Chapter IV: Fasciæ and Muscles of the Forearm and Manus

The forearm is covered on three sides by the muscles of this group, leaving the inner surface of the radius for the most part subcutaneous. The extensors of the carpus and digit lie on the anterior (dorsal) and external part of the region, while the flexors occupy the posterior (volar) surface.

The =fascia= of the =forearm= (Fascia antibrachii) forms a very strong and complete investment for all the muscles of the region. The =superficial fascia= is thin, and disappears at the carpus by fusing with the deep fascia. The =deep fascia= is very strong and tendinous in character. It furnishes insertion at its upper and inner part to the tensor fasciæ antibrachii and posterior superficial pectoral muscles; at its upper anterior and outer part, to aponeuroses from the mastoido-humeralis and biceps. It is attached at the elbow by its deep face to the outer tuberosities of the humerus and radius, to the ulna, and to the lateral ligaments. On the inner surface of the forearm it blends with the periosteum on the subcutaneous surface of the radius. It is closely adherent to the surface of the extensor muscles, but is rather loosely attached to the flexors. From its deep face are detached =intermuscular septa=, which form sheaths for the muscles and are attached to the underlying bones. The principal septa are: (_a_) One which passes between the common extensor (in front) and the lateral extensor and flexor carpi externus (behind); (_b_) one between the common extensor and the extensor carpi radialis; (_c_) one between the inner and middle flexors of the carpus.

The =carpal fascia= (Fascia carpi) is a direct continuation of that of the forearm. It is attached chiefly to the tuberosities at the distal end of the radius, to the accessory carpal (pisiform) bone, and to the lateral ligaments. In front it forms the so-called =anterior annular ligament= (Ligamentum carpi dorsale), bridging over the grooves and binding down the extensor tendons and their synovial sheaths. Behind it is greatly thickened and forms the =posterior annular= or =transverse ligament= of the carpus (Ligamentum carpi transversum). This stretches across from the accessory carpal bone to the internal lateral ligament and the proximal extremity of the inner metacarpal bone. It thus completes the carpal canal, in which lie the flexor tendons, their synovial sheath, and the principal vessels and nerves of the region.

The =fascia= of the =metacarpus= and =digit= (Fascia metacarpea et digitalis) is, in general, thinner than the preceding. It is attached to the tendons, ligaments, and the exposed bony prominences—especially to the small metacarpal bones. On the flexion surface of the fetlock joint it is much thickened by fibers passing transversely from one sesamoid bone to the other, forming an =annular ligament= which binds down the flexor tendons in the sesamoid groove or canal. Distal to this is a second thick quadrilateral sheet (Ligamentum vaginale) which covers and is adherent to the tendon of the flexor perforatus. It is attached on either side by two bands to the borders of the first phalanx, thus firmly binding down the flexor tendons. A little further down a crescentic =fibro-elastic sheet= covers the terminal expansion of the deep flexor tendon. It is attached on either side by a strong band to the side of the first phalanx about its middle.

A. EXTENSOR DIVISION

1. =Extensor carpi radialis= (M. extensor carpi radialis s. radialis dorsalis; extensor metacarpi magnus; anterior extensor of the metacarpus).—This is the largest muscle of the extensor division, and lies on the anterior (dorsal) surface of the radius.

_Origin._—(1) The external condyloid crest of the humerus; (2) the coronoid fossa.

_Insertion._—The tuberosity on the anterior (dorsal) surface of the proximal extremity of the large (third) metacarpal bone.

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

_Structure._—The belly of the muscle is rounded, and runs out to a point at the distal third of the forearm. The tendon, which runs nearly the whole length of the fleshy portion, appears on the surface of the latter about its middle; here the muscle shows a distinctly pennate arrangement. The tendon passes through the middle groove at the distal extremity of the radius and over the capsule of the carpal joint, bound down by the anterior annular ligament and invested with a =synovial sheath=. The latter begins three to four inches (ca. 8 to 10 cm.) above the carpus and extends almost to the insertion of the tendon. In the lower half of the forearm the deep fascia blends with the tendon, and here the latter is joined by the long tendon of the biceps.

_Relations._—Superficially, the skin, fascia, and the oblique extensor; deeply, the capsule of the elbow joint, the biceps tendon, the radius, the carpal joint capsule, the anterior radial artery, and the radial nerve; externally, the anterior or common extensor; internally, at the elbow, the brachialis and biceps.

_Blood-supply._—Anterior radial artery.

_Nerve-supply._—Radial nerve.

