HORSES

THE HORSE

Transverse Section of the Horse Middle Scapula

Structure 1Long head of the Triceps muscleThe long head of the triceps brachii muscle is the largest and differs from the other tricipital heads in that it originates from the caudal border of the scapula rather than the humerus. Its origin from the scapula gives it the ability to flex the shoulder, although its main action is elbow extension. All parts of the triceps brachii muscle insert upon the olecranon tuber, are innervated by the radial nerve, and are the most important elbow extensors. Without elbow extension the forelimb would collapse under the weight of the standing horse. A synovial bursa is commonly found between the tendon of insertion of the triceps brachii and the expanded part of the olecranon tuber. Inflammation of this bursa may result in a condition known as capped elbow.

Structure 2Subscapularis muscleThe subscapularis muscle arises from and occupies the subscapular fossa on the costal surface of the scapula and inserts on the lesser tubercle of the humerus. The shoulder joint of the horse lacks collateral ligaments but the strong tendons of insertions of the subscapularis and infraspinatus muscles have an analagous function. The subscapularis muscle functions primarily as a shoulder joint stabilizer but it can also adduct that joint. It is innervated by the subscapular nerve.

Structure 3Teres Major muscleThe teres major muscle along with the deltoideus and the teres minor are the primary flexors of the shoulder joint. The teres major originates from the caudal angle of the scapula and shares its insertion on the teres major tuberosity with the latissimus dorsi. It is innervated by the axillary nerve.

Structure 4Infraspinatous muscleThe infraspinatus muscle arises from and occupies the infraspinous fossa of the scapula. It has deep and superficial tendons of insertion, both of which attach upon the greater tubercle of the humerus. The shoulder joint of the horse lacks collateral ligaments but the strong tendons of insertions of the infraspinatus and subscapularis muscles have an analagous function. An important bursa lies between the superficial tendon of insertion and the humerus.

Structure 5Subclavis muscleThe subclavius is a deep pectoral muscle that originates on the manubrium and insert upon the epimysium of the supraspinatus. Its distinctive crescent shape and position relative the supraspinatus make it easy to identify on transverse sections of the scapula. The subclavius is not present in the dog. It is innervated by pectoral nerves from the brachial plexus.

Structure 6Supraspinatous muscleThe supraspinatus muscle originates from and occupies the supraspinous fossa of the scapula. Its epimysium provides an insertion site for the subclavius muscle. The split tendon of insertion attaches to the cranial parts of the greater and lesser tubercles of the humerus and its action is to stabilize and extend the shoulder joint. Inervation is provided by the suprascapular nerve. As this nerve crosses the cranial border of the scapula it is exposed to trauma. Damage to the suprascapular nerve results in a condition known as "Sweeny" and a resulting atrophy of the muscle.

Structure 7Deltoideus muscleThe deltoideus muscle of the horse has muscular and aponeurotic parts and is sometimes difficult to separate from the infraspinatus muscle during dissections. Unlike the dog and domestic ruminants, the deltoideus of the horse does not have two distinct divisions. The deltoideus arises from the spine and the caudal border of the scapula and inserts upon the prominent (and palpable) deltoid tuberosity of the humerus. Its primary action is to flex the shoulder joint.

Structure 8ScapulaThe large, flattened, triangular-shaped scapula is the most proximal bone of the thoracic limb. It articulates with the humerus at approximately a right angle to form the shoulder joint. The scapula does not have an osseus articulation with the axial skeleton but is held in place lateral to the thorax by a complex series of extrinsic forelimb muscles.

Structure 9Brachiocephalicus muscleThe brachiocephalicus of the horse is composed of two parts. Part one is the most cranial and is called the cleidomastoideus. The cleidomastoideus runs from the mastoid region of the skull to the clavicular insertion (the horse is devoid of a clavicle but has a small tendinous band embedded in muscle at the point of shoulder called the clavicular insertion. This is regarded by some as a vestige of the clavicle bone). Part two is more caudal and called the cleidobrachialis. It runs from the clavicular insertion to the deltoid tuberosity and crest of the humerus. The brachiocephalicus has a complex origin from the mastoid process, nuchal crest, wing of atlas, and the transverse processes of C2, C3, and C4. It inserts upon the deltoid tuberosity and crest of the humerus. Its action is to advance the limb, extend the shoulder, draw neck and head to side. It is one of four muscles innervated by the accessory nerve, but is also receives some parasympathetic innervation from dorsal branches of the spinal nerves.

