Adaptations to cursoriality and digit reduction in the forelimb of the African wild dog (Lycaon pictus)

M. trapezius

This muscle was extensively damaged during necropsy, so its origin and the morphology of its proximal fibers could not be evaluated. Its distal fibers consist of a thin muscle belly with muscle fibers coursing inferolaterally towards the spine of the scapula (Fig. 2). The muscle lies superficial to the supraspinatus (Fig. 2). Near its insertion, it courses adjacent to the m. omotransversarius, and their bellies adhere to each other by a thin band of fascia. It inserts onto the proximal two-thirds of the cranial aspect of the scapular spine via fleshy fibers (Figs. 2 and 3A). A separable pars thoracica and pars cervicalis could not be identified in this specimen, likely due to damage during necropsy. M. trapezius acts to move the scapula cranially and dorsally, thus elevating the forelimb, as well as stabilize the scapula.

M. latissimus dorsi

The origin of this muscle was damaged during necropsy. It is a thick sheet of muscle, slightly thicker than m. teres major (Fig. 4). Remaining muscle fibers run perpendicular to the length of the humerus (Fig. 4). There is an aponeurosis between m. latissimus dorsi and m. teres major, and the muscles fuse distally to insert on the teres major tuberosity of the humerus via a thick conjoined tendon (Figs. 3A and 5). M. latissimus dorsi acts to retract and abduct the forelimb. When the forelimb is fixed, it moves the trunk cranially.

M. omotransversarius

The origin of this muscle was damaged during necropsy. It is a thin muscle belly similar in appearance to the m. trapezius but narrower (Fig. 2). Muscle fibers course caudolaterally, perpendicular to the spine of the scapula (Fig. 2). The muscle lies superficial to the craniodistal edge of m. supraspinatus and is proximal to the origin of m. deltoideus pars acromialis (Fig. 2). It inserts onto the distal one-third of the scapular spine adjacent to the insertion of m. trapezius, and onto the proximal acromion just proximal to the insertion of m. deltoideus pars acromialis (Figs. 2 and 3A). M. omotransversarius pulls the scapula cranially, advancing the forelimb. When the forelimb is fixed, it flexes the neck.

M. cleidobrachialis

The origin of this muscle was damaged during necropsy. It consists of a long round muscle belly on the cranial portion of the brachium which tapers towards its insertion (Figs. 2 and 4). Muscle fibers run parallel to the long axis of the muscle (Figs. 1 and 3). The muscle travels cranial to m. deltoideus pars acromialis and lateral to m. pectoralis superficialis (Figs. 2 and 4). M. cleidobrachialis inserts onto a small area of the craniodistal humerus between m. biceps brachii and m. brachialis muscle bellies (Figs. 2, 4 and 5A). M. cleidobrachialis draws the forelimb cranially, or flexes the neck if the forelimb is fixed.

M. pectoralis superficialis

This muscle was extensively damaged during necropsy; however, its attachment to the humerus was preserved. Muscle fibers are positioned medial to m. cleidobrachialis and lateral to m. biceps brachii (Figs. 2 and 4). Muscle fibers course perpendicular to the long axis of the humerus (Figs. 2 and 4). It inserts along the proximal two-thirds of the cranial surface of the humerus (Figs. 2, 4, 5A and 5B). M. pectoralis superficialis adducts the forelimb and may draw the forelimb cranially or caudally depending on the portion of the muscle that is active and the position of the limb.

M. deltoideus

Pars scapularis: This muscle originates from the scapular spine indirectly via a connection to the aponeurosis of m. infraspinatus (Fig. 2). It is a flat fusiform muscle which lies superficial to the inferior portion of the infraspinatus and completely overlies m. teres minor (Fig. 1). The muscle fibers run parallel to the length of the spine of the scapula (Fig. 2). It inserts onto the m. deltoideus pars acromialis and has no direct connection to the humerus (Fig. 2).

Pars acromialis: The origin of this muscle is from the acromion process of the scapula distal to the insertion of m. omotransversarius (Figs. 2 and 3A). Its muscle belly is fusiform and has a prominent aponeurosis covering it proximally (Fig. 2). The proximal muscle fibers course parallel to the scapular spine, and then slowly curve distally to become parallel to the humerus (Fig. 2). It has a large insertion onto the deltoid tuberosity on the lateral humerus (Figs. 2 and 5B).