2. =Anterior= or =common digital extensor= (M. extensor digitalis communis; anterior extensor of the phalanges; extensor pedis).—This muscle lies external to the foregoing, which it resembles in general form, although less bulky.

_Origin._—(1) The front of the distal extremity of the humerus, just external to the coronoid fossa; (2) the external tuberosity on the proximal extremity of the radius, the external lateral ligament of the elbow, and the external border of the radius at the junction of its proximal and middle thirds; (3) the external surface of the shaft of the ulna; (4) the fascia of the forearm.

FIG. 190.—MUSCLES OF LEFT THORACIC LIMB OF HORSE, FROM ELBOW DOWNWARD,
ANTERIOR VIEW.

_a_, Extensor carpi radialis; _g′_, superficial pectoral muscle.
(After Ellenberger-Baum, Anat. für Künstler.)
]

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

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

_Structure._—The muscle is a compound one, representing the common extensor, together with vestiges of the proper extensors of the digits. Usually at least =two heads= may be distinguished. The principal or humeral head (Caput humerale) arises from the front of the extensor epicondyle of the humerus in common with the extensor carpi. Its belly is fusiform, and terminates in a point near the distal third of the radius. The tendon appears on the surface of the muscle about the middle of the belly, the arrangement being pennate. The tendon passes downward through the outer of the two large grooves on the front of the distal end of the radius, and over the capsule of the carpal joint. Passing down over the front of the metacarpus, it inclines gradually inward, reaching the middle line of the limb near the fetlock. A little below the middle of the first phalanx it is joined by the branches of the suspensory ligament, and the tendon thus becomes much wider. Two =synovial membranes= facilitate the play of the tendon. The proximal one is a =sheath= which begins about three inches (ca. 7 to 8 cm.) above the carpus, and terminates at the proximal end of the metacarpus. At the fetlock a =bursa= occurs between the tendon and the joint capsule, but otherwise the two are adherent. The smaller head, arising chiefly from the radius and ulna, is often divisible into two parts (Fig. 443). The larger of these (Caput radiale, muscle of Phillips)[59] arises from the outer tuberosity and outer border of the radius, and from the lateral ligament of the elbow joint. The flat belly is succeeded by a delicate tendon, which accompanies the principal tendon over the carpus (included in the same sheath), and then passes outward to fuse with the tendon of the lateral extensor. Usually a slip is detached which is inserted on the proximal extremity of the first phalanx, or ends in the fascia here. The smaller and deeper division (Caput ulnare, muscle of Thiernesse)[60] is usually somewhat difficult to isolate. It arises from the ulna close to the interosseous space. It has a small rounded belly and is provided with a delicate tendon which may fuse with the principal tendon or may be inserted into the capsule and the fascia in front of the fetlock joint.

_Relations._—The chief relations of the belly of the muscle are: superficially, the skin and fascia; deeply, the elbow joint, the radius and ulna, the extensor carpi obliquus, and the anterior radial vessels and radial nerve; in front and internally, the extensor carpi radialis; behind, the lateral extensor and the interosseous vessels.

_Blood-supply._—Radial and interosseous arteries.

_Nerve-supply._—Radial nerve.

3. =Lateral digital extensor= (M. extensor digitalis lateralis s. digiti quinti proprius; lateral extensor of the phalanges; extensor suffraginis).—This muscle is much smaller than the preceding, behind which it is situated.

_Origin._—The external tuberosity of the radius and the lateral ligament of the elbow joint, the shaft of the ulna, the outer border of the radius, and the intermuscular septum.

_Insertion._—An eminence on the front of the proximal extremity of the first phalanx.

_Action._—To extend the digit and carpus.

_Structure._—The muscle is pennate, and is inclosed in a sheath formed by the deep fascia, from which many fibers arise. The belly is thin and fusiform and terminates at the lower third of the forearm. From here the tendon (at first small and round) passes downward through the groove on the outer tuberosity of the distal end of the radius, then over the carpus, and, gradually inclining toward the front, but not reaching the middle line of the limb, it passes over the metacarpus and fetlock to its insertion. Two =synovial membranes= occur in connection with the tendon. A =sheath= envelops the tendon, beginning about three inches (ca. 6 to 8 cm.) above the carpus, and reaching to the proximal end of the metacarpus. At the fetlock a small bursa lies between the tendon and the joint capsule, but otherwise the tendon is adherent to the capsule. The tendon becomes flat and much larger below the carpus, having received the tendon of the radial head of the anterior extensor and a strong band from the accessory carpal bone.

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=.

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.

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.

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.

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.
]

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.

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.

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