _14_, fetlock joint; _15_, lateral cartilage; _16_, band from first phalanx to lateral cartilage. (After Ellenberger, in Leisering’s Atlas.)

The synovial sheaths (colored yellow) and the joint capsules (colored pink) are injected. ]

[Illustration:

FIG. 196.—SYNOVIAL SHEATHS AND BURSÆ OF LOWER PART OF RIGHT FORE LIMB OF HORSE, EXTERNAL VIEW.

_a_, Sheath of extensor carpi radialis; _b_, sheath of anterior extensor; _c_, sheath of lateral extensor; _d_, sheath of outer tendon of flexor carpi externus; _e_, _e′_, carpal sheath; _f_, _f′_ _f″_, digital sheath; _g_, bursa under anterior extensor tendon; _h_, bursa under lateral extensor tendon; _i_, capsule of fetlock joint; _1_, extensor carpi radialis; _2_, anterior digital extensor; _3_, lateral digital extensor; _4_, flexor carpi externus; _4′_ _4″_, tendons of _4_; _5_, superficial flexor tendon; _6_, deep flexor tendon; _7_, suspensory ligament; _8_, external metacarpal bone; _9_, large metacarpal bone; _10_, posterior annular ligament of fetlock; _11_, digital annular ligament; _12_, fetlock joint; _13_, lateral cartilage; _14_, band from first phalanx to lateral cartilage. (After Ellenberger, in Leisering’s Atlas.) ]

_Structure._—The fleshy portion of the muscle or =humeral head= takes origin from the humerus. The =radial head= (Caput tendineum) consists of a strong fibrous band, usually termed the =radial= or =superior check ligament=, which fuses with the tendon near the carpus. The belly of the muscle is intersected by tendinous strands, and fuses more or less with that of the deep flexor. Near the carpus it is succeeded by a strong, thick tendon which passes down through the carpal canal, formed by the posterior ligament of the carpus, the accessory carpal bone, and the posterior annular ligament (Ligamentum carpi transversum). Here it is enveloped by a synovial sheath, in common with the deep flexor. This =carpal sheath= (Vagina carpea) begins three or four inches (8 to 10 cm.) above the carpus, and extends downward nearly to the middle of the metacarpus. Below the carpus the tendon becomes flattened and broader. Near the fetlock it forms a ring through which the tendon of the deep flexor passes (Fig. 150). Here the two tendons are bound down in the sesamoid groove by the posterior annular ligament, which fuses more or less with the superficial flexor tendon. At the distal end of the first phalanx the tendon divides into two branches which diverge to reach their points of insertion, and between these branches the tendon of the deep flexor emerges (Fig. 189). A second =synovial sheath=, the =digital= or =sesamoidean= (Vagina digitalis), begins at the distal third of the metacarpus, three to four inches (8 to 10 cm.) above the fetlock, and extends to the middle of the second phalanx.

[Illustration:

FIG. 197.—CROSS-SECTION OF DISTAL PART OF LEFT METACARPUS OF HORSE, JUST ABOVE SESAMOIDS. ]

_Relations._—The belly of the muscle is related superficially to the ulnar head of the deep flexor, the flexor carpi medius, and, at its origin, to the ulnar vessels and nerve; deeply to the humeral head of the deep flexor. The tendon is related superficially to the skin and fascia; deeply to the deep flexor tendon.

_Blood-supply._—Posterior radial artery.

_Nerve-supply._—Median nerve.

5. =Deep digital flexor= or =flexor perforans= (M. flexor digitalis profundus; deep flexor of the phalanges).—The fleshy part of this muscle lies on the posterior surface of the radius, under cover of the preceding muscles. It is the largest muscle of the flexor group.

_Origin._—(1) The flexor (internal) epicondyle of the humerus; (2) the inner surface of the olecranon; (3) the middle of the posterior surface of the radius and a small adjacent area of the ulna.

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

_Action._—To flex the digit and carpus, and to extend the elbow.

_Structure._—This muscle consists of three principal heads. The =humeral head= (Caput humerale) constitutes the bulk of the muscle. It is marked by tendinous intersections, and is separable into three secondary heads. The =ulnar head= (Caput ulnare, ulnaris accessorius) is much smaller, and is at first superficially situated between the outer and middle flexors of the carpus, the =radial head= (Caput radiale, radialis accessorius) is the smallest, and is not always present; it is situated on the distal two-thirds of the posterior surface of the radius, under the humeral head. Each of these heads is provided with a tendon. The principal tendon—that of the humeral head—begins about three or four inches (8 to 10 cm.) above the carpus. It is soon joined by the small tendon of the ulnar head, which begins about the middle of the forearm. The tendon of the radial head fuses with the principal tendon close to the carpus. The conjoined tendon passes downward through the carpal canal, being included in the =carpal synovial sheath= with the superficial flexor tendon, as previously described. Continuing downward, it is joined about the middle of the metacarpus by a strong fibrous band, the so-called =inferior= or =subcarpal check ligament= (Caput tendineum). This is a direct continuation of the posterior ligament of the carpus. Below this the tendon passes through the ring formed by the perforatus, then in succession over the sesamoid groove, the inferior sesamoidean ligaments, and the tendon surface of the third sesamoid, to its insertion (Figs. 150 and 151). Its terminal part is much widened. From the distal third of the metacarpus to the distal end of the second phalanx it is inclosed in the =digital synovial sheath= described in connection with the perforatus. The =navicular bursa= or =bursa podotrochlearis= is found between the tendon and the third sesamoid or navicular bone. The terminal part of the tendon is bound down by the fibrous sheet described with the fascia.