M. deltoideus flexes the glenohumeral joint and abducts the humerus.

M. supraspinatus

This muscle has a large, thick muscle belly originating from and fully filling the supraspinous fossa to the tip of the spine of the scapula (Figs. 2 and 3A). The muscle is elliptical in shape, similar to, but wider than m. infraspinatus. Muscle fibers run obliquely from the spine of the scapula to the distal margins of the muscle belly. An aponeurosis covers the proximal one-third of the muscle surface. There is a thick tendon in the middle of the muscle which travels two-thirds proximally up the muscle from the glenohumeral joint to the proximal point and dissipates, similar to the pattern in m. infraspinatus. It inserts onto the greater tubercle of the humerus along its cranial aspect, proximal to the insertion of m. infraspinatus (Figs. 2, 3 and 5B). As with the other muscles of the rotator cuff, m. supraspinatus stabilizes the shoulder. It also extends the glenohumeral joint, resulting in advancement of the forelimb.

M. infraspinatus

This muscle originates from and occupies the infraspinous fossa (Fig. 3A). It has a thick muscle belly with an elliptical shape that tapers on its proximal and distal ends. Distal portions of the muscle are deep to the m. deltoideus pars scapularis. The muscle has various muscle fiber directions with the cranial and caudal fibers coursing diagonally to converge on the midline of the muscle. The tendon adhering to the humerus is located within the muscle belly and travels three-quarters of the length of m. infraspinatus before widening and inserting on the greater tubercle of the humerus (Fig. 5B). The flat portion of the tendon parallels the spine of the scapula. M. infraspinatus has an extensive aponeurosis on its cranial portion covering about one-third of the superficial muscle. It inserts onto the greater tubercle of the humerus inferior to the insertion of m. supraspinatus (Fig. 5B). M. infraspinatus acts to laterally rotate the humerus and stabilize the glenohumeral joint. It can also contribute to flexion or extension of the shoulder, depending on the position of the humeral head relative to the glenoid when the muscle contracts.

M. teres minor

M. teres minor originates from the distal three-quarters of the infraspinous fossa margin (Figs. 2 and 3A). Proximally, the muscle is thin and broad, and then it thickens quickly to a spindle shape and tapers distally (Figs. 2 and 5B). Its insertion is the teres minor tuberosity on the proximolateral aspect of the greater tubercle of the humerus (Figs. 2 and 5B). M. teres minor flexes the glenohumeral joint.

M. teres major

The origin of this muscle was damaged during necropsy. The muscle forms a thick sheet which courses adjacent to m. latissimus dorsi and adheres to it near the origin of m. tensor fascia antebrachii on m. latissimus dorsi (Fig. 3). It inserts broadly onto the teres major tuberosity (Figs. 3 and 5A) cranial to the insertion of m. triceps brachii caput mediale. The attachment of m. teres major onto the humerus is thicker and stronger than the attachments of the rotator cuff muscles. M. teres major flexes the glenohumeral joint, drawing the humerus caudally. It may also medially rotate the humerus, resisting lateral rotation.

M. subscapularis

This is a large, thick muscle which originates from the distal two-thirds of the subscapular fossa (Fig. 3B) distal to the presumed insertion of m. serratus ventralis. Its broad fibers are divided into four pennate portions which occupy much of the subscapular fossa. The two cranialmost pennations are broad and triangular, while the two caudalmost pennations are elongated and course parallel to the caudal border of the scapula. Its tendon contributes to the glenohumeral joint capsule and inserts onto the tip of the lesser tubercle (Fig. 5A). M. subscapularis adducts and extends the glenohumeral joint. It also stabilizes the glenohumeral joint by medially rotating the humerus in order to prevent undesired lateral rotation.