Structure 10Cutaneous trunci muscleThe cutaneous muscles of the horse, like most domestic mammals, are a thin muscular layer developed in the superfacial fascia. They attach to the skin but have little attachment to the skeleton. In the horse they are divided into several parts. The cutaneus fasciei covers part of the face, the cutaneus colli the sides of the neck, the cutaneus omobrachialis covers the lateral surface of the shoulders, and the cutaneus trunci covers almost the entire lateral surface of the trunk. The cutaneus fasciei and the cutaneus colli together comprise the platysma. The fibers of the cutaneous truncii have a general longitudinal orientation along the lateral aspect of the trunk. The cutaneous trunci typically adheres to the skin during dissections and is usually reflected along with it. A thin tendon from it passes cranially with the pectoral muscles to the lesser tubercle of the humerus and blends with the tendon of the latissimus dorsi. Caudally it forms a skin covered fold (fold of flank) and ends on the fascia above the stifle. The cutaneous trunci muscle allows the horse to quiver the skin of the trunk vigorously to dislodge insects and other irritants. It is innervated by the lateral thoracic nerve. This nerve is clinically important because it is the efferent limb of the cutaneus trunci reflex which can be used to localize spinal lesions.

Structure 11Tensor Fascia Antibrachii muscleThe tensor fascia antibrachii muscle derives its name from its action of stretching (tensing) the fascia of the forearm. It can also extend the elbow joint. The muscle is very thin and flat and covers the medial aspect of the long head of the triceps. It originates from the caudal border of the scapula and also from the tendon of insertion of the latissimus dorsi. It inserts on the deep fascia of the forearm and also on the olecranon, which gives it its ability to extend the elbow. It is innervated by the radial nerve.

Structure 12Cutaneous omobrachialis muscleThe cutaneous muscles of the horse, like most domestic mammals, are a thin muscular layer developed in the superfacial fascia. They attach to the skin but have little attachment to the skeleton. In the horse they are divided into several parts. The cutaneus fasciei covers part of the face, the cutaneus colli the sides of the neck, the cutaneus omobrachialis covers the lateral surface of the shoulders, and the cutaneus trunci covers almost the entire lateral surface of the trunk. The cutaneus fasciei and the cutaneus colli together comprise the platysma. The fibers of the cutaneous trunci have a general longitudinal orientation along the lateral aspect of the trunk. The cutaneous trunci typically adheres to the skin during dissections and is usually reflected along with it. A thin tendon from it passes cranially with the pectoral muscles to the lesser tubercle of the humerus and blends with the tendon of the latissimus dorsi. Caudally it forms a skin covered fold (fold of flank) and ends on the fascia above the stifle. The cutaneous trunci muscle allows the horse to quiver the skin of the trunk vigorously to dislodge insects and other irritants. It is innervated by the lateral thoracic nerve. This nerve is clinically important because it is the efferent limb of the cutaneus trunci reflex which can be used to localize spinal lesions.

Transverse Section of the Horse Shoulder

Structure 1HumerusThe humerus is the primary skeletal component of the brachium. Its large rounded head articulates with the glenoid cavity of the scapula to form the shoulder joint. Together, these two bones approximate a right angle in the horse. At its distal end the humerus articulates with the radius and ulna to form the elbow joint. The greater tubercle (point of shoulder) and the deltoid tuberosity are conspicuous palpable features of this bone.

Structure 2Greater Tubercle of the HumerusThe greater tubercle is a conspicuous palpable feature of the horse's humerus. It has two parts; the more cranial part is commonly referred to as the point of the shoulder and provides an attachment site for the lateral tendon of insertion of the supraspinatus muscle. The caudal part of the greater tubercle is the attachment site for part of the infraspinatus muscle. The shoulder joint of the horse lacks collateral ligaments but the strong tendons of insertions of the infraspinatus and subscapularis muscles have an analagous function.

Structure 3Lesser Tubercle of the HumerusThe lesser tubercle of the humerus, like its more massive lateral counterpart, the greater tubercle, has cranial and caudal parts. The cranial part of the lesser tubercle provides an attachment site for the supraspinatus muscle. The caudal part is the attachment site for the subscapularis muscle. The shoulder joint of the horse lacks collateral ligaments but the strong tendons of insertions of the infraspinatus and subscapularis muscles have an analagous function.