[Illustration:

FIG. 198.—DIGIT OF HORSE, LATERAL VIEW.

_1_, Large metacarpal bone; _2_, distal end of small metacarpal bone; _3_, fetlock joint; _5_, first phalanx; _6_, pastern joint; _7_, second phalanx; _8_, coffin joint; _9_, third phalanx; _11_, lateral cartilage; _11′_, upper (subcutaneous) border of cartilage; _12_, suspensory ligament; _13_, branch of _12_ to extensor tendon; _14_, deep flexor tendon; _15_, superficial flexor tendon; _15′_, branch of _15_; _16_, posterior annular ligament of fetlock; _17_, proximal annular or vaginal ligament of digit; _18_, distal annular ligament or reinforcing sheath of deep flexor tendon; _19_, lateral ligament of coffin joint; _21_, tendon of common or anterior extensor; _22_, tendon of lateral extensor; _23_, digital vein. (After Ellenberger-Baum, Anat. für Künstler.) ]

_Relations._—The belly of the muscle is related posteriorly to the perforatus and the middle flexor of the carpus; internally, to the internal carpal flexor, the radial check ligament, and the posterior radial vessels and median nerve; externally, to the external flexor of the carpus; anteriorly, to the radius and ulna and branches of the posterior radial artery and median nerve. Below the carpus the tendon is accompanied by the vessels and nerves of the digit. It may also be noted that the muscle is not entirely covered by the other flexors; it comes in contact with the skin and fascia on the postero-external aspect of the proximal half of the forearm, and also on the outer aspect of the distal fourth.

_Blood-supply._—Posterior radial and ulnar arteries.

_Nerve-supply._—Median and ulnar nerves.

METACARPAL MUSCLES

The five muscles of this group are either reduced to vestiges or modified greatly in structure.

1, 2. =Lumbricales= (internus et externus).—These are two small muscles which lie on either side of the flexor tendons above the fetlock. They arise from the deep flexor tendon, and are lost in the fibrous tissue which lies under the nodule of horn or ergot at the fetlock (Fig. 447). Their action is inappreciable. The size of these muscles is subject to much variation. Often very little muscular tissue can be found, but the small tendon is constantly present.

_Blood-supply._—Metacarpal arteries.

_Nerve-supply._—Median and ulnar nerves.

3, 4, 5. =Interossei.=—These are three in number in the horse, and are situated chiefly in the metacarpal groove. Two, the =internus= and =externus=, are very small muscles, each of which arises from the corresponding small metacarpal bone near its proximal extremity, and is provided with a delicate tendon which is usually lost in the fascia at the fetlock (Fig. 150). They have no appreciable action. Their blood- and nerve-supply is the same as that of the preceding muscles.

The =interosseus medius= is so much modified that it is usually termed the =suspensory=, or =superior sesamoidean ligament=. It contains little muscular tissue, being transformed very largely into a strong tendinous band, bifurcate below, and having for its chief function the supporting of the fetlock. It has been described, in deference to common usage, with the ligaments.

FASCIÆ AND MUSCLES OF THE PELVIC LIMB

THE FASCIÆ

The =iliac fascia= (Fascia iliaca) covers the ventral surface of the iliacus and psoas muscles, over which it is tightly stretched (Fig. 450). It is attached internally to the tendon of the psoas minor, externally to the inguinal (Poupart’s) ligament and the external angle of the ilium. Its anterior part is thin. Posteriorly, it is in part attached to the ilium, in part becomes continuous with the pelvic fascia. It furnishes surfaces of origin for the sartorius, cremaster externus, and transversus abdominis muscles.

The =pelvic fascia= (Fascia pelvis) lines the cavity (parietal layer) and is reflected on the viscera at the pelvic outlet (visceral layer). Laminæ are detached from it to strengthen the various peritoneal folds.