M. biceps brachii

M. biceps brachii has a single, spindle-shaped belly that originates from the supraglenoid tubercle of the scapula (Figs. 3A and 4). Its thick tendon crosses over the head of the humerus through the intertubercular groove of the humerus (Fig. 4), and it courses the length of the muscle. An aponeurosis covers the muscle superficially and also the proximal, deep two-thirds of the muscle (Figs. 2, 4, 6, 7 and 8). The muscle belly lies on the medial side of the humerus with fiber orientation parallel to the humerus (Figs. 4 and 7). The tendon of m. biceps brachii combines with the m. brachialis tendon before bifurcating and inserting on the radial and ulnar tuberosities (Figs. 9B and 10A). M. biceps brachii flexes the elbow and extends the glenohumeral joint. It also acts to stabilize the glenohumeral joint when the forelimb is fixed.

M. tensor fasciae antebrachii

This is a thin, flat, wide muscle that arises via a thin aponeurosis on the surface of m. latissimus dorsi (Fig. 4). The muscle belly tapers distally and inserts onto the medial ulna and the antebrachial fascia (Figs. 4 and 9B). This muscle tenses the antebrachial fascia and assists m. triceps brachii in extending the elbow.

M. articularis humeri (coracobrachialis)

This is a thin, delicate fusiform slip of muscle whose origin cannot be determined definitively due to necropsy damage (Fig. 4). The muscle belly lies over the lesser tubercle of the humerus, and is medial to the m. teres major insertion, m. triceps brachii medial head origin, and belly of m. brachioradialis (Fig. 4). It inserts onto the craniomedial aspect of the proximal humerus (Fig. 5A). M. articularis humeri stabilizes the glenohumeral joint.

M. brachialis

The origin site of this muscle begins at a caudolateral divot on the proximal greater tubercle of the humerus (Fig. 5B). Its two lines of origin diverge distally from this divot. One origin follows the lateral side of the tricipital line along the deltoid tuberosity in the proximal half of the humerus and courses medially and distally to the cranial side, half-way down the humerus (Fig. 5B). The other line of origin is straight on the caudal aspect of the proximal two-thirds of the humerus, ending at the origin of the anconeus (Figs. 5A and 5B). The fibers of the muscle belly are minimally apparent, giving the belly a flat appearance (Figs. 2, 4, 6 and 7). It occupies the entire lateral brachium and passes craniomedially in its distal half (Figs. 4, 6 and 7). The muscle becomes tendinous and fuzes with the tendon of m. biceps brachii before bifurcating and inserting onto the radial and ulnar tuberosities (Figs. 9B and 10A). M. brachialis flexes the elbow joint.

M. triceps brachii

Caput magnum: It should be noted that this head is not listed in the Nomina Anatomica Veterinaria, because it was previously undescribed. We define the m. triceps brachii caput magum here as a large head of m. triceps brachii that is positioned caudally and laterally in the m. triceps brachii complex. It originates from the caudal two-thirds of the scapular margin and neck, medial to the origin of m. teres minor (Fig. 3B). The muscle belly has a tear-drop appearance, with the tip oriented distally towards the insertion (Fig. 2). It fuses via a strong tendon with the caput longum along the distal half of the lateral aspect of the humerus (Figs. 2 and 4).

Caput longum: This head of m. triceps brachii originates from the inferior border of the scapula lateral to the origin of m. triceps brachii caput magnum (Fig. 3B). It has an elongated spindle-shaped appearance that tapers as it travels distally towards its insertion. It fuses with the caput magnum distally (Figs. 2 and 4).

Caput mediale: This head of m. triceps brachii originates from the intertubercular groove cranial to the insertion of m. teres major (Fig. 5A). The muscle is long and spindle-shaped, narrow near its origin and thicker distally (Fig. 4).

Caput laterale: The lateral head of m. triceps brachii has a thick parallelogram shape (Fig. 2). It originates via a thin, tendinous band from the proximolateral third of the humeral shaft (Figs. 2 and 9A). The muscle belly decreases in width but increases in thickness toward its insertion (Figs. 2 and 9A).

Caput accessorium: The accessory head of m. triceps brachii is long and thin with a broad origin from the lateral aspect of the humeral neck (Figs. 2 and 5B). The attachment resembles an inverted tick mark (Fig. 5B). The muscle is situated between the caput mediale and caput magnum of the m. triceps brachii (Fig. 2). Most of the muscle is on the caudal aspect of the humerus and it slightly covers m. articularis humeri and m. brachialis proximally and m. anconeus distally (Fig. 2).