Structure 4Intermediate Tubercle of the HumerusThe cranial aspect of the head of the equine humerus presents three prominent tubercles: greater, lesser, and intermediate. The greater and lesser tubercles are separated by the intertubercular groove. The intertubercular groove is occupied by the intermediate tubercle. The tough fibrocartilagenous tendon of the biceps brachii muscle is stabilized as it passes over the shoulder joint by the prominent intermediate tubercle which closely conforms to the tendon's caudal surface.

Structure 5Tendon of the Biceps brachii muscleThe massive, fibrocartilagenous tendon of the biceps brachii muscle passes through the intertubercular groove as it crosses the cranial aspect of the shoulder joint. Its caudal surface closely conforms to the ridge-like intermediate tubercle. The tendon is relatively inelastic and therefore passively keeps the shoulder joint from collapsing under the weight of the standing horse. Shoulder lameness may result when the intertubercular bursa (which protects the tendon at the level of the shoulder joint) becomes inflamed.

Structure 6Long head of the Triceps muscleThe long head of the triceps brachii muscle is the largest and differs from the other tricipital heads in that it originates from the caudal border of the scapula rather than the humerus. Its origin from the scapula gives it the ability to flex the shoulder, although its main action is elbow extension. All parts of the triceps brachii muscle insert upon the olecranon tuber, are innervated by the radial nerve, and are the most important elbow extensors. Without elbow extension the forelimb would collapse under the weight of the standing horse. A synovial bursa is commonly found between the tendon of insertion of the triceps brachii and the expanded part of the olecranon tuber. Inflammation of this bursa may result in a condition known as capped elbow.

Structure 7Lateral head of the Triceps muscleThe lateral head of the triceps brachii muscle originates from the deltoid tuberosity and a distinctive roughened curved line on the proximal end of the humerus. All parts of the triceps brachii muscle insert upon the olecranon tuber, are innervated by the radial nerve, and are the most important elbow extensors. Without elbow extension the forelimb would collapse under the weight of the standing horse. A synovial bursa is commonly found between the tendon of insertion of the triceps brachii and the expanded part of the olecranon tuber. Inflammation of this bursa may result in a condition known as capped elbow.

Structure 8Subscapularis muscleThe subscapularis muscle arises from and occupies the subscapular fossa on the costal surface of the scapula and inserts on the lesser tubercle of the humerus. The shoulder joint of the horse lacks collateral ligaments but the strong tendons of insertions of the subscapularis and infraspinatus muscles have an analagous function. The subscapularis muscle functions primarily as a shoulder joint stabilizer but it can also adduct that joint. It is innervated by the subscapular nerve.

Structure 9Brachiocephalicus muscleThe brachiocephalicus of the horse is composed of two parts. Part one is the most cranial and is called the cleidomastoideus. The cleidomastoideus runs from the mastoid region of the skull to the clavicular insertion (the horse is devoid of a clavicle but has a small tendinous band embedded in muscle at the point of shoulder called the clavicular insertion. This is regarded by some as a vestige of the clavicle bone). Part two is more caudal and called the cleidobrachialis. It runs from the clavicular insertion to the deltoid tuberosity and crest of the humerus. The brachiocephalicus has a complex origin from the mastoid process, nuchal crest, wing of atlas, and the transverse processes of C2, C3, and C4. It inserts upon the deltoid tuberosity and crest of the humerus. Its action is to advance the limb, extend the shoulder, draw neck and head to side. It is one of four muscles innervated by the accessory nerve, but is also receives some parasympathetic innervation from dorsal branches of cervical spinal nerves. The brachiocephalicus is an important landmark in the neck of the horse and forms the dorsal border of the jugular groove.

Structure 10Tendon of the Supraspinatus muscleThe supraspinatus muscle originates from and occupies the supraspinous fossa of the scapula. Its epimysium provides an insertion site for the subclavius muscle. The split tendon of insertion attaches to the cranial parts of the greater and lesser tubercles of the humerus and its action is to stabilize and extend the shoulder joint. Inervation is provided by the suprascapular nerve. As this nerve crosses the cranial border of the scapula it is exposed to trauma. Damage to the suprascapular nerve results in a condition known as "Sweeny" and a resulting atrophy of the muscle.

Structure 11Tendon of the Infraspinatous muscleThe infraspinatus muscle arises from and occupies the infraspinous fossa of the scapula. It has deep and superficial tendons of insertion, both of which attach upon the greater tubercle of the humerus. The shoulder joint of the horse lacks collateral ligaments but the strong tendons of insertions of the infraspinatus and subscapularis muscles have an analagous function. An important bursa lies between the superficial tendon of insertion and the humerus. Inflammation here may lead to animal standing with affected limb abducted at the shoulder.

Structure 12Deltoideus muscleThe deltoideus muscle of the horse has muscular and aponeurotic parts and is sometimes difficult to separate from the infraspinatus muscle during dissections. Unlike the dog and domestic ruminants, the deltoideus of the horse does not have two distinct divisions. The deltoideus arises from the spine and the caudal border of the scapula and inserts upon the prominent (and palpable) deltoid tuberosity of the humerus. Its primary action is to flex the shoulder joint.

Structure 13Teres Major muscleThe teres major muscle along with the deltoideus and the teres minor are the primary flexors of the shoulder joint. The teres major originates from the caudal angle of the scapula and shares its insertion on the teres major tuberosity with the latissimus dorsi. It is innervated by the axillary nerve.

Structure 14Corabrachialis muscleAs its name implies this muscle originates from the coracoid process of the scapula and inserts on the proximal aspect of the shaft of the humerus. It lies medial to the shoulder joint. It can flex the shoulder joint but it is primarily an adductor of the arm. It is innervated by the musculocutaneous nerve.

Structure 15Brachialis muscleThe brachialis muscle occupies a conspicuous spiral shaped groove on the lateral aspect of the humerus. Its caudal border has a close topographical relationship with the radial nerve. It originates on the caudal aspect of the proximal humerus and inserts in a conspicuous depression on the medial aspect of the proximal radius beneath the medial collateral ligament. This muscle's only action is to flex the elbow. It is innervated by both the musculocutaneous and radial nerves.

Structure 16Cutaneous Colli MuscleThe cutaneous muscles of the horse, like those of most domestic mammals, are a thin muscular layer developed in the superficial fascia. They attach to the skin but have little attachment to the skeleton. In the horse they are divided into several parts. The cutaneous fasciei covers part of the face, the cutaneous colli the sides of the neck, the cutaneous omobrachialis covers the lateral surface of the shoulders, and the cutaneous trunci covers almost the entire lateral surface of the trunk. The cutaneous fasciei and the cutaneous colli together comprise the platysma. Cutaneous muscles allow mammals to twitch the skin in order to dislodge insects and other irritants. Cutaneous muscles are innervated by ventral branches of cervical spinal nerves but the cutaneous trunci muscle is innervated by the lateral thoracic nerve. The lateral thoracic nerve is clinically important because it is the efferent limb of the cutaneous trunci reflex which is used to localize spinal lesions.

Structure 17Subclavis muscleThe subclavius is a deep pectoral muscle that originates on the manubrium and insert upon the epimysium of the supraspinatus. Its distinctive crescent shape and position relative the supraspinatus make it easy to identify on transverse sections of the shoulder. The subclavius is not present in the dog. It is innervated by pectoral nerves from the brachial plexus.

Structure 18Supraspinatous muscleThe supraspinatus muscle originates from and occupies the supraspinous fossa of the scapula. Its epimysium provides an insertion site for the subclavius muscle. The split tendon of insertion attaches to the cranial parts of the greater and lesser tubercles of the humerus and its action is to stabilize and extend the shoulder joint. Inervation is provided by the suprascapular nerve. As this nerve crosses the cranial border of the scapula it is exposed to trauma. Damage to the suprascapular nerve results in a condition known as "Sweeny" and a resulting atrophy of the muscle.

Transverse Section of the Horse Humerus

Structure 1HumerusThe humerus is the primary skeletal component of the brachium. Its large rounded head articulates with the glenoid cavity of the scapula to form the shoulder joint. Together, these two bones approximate a right angle in the horse. At its distal end the humerus articulates with the radius and ulna to form the elbow joint. The greater tubercle (point of shoulder) and the deltoid tuberosity are conspicuous palpable features of this bone.

Structure 2Deltoid Tuberosity of HumerusThe deltoid tuberosity is a conspicuous palpable and radiological feature of the proximolateral aspect of the equine humerus. It provides an insertion site for the deltoideus muscle, a prominent shoulder flexor.

Structure 3Biceps Brachii MuscleThe biceps brachii muscle takes origin from the supraglenoid tubercle of the scapula, passes through the intertubercular groove of the humerus and has its primary insertion on the radial tuberosity. A large intertubercular bursa lies between its tendon as it passes through the groove. The strong fibrous internal tendon of this muscle prevents shoulder joint flexion and is an important part of the thoracic limb passive stay-apparatus. It is also an extensor of the shoulder and a primary flexor of the elbow. Innervation is provided by the musclocutaneous nerve.

Structure 4Internal tendon of the Biceps Brachii MuscleThe massive, fibrocartilagenous tendon of the biceps brachii muscle passes through the intertubercular groove as it crosses the cranial aspect of the shoulder joint. Its caudal surface closely conforms to the ridge-like intermediate tubercle. The tendon is relatively inelastic and therefore passively keeps the shoulder joint from collapsing under the weight of the standing horse. Shoulder lameness may result when the intertubercular bursa (which protects the tendon at the level of the shoulder joint) becomes inflamed.

Structure 5CleidobrachialisThe brachiocephalicus of the horse is composed of two parts. Part one is the most cranial and is called the cleidomastoideus. The cleidomastoideus runs from the mastoid region of the skull to the clavicular insertion (the horse is devoid of a clavicle but has a small tendinous band embedded in muscle at the point of the shoulder called the clavicular insertion). Part two is more caudal and called the cleidobrachialis. It runs from the clavicular insertion to the deltoid tuberosity and crest of the humerus. The action of the brachiocephalicus is to advance the limb, extend the shoulder, and draw the neck and head to the side. It is one of four muscles innervated by the accessory nerve, but receives some parasympathetic innervation from dorsal branches of cervical spinal nerves. The brachiocephalicus is an important landmark in the neck of the horse and forms the dorsal border of the jugular groove.

Structure 6Cephalic VeinThe cephalic vein originates from the external jugular vein and is very conspicuous on the living horse. It lies in the groove formed by the brachiocephalicus and the pectoralis descendens muscles. Because it is so superficial it is sometimes lacerated on fences or other sharp objects and may bleed profusely.

Structure 7Pectoralis Descendens MuscleThe pectoralis descendens muscle, along with the pectoralis transversus muscle form the superficial pectoral muscles of the horse. The pectoralis descendens originates from the manubrium and inserts on the crest of the humerus and the fascia of the arm. The action of this muscle adducts and advances the limb. It is innervated by pectoral nerves from the brachial plexus. On the living horse this muscle is very conspicuous and is located just ventral to the cephalic vein.

Structure 8Subclavius MuscleThe subclavius is a deep pectoral muscle that originates on the manubrium and inserts upon the epimysium of the supraspinatus. Its distinctive crescent shape and position relative to the supraspinatus make it easy to identify on shoulder joint transverse sections. The subclavius is innervated by nerves from the brachial plexus. This muscle is not found in the dog.

Structure 9Coracobrachialis MuscleAs its name implies this muscle originates from the coracoid process of the scapula and inserts on the proximal aspect of the shaft of the humerus. It lies medial to the shoulder joint. It can flex the shoulder joint but it is primarily an adductor of the arm. It is innervated by the musculocutaneous nerve

Structure 10Medial Head of the Triceps Brachii MuscleAll members of this muscle group insert on the olecranon tuber. As its name implies this head of the triceps brachii muscle has the most medial origin. It is the smallest of the tricipital heads and originates from a distinctive rough line on the caudal surface of the humerus near the teres major tuberosity. All parts of the triceps brachii muscle insert upon the olecranon tuber, are innervated by the radial nerve, and are the most important elbow extensors. Without elbow extension the forelimb would collapse under the weight of the standing horse. A synovial bursa is commonly found between the tendon of insertion of the triceps brachii and the expanded part of the olecranon tuber. Inflammation of this bursa may result in a condition known as capped elbow

Structure 11Brachialis MuscleThe brachialis muscle occupies a conspicuous spiral shaped groove on the lateral aspect of the humerus. Its caudal border has a close topographical relationship with the radial nerve. It originates on the caudal aspect of the proximal humerus and inserts in a conspicuous depression on the medial aspect of the proximal radius beneath the medial collateral ligament. This muscle's only action is to flex the elbow. It is innervated by both the musculocutaneous and radial nerves.

Structure 12Radial NerveThe radial nerve starts at the brachial plexus and is formed by branches from the last two cervical and first thoracic spinal nerves. The radial nerve provides innervation to all the forearm extensors and to the skin on the lateral surface of the brachium. Damage to the radial nerve can occur as a result of traumatic injuries to the arm which are common. Radial nerve damage may prohibit the limb from bearing weight.

Structure 13Brachial ArteryThe brachial artery is the continuation of the axillary artery into the arm. It is the main arterial axis of the brachium and its branches supply the major muscles in this region. The brachial artery lies mostly on the medial aspect of the humerus. Near the proximomedial aspect of the antibrachium it gives off the common interosseus artery which passes through a space between the ulna and radius. After giving off the common interosseus artery the brachial artery becomes the median artery, the main arterial axis of the antibrachium.

Structure 14Lateral Head of the Triceps Brachii MuscleAll members of this muscle group insert upon the olecranon tuber. The lateral head of the triceps brachii muscle originates from the deltoid tuberosity and a distinctive roughened curved line on the proximal end of the humerus. All parts of the triceps brachii muscle insert upon the olecranon tuber, are innervated by the radial nerve, and are the most important elbow extensors. Without elbow extension the forelimb would collapse under the weight of the standing horse. A synovial bursa is commonly found between the tendon of insertion of the triceps brachii and the expanded part of the olecranon tuber. Inflammation of this bursa may result in a condition known as capped elbow.

Structure 15Deltoideus MuscleThe deltoideus muscle of the horse has muscular and aponeurotic parts and is sometimes difficult to separate from the infraspinatus muscle during dissections. Unlike the dog and domestic ruminants, the deltoideus of the horse does not have two distinct divisions. The deltoideus arises from the spine and the caudal border of the scapula and inserts upon the prominent (and palpable) deltoid tuberosity of the humerus. Its primary action is to flex the shoulder joint.

Structure 16Cutaneous Omobrachialis MuscleThe cutaneous muscles of the horse, like those of most domestic mammals, are a thin muscular layer developed in the superficial fascia. They attach to the skin but have little attachment to the skeleton. In the horse they are divided into several parts. The cutaneous fasciei covers part of the face, the cutaneous colli the sides of the neck, the cutaneous omobrachialis covers the lateral surface of the shoulders, and the cutaneous trunci covers almost the entire lateral surface of the trunk. The cutaneous fasciei and the cutaneous colli together comprise the platysma. Cutaneous muscles allow mammals to twitch the skin in order to dislodge insects and other irritants. Cutaneous muscles are innervated by ventral branches of cervical spinal nerves but the cutaneous trunci muscle is innervated by the lateral thoracic nerve. The lateral thoracic nerve is clinically important because it is the efferent limb of the cutaneous trunci reflex which is used to localize spinal lesions.

Structure 17Long Head of the Triceps MuscleAll members of this muscle group insert upon the olecranon tuber. The long head of the triceps brachii muscle is the largest and differs from the other tricipital heads in that it originates from the caudal border of the scapula rather than the humerus. Its origin from the scapula gives it the ability to flex the shoulder, although its main action is elbow extension. All parts of the triceps brachii muscle insert upon the olecranon tuber, are innervated by the radial nerve, and are the most important elbow extensors. Without elbow extension the forelimb would collapse under the weight of the standing horse. A synovial bursa is commonly found between the tendon of insertion of the triceps brachii and the expanded part of the olecranon tuber. Inflammation of this bursa may result in a condition known as capped elbow.

Structure 18Tensor Fascia Antebrachii MuscleThe tensor fascia antibrachii muscle derives its name from its action of stretching (tensing) the fascia of the forearm. It can also extend the elbow joint. The muscle is very thin and flat and covers the medial aspect of the long head of the triceps. It originates from the caudal border of the scapula and also from the tendon of insertion of the latissimus dorsi. It inserts on the deep fascia of the forearm and also on the olecranon, which gives it its ability to extend the elbow. It is innervated by the radial nerve.

Structure 19Pectoralis Transversus MuscleThe pectoralis transversus along with the pectoralis descendens muscle form the superficial pectorals of the horse. The pectoralis transversus originates on the more cranial sternebrae and insert on the crest of the humerus and the fascia of the forearm. This muscle adducts the limb and is innervated by pectoral nerves from the brachial plexus.

Transverse Section of the Horse Elbow

Structure 1Brachialis MuscleThe brachialis muscle occupies a conspicuous spiral shaped groove on the lateral aspect of the humerus. Its caudal border has a close topographical relationship with the radial nerve. It originates on the caudal aspect of the proximal humerus and inserts in a conspicuous depression on the medial aspect of the proximal radius beneath the medial collateral ligament. This muscle's only action is to flex the elbow. It is innervated by both the musculocutaneous and radial nerves.

Structure 2Biceps Brachii MuscleThe biceps brachii muscle takes origin from the supraglenoid tubercle of the scapula, passes through the intertubercular groove of the humerus and has its primary insertion on the radial tuberosity. A large intertuburcular bursa lies between its tendon as it passes through the groove. The strong fibrous internal tendon of this muscle prevents shoulder joint flexion and is an important part of the thoracic limb passive stay-apparatus. It is also an extensor of the shoulder and a primary flexor of the elbow. Innervation is provided by the musclocutaneous nerve.

Structure 3HumerusThe humerus is the primary skeletal component of the brachium or arm. Its large rounded head articulates with the glenoid cavity of the scapula to form the shoulder joint. Together, these two bones approximate a right angle. At its distal end the humerus articulates with the radius and ulna to form the elbow joint. The greater tubercle (point of shoulder) and the deltoid tuberosity are conspicuous palpable features of this bone.

Structure 4Lacertus FibrosusThe lacertus fibrosus is a tough relatively inelastic band of tissue that connects the internal tendon of the biceps brachii with the extensor carpi radialis. It is stretched tightly in the standing horse and can be palpated where it crosses the cranial aspect of the elbow joint. The lacertus fibrosus plays an important role in the thoracic limb passive stay-apparatus by helping maintain the carpal joint in extension

Structure 5Extensor Carpi Radialis MuscleThe extensor carpi radialis muscle is a conspicuous feature of the craniolateral aspect of the forearm in the living horse. It has two heads; one originates from the lateral epicondyloid crest of the humerus and the other from a roughened area proximal to the humeral capitulum. It inserts on the metacarpal tuberosity of McIII along with the lacertus fibrosus. The extensor carpi radialis is an important part of the thoracic limb passive stay-apparatus because it passively prevents flexion of the carpal joint when weight is on the limb. It is innervated by the radial nerve.

Structure 6Common Digital Extensor MuscleThe bulk of the mass of the common digital extensor muscle is provided by its large humeral head which originates from the lateral epicondyloid crest of the humerus and a roughened area proximal the humeral capitulum. The relatively insignificant radial head is separate from the humeral head and originates from the lateral tuberosity of that bone. The main tendon of this muscle is conspicuous on the dorsal surface of the cannon bone and just proximal to its main insertion on the on the extensor process of the third phalanx it is joined by branches of the suspensory ligament. The common digital extensor extends the carpus, the joints of the digits, and can flex the elbow. Innervation, like all forearm extensors is provided by the radial nerve.

Structure 7Medial Epicondyle of the HumerusThe medial epicondyle of the humerus is a bony protrubance at this bone's distal end that provides attachment sites for the flexors of the forearm and the medial collateral ligament of the elbow joint. The name epicondyle literally means "above the joint". Epicondyles are prominent usually palpable features on the distal extremities of the humerus and the femur and are important for ligiment and muscle attachment.

Structure 8OlecranonThe olecranon is part of the ulna, which in the horse is greatly reduced in size and typically fused to the caudal aspect of the radius. The olecranon tuber is a conspicuous palpable feature of the horse and is known as the point of the elbow. It provides an attachment site for the triceps brachii muscles which are the main elbow joint extensors. Trauma induced to the point of the elbow (over the olecranon) results in Hygroma of the elbow, also know as capped elbow or shoe boil.

Structure 9Lateral Collateral