The =superficial fascia= of the =gluteal region= is continuous with the aponeurosis of the panniculus. It covers and partly blends with the deep fascia. The =gluteal fascia= (Fascia glutea) covers the superficial muscles of the region, and detaches intermuscular septa, which pass between the muscles. It is attached to the sacral spines, the dorsal sacro-iliac ligament, and the angles of the ilium, and is continuous in front with the lumbo-dorsal fascia, behind with the coccygeal fascia. Its deep face gives origin to fibers of the superficial and middle glutei, the biceps femoris, and the semitendinosus, so that care is necessary in dissecting it off these muscles. The chief =intermuscular septa= are: (1) One which passes between the superficial gluteus and the biceps femoris; (2) one between the biceps and semitendinosus, from which a lamella is detached which passes between the middle and posterior portions of the biceps and is attached to the tuber ischii; (3) one between the semitendinosus and semimembranosus, which is attached to the sacro-sciatic ligament and tuber ischii; it furnishes origin for fibers of the long head of the semimembranosus.

The =fascia lata= is continuous with the preceding, and covers the muscles on the outer surface and front of the thigh. It is tendinous and very strong, and for the most part easily separable from the underlying muscles. It furnishes insertion to the tensor fasciæ latæ and to the biceps femoris (in part), by both of which it is tensed. At the stifle it is attached to the patella and the inner and outer straight ligaments. Internally it is continuous with the internal femoral fascia. It furnishes the following =intermuscular septa=: (1) One which passes between the vastus externus and biceps femoris to be attached to the external trochanter of the femur; (2) two which pass between the three branches of the biceps femoris; (3) a fourth between the biceps femoris and semitendinosus.

The =internal femoral fascia= (Fascia femoralis medialis) covers the superficial muscles on the inner surface of the thigh. At its upper part it is joined by part of the aponeurosis of the external oblique muscle (Lamina femoralis) (Fig. 450). The posterior part is thin. It is continuous with the fascia lata in front and the crural fascia below. At the stifle it fuses with the tendons of the sartorius and gracilis.

The =crural fascia=, or fascia of the leg (Fascia cruris), consists of three layers. Two of these invest the entire region and may, therefore, be termed the =common fasciæ=. The superficial layer is a continuation of the fasciæ of the thigh, while the second layer may be regarded chiefly as a continuation of the tendons of the superficial muscles of the hip and thigh (biceps femoris, semitendinosus, tensor fasciæ latæ, sartorius, and gracilis). The two layers frequently fuse, and are attached chiefly to the patellar ligaments and the crest and internal surface of the tibia. About the middle of the leg the two layers unite behind the deep flexor of the digit and form a strong band which passes downward in front of the tendons of the gastrocnemius and superficial flexor, to be attached with the latter to the anterior and inner part of the tuber calcis. This may be regarded as an accessory or tarsal tendon of insertion of the biceps femoris and semitendinosus. The third layer forms sheaths for the muscles, furnishing origin in part to their fibers. Two important =intermuscular septa= are detached, viz.: (1) One which passes between the anterior and lateral digital extensors to be attached to the fibula and the external border of the tibia; (2) one between the lateral extensor and the deep flexor.

The =tarsal fascia= (Fascia tarsi) fuses with the ligaments and bony prominences of the region. It is strong and tendinous in front, and joins the tendon of the anterior extensor below the joint. Laterally, it is thin and fuses with the ligaments. Posteriorly, it is very thick and strong, forming an annular ligament which stretches from the internal lateral ligament to the fibular tarsal bone and the plantar ligament. This converts the groove at the back of the hock into a canal, in which are the deep flexor tendon with its synovial sheath and the plantar nerves. In front of the tarsus and above and below it there are three =annular ligaments= (Ligamenta transversa). The proximal one binds down the tendons of the anterior extensor, peroneus tertius, and tibialis anterior on the distal end of the tibia. The middle one stretches from the fibular tarsal bone to the outer tendon of the peroneus tertius, forming a loop around the tendon of the anterior extensor. The distal band stretches across the proximal extremity of the large metatarsal bone and incloses the tendons (and sheaths) of the two extensors of the digit.

The =metatarsal= and =digital fasciæ= do not differ materially from those of the corresponding regions of the thoracic limb.

THE MUSCLES

I. THE SUBLUMBAR MUSCLES (Figs. 450, 456)

The muscles of this group are not confined to the sublumbar region, but extend beyond it both before and behind. Their chief function is to flex the hip joint. Two, however,—the psoas minor and the quadratus lumborum,—have not this action.

1. =Psoas minor= (s. parvus).—This is a fusiform, flattened, pennate muscle, which lies along the ventro-lateral aspect of the bodies of the last three thoracic and the lumbar vertebræ.

_Origin._—The bodies of the last three thoracic and first four or five lumbar vertebræ, and the vertebral ends of the sixteenth and seventeenth ribs.

_Insertion._—The psoas tubercle on the shaft of the ilium.

_Action._—To flex the pelvis on the loins, or to incline it laterally.