The heads of m. triceps brachii fuse distally and share a stout tendon of insertion onto the entirety of the proximal portion of the olecranon process, and part of the olecranon tuber on the lateral side of the ulna (Figs. 9A and 9B). General contributions to this large insertion site include: a strong round tendon formed by the caput accessorium that enters the olecranon groove, the caput laterale insertion on the lateral side of the olecranon, and a crescent-shaped caput magnum+longum tendon that covers the tendon of the caput accessorium (Figs. 9A and 9B). Together, the m. triceps brachii are powerful extensors of the elbow and stabilize the joint during standing.

M. anconeus

The muscle belly of m. anconeus is triangular and generally thin (Figs. 2, 4 and 6). Its origin is V-shaped on the caudal aspect of the distal humerus and lies between the midpoint of the caudal shaft to the medial and lateral supracondylar crests (Figs. 5A and 5B). On the lateral supracondylar crest, it is adjacent to m. extensor carpi radialis and m. brachialis (Fig. 5B). There is a large fat deposit in the olecranon fossa deep to the muscle at the caudal humerus. M. anconeus has a broad, fleshy insertion from the lateral side of the olecranon to the lateral aspect of the coronoid process (Figs. 5A and 5B). M. anconeus may assist the m. triceps brachii with extension of the elbow. However, it is likely that its more important role maybe in resisting elbow flexion during standing and maintaining stability at the joint.

M. brachioradialis

This muscle has a long, thin, flat belly overlying mm. extensor carpi radialis longus and brevis (Figs. 6 and 8). It originates from approximately the lateral aspect of the distal quarter of the humerus (Figs. 5B and 6). The muscle belly is covered with an extensive fascia, and crosses from lateral to medial, distal to m. extensor carpi radialis (Fig. 6). It inserts onto the distal quarter of radius (Fig. 10B). M. brachioradialis flexes the elbow joint. It may also weakly supinate the antebrachium, but the tight attachment between the radius and ulna limits such rotatory movements.

M. extensor carpi radialis

This muscle is generally spindle-shaped and has a fanning origin from the lateral supracondylar ridge of the humerus (Figs. 5B and 6). An aponeurosis covers the proximal half of the caudal portion of the muscle (Figs. 6 and 8). It has two distinct bellies which are fused at the origin and separate in its distal half (Fig. 6). Each belly gives rise to a tendon in the distal third of the radius and insert as described below (Fig. 6).

M. extensor carpi radialis longus: The portion comprising m. extensor carpi radialis longus is approximately a third the size of m. extensor carpi radialis brevis (Figs. 6 and 8). Its muscle fibers are parallel with the radius, and it is superficial to the m. extensor carpi radialis brevis (Fig. 6). M. extensor carpi radialis longus has a smaller tendon than m. extensor carpi radialis brevis, which inserts onto the base of metacarpal II (Fig. 11).

M. extensor carpi radialis brevis: M. extensor carpi radialis brevis is approximately three times larger than its counterpart (Fig. 6). Its belly originates in a fan-like arrangement and gives rise to a tendon that is significantly larger than the tendon for m. extensor carpi radialis longus (Fig. 6). M. extensor carpi radialis brevis lies deep to the belly of m. extensor carpi radialis longus and has a tendinous insertion onto the base of metacarpal III (Fig. 11).

Both mm. extensor carpi radialis muscles primarily extend the carpal joint and may weakly flex the elbow.

M. extensor digitorum communis

This muscle originates from the proximal portion of the lateral epicondyle of the humerus (Figs. 5B and 6). A thick band of fascia covers the edges of the muscle belly proximally and also connects to m. extensor digitorum lateralis near its insertion (Fig. 6). The muscle belly is triangular and has an aponeurosis on its cranial aspect which lies deep to the m. extensor carpi radialis (Fig. 6). Its fibers converge in the middle of the muscle, and give rise to a tendon in the distal half of the radius (Fig. 6). The tendon of m. extensor digitorum communis is approximately 50% thicker than that of m. extensor digitorum lateralis (Fig. 6). The muscle remains undivided until reaching the wrist, where it divides into four tendons serving digits II–V. A dorsal sesamoid bone is embedded in each tendon as it crosses the metacarpophalangeal joint. Each sesamoid is a small, flat, elliptical bone and those of digits III and IV are largest. The tendons each insert on the extensor expansion at the distal phalanx of digits II–V (Fig. 11). The tendons of m. extensor digitorum communis attaching to metacarpal IV and V are fused with the tendons of m. extensor digitorum lateralis. M. extensor digitorum communis extends the carpal joint, metacarpophalangeal, and interphalangeal joints of digits II–V.

M. extensor digitorum lateralis

This muscle is fusiform and its fibers originate from the lateral epicondyle of the humerus and lateral collateral ligament (Figs. 5B and 6). Its muscle belly gives rise to a tendon in the distal half of the radius along its caudal edge (Fig. 6). The muscle is covered by an aponeurosis on its cranial and caudal edges, and it lies between m. extensor digitorum communis and m. extensor carpi ulnaris, superficial to m. extensor digiti I et II (Fig. 6). The tendons of m. extensor digitorum lateralis travel deep to m. extensor digitorum communis over metacarpal III and IV. The tendons insert on the extensor expansion at the distal phalanx of their respective digits IV–V (Fig. 11). As they do so, the two tendons fuse with m. extensor digitorum communis on the distal aspect of the proximal phalanges of digits IV and V (Fig. 11). M. extensor digitorum lateralis does not serve digit III in either limb, which differ from the domestic dog (Evans & De Lahunta, 2013). This muscle extends the carpal joint, metacarpophalangeal, and interphalangeal joints of digits IV–V.

M. extensor digiti I et II (m. extensor pollicis longus et indicis proprius)

This muscle has an elongated, thin belly on the lateral side of the antebrachium located between the ulna and radius. It originates from the middle third of the ulna on its craniolateral aspect. The muscle lies deep to m. digitorum lateralis and travels through the concavity between the radius and ulna superficial to the interosseous membrane (Fig. 7). M. extensor digiti I et II lies immediately lateral to m. abductor digiti I longus, and its fibers travel approximately parallel to the ulna. It becomes tendinous in the distal third of the ulna (Fig. 7). Its tendon is thin and crosses medially over the dorsal aspect of the manus beginning at the wrist. The tendon bifurcates on the proximal dorsal surface of metacarpal III. One branch travels medially along metacarpal II, and at its midpoint the tendon courses deep to the tendon of m. extensor digitorum communis, before it fuses on the lateral side of m. extensor digitorum communis, on the proximal phalanx of digit II. The other belly travels further medially and also inserts on metacarpal II (Fig. 11). There is no insertion onto the vestigial digit I. M. extensor digiti I et II therefore acts to weakly extend the carpal joint, metacarpophalangeal, and interphalangeal joints of digit II, but does not act on vestigial digit I.

M. extensor carpi ulnaris

This is a spindle-shaped muscle originating from the lateral epicondyle of the humerus (Figs. 5B, 6 and 7). Muscle fibers are oriented caudally near the origin but become distally parallel with the ulna in its distal half (Figs. 6 and 7). The tendon arises superficially in the proximal third of the ulna but appears distally from the deep aspect in the distal quarter of the ulna (Fig. 6). The belly is covered by an aponeurosis that is thicker on its deep surface. The tendon of insertion is more robust and stiffer than those of the other antebrachial muscles. It inserts onto the lateral side of the pisiform and the base of metacarpal V (Figs. 6 and 11). In the domestic dog, the insertion on the pisiform (accessory carpal) is described as having two fiber bundles that leave the pisiform to fuse with m. extensor carpi ulnaris (Evans & De Lahunta, 2013). We did not observe this pattern in L. pictus. M. extensor carpi ulnaris abducts the carpal joint and supports the carpus, especially when it is extended and weight-bearing.

M. abductor digiti I longus

The muscle belly of m. abductor digiti I longus is long and flat, filling the entire interosseous space between the radius and ulna (Fig. 7). The muscle originates from the caudolateral radius, craniolateral ulna, and interosseous membrane (Figs. 9A and 10). It lies deep to m. extensor digiti I et II on the lateral side of the antebrachium in the concavity between the radius and ulna. Muscle fibers travel diagonally, oriented cranially across the forearm. A wide, flat tendon arises and tapers distally at the cranial-most aspect of the radius. The tendon crosses obliquely from lateral to medial over the tendon of m. extensor carpi radialis toward the medial side of the manus. There is a sesamoid bone in its tendon as it becomes thinner just before its insertion onto the medial base of vestigial metacarpal I (Fig. 7). M. abductor digiti I longus primarily acts to abduct the manus. While it attaches to the vestigial metacarpal I, the bone is tightly adhered to the carpus and is unlikely to experience any notable movement at the joint between MC1 and carpals.

M. supinator

This deep muscle completely covers the proximal quarter of the cranial radius. It is thin, flat and fan-shaped with a heavy aponeurosis across most of its superficial surface (Fig. 7). The muscle originates from the lateral epicondyle of the humerus and lateral collateral ligament (Fig. 5B). Its fibers are oriented medially, oblique to the radius. The muscle insertion is “U”-shaped on the proximal quarter of the radius (Fig. 10B). The medial portion of the insertion extends approximately twice that of the lateral portion (Fig. 10B). Unlike the domestic dog, there is no sesamoid bone within the tendon, (Evans & De Lahunta, 2013). M. supinator acts to stabilize the elbow and weakly supinates the antebrachium, but limited rotatory movement is possible due to a tightly connected radius and ulna.

M. pronator teres

The belly of m. pronator teres is spindle-shaped proximally and flatter distally (Fig. 8). An aponeurosis covers its distal half, and the muscle slightly overlaps m. supinator medially (Fig. 8). The muscle originates from the craniodistal aspect of the medial epicondyle (Figs. 5A and 8). Its fibers are obliquely oriented in a cranial direction, and it inserts onto the proximal third of the craniomedial radius, distal to the insertion of m. supinator (Fig. 10B). As its name implies, m. pronator teres weakly pronates the antebrachium. However, since antebrachial rotation is limited in L. pictus, its primary action may be to flex the antebrachium at the elbow.

M. flexor carpi radialis

This muscle is spindle-shaped. The cranial aspect of its distal half is covered by a thick aponeurosis near its origin (Fig. 8). The muscle originates from the medial epicondyle of the humerus (Fig. 5A). The caudal surface of the muscle is also covered by aponeurosis, adjacent to m. flexor digitorum profundus caput humerale (Fig. 8). The muscle becomes tendinous at the distal half of the radius (Fig. 8). The proximal muscle fibers travel parallel to the radius, but at the midline of the muscle they converge distally as they form a tendon (Fig. 8). The tendon inserts onto the palmar aspects of the bases of metacarpals II and III. M. flexor carpi radialis flexes and stabilizes the carpal joint.

M. flexor carpi ulnaris

This muscle includes two heads.

M. flexor carpi ulnaris caput ulnare is short and flat, originating from the distomedial aspect of the olecranon (Figs. 6 and 7). The muscle belly gives rise to a tendon at the level of the distal half of the ulna. The muscle fibers are oriented slightly obliquely toward the medial side. M. flexor carpi ulnaris caput ulnare caput ulnare lies superficial to m. flexor digitorum profundus caput ulnare and is lateral to m. flexor digitorum superficialis (Figs. 6 and 7). It inserts on the pisiform (accessory carpal bone), cranial to the insertion of m. flexor carpi ulnaris caput humerale (Figs. 11 and 12).

M. flexor carpi ulnaris caput humerale is flat proximally, becoming spindle-shaped in its distal half (Figs. 6 and 7). It originates from the caudal aspect of the medial epicondyle of the humerus (Figs. 4 and 5). Fascia covers most of the muscle on its superficial and deep surfaces (Figs. 6 and 7). The muscle fibers are oriented parallel to the ulna, and the muscle inserts onto the pisiform, cranial to the insertion of m. flexor carpi ulnaris caput humerale (Figs. 11 and 12).

Together the two heads of m. flexor carpi ulnaris flex and abduct the manus.

M. flexor digitorum superficialis

This muscle is spindle-shaped and somewhat flat (Figs. 8 and 12). It is superficial to m. flexor digitorum profundus and m. flexor carpi ulnaris caput humerale (Fig. 8). The muscle originates from the medial epicondyle of the humerus (Fig. 5). The muscle becomes tendinous at the wrist and travels deep to the flexor retinaculum before passing medial to the pisiform (Figs. 11 and 12). It then bifurcates over the proximal phalanges of digits II–V, allowing passage of m. flexor digitorum profundus. The muscle inserts bilaterally on the palmar aspects of the middle phalanges of digits II–V (Figs. 11 and 12). M. flexor digitorum superficialis flexes the carpal, metacarpophalangeal, and proximal interphalangeal joints of digits II–V.

M. flexor digitorum profundus

M. flexor digitorum profundus has five heads of various sizes, including three with a shared humeral origin, one ulnar, and one radial. It serves digits II–V and does not contact the vestigial digit I (Figs. 5, 8, 9, 10 and 12).

Caput humerale laterale: This is the largest of the humeral heads, and accounts for approximately half of the entire humeral component, which has a fused origin (Fig. 8). It is spindle-shaped, with fibers that fan out from its origin at the caudodistal aspect of the medial epicondyle (Fig. 5). It lies adjacent to the caput humerale mediale, to the caput profundus, and deep to m. flexor digitorum superficialis (Fig. 8). Approximately three-quarters of the medial surface of the muscle is covered by parallel-fibered aponeurosis.

Caput humerale mediale: This is the most superficial of the humeral heads (Figs. 5 and 8). It is spindle-shaped, covered by a thick aponeurosis, and about half the size of the caput humerale laterale. This medial head lies lateral to m. flexor carpi radialis, medial to caput humerale laterale and m. flexor digitorum superficialis, and superficial to caput humerale profundus.

Caput humerale profundus: This is the smallest humeral head. It is spindle-shaped and its fibers travel parallel to the ulna (Fig. 5). The medial aspect of its proximal half is covered by an aponeurosis. It gives rise to a tendon along the distal third of the radius, which is enclosed by the capita humerale laterale and mediale.

Caput ulnare: This belly lies caudal to the ulna (Fig. 9B). It originates from the base of the olecranon and the proximal quarter of the ulnar shaft (Fig. 9B). The entire muscle belly is slightly adhered via fascia to the ulna. Its tendon arises unattached to the bone, and joins the three humeral heads. The muscle’s fibers are parallel to the ulna, and a thick aponeurosis covers the medial half of the superficial surface.

Caput radiale: This is the smallest head of m. flexor digitorum profundus, located on the medial side of the antebrachium, superficial to m. pronator quadratus. Its thin muscle belly originates from the proximal third of the caudomedial radius (Fig. 10A). The muscle fibers travel parallel to the radius, and the insertion tendon arises at the level of the distal quarter of the bone.

Tendons of the flexor digitorum profundus fuse at the distalmost radius, and the combined tendon passes deep to the flexor retinaculum and medial to the pisiform. It divides into four smaller parts deep to the retinaculum, which serve digits II–V (Figs. 10 and 11). The tendon of caput ulnare primarily serves digit V, but such a distinction cannot be made for the tendons serving digits II–IV. M. flexor digitorum profundus flexes the carpal, metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints of digits II–V.

M. pronator quadratus

This muscle lies deep to m. flexor digitorum profundus, occupying the entire space between the radius and ulna on the medial side of the antebrachium. It originates from the craniomedial aspect of the distal ulna and interosseous membrane (Fig. 9B). Its fibers are oriented obliquely to perpendicular to the radius and ulna. An aponeurosis covers the superficial surface of the proximal three quarters of the muscle belly. The muscle inserts on almost the entire caudomedial aspect of the radial shaft (Fig. 10). Unlike the muscle attachments in the domestic dog, these are situated more proximally on the radius and ulna (Evans & De Lahunta, 2013). M. pronator quadratus weakly pronates, but likely primarily acts to stabilize the antebrachium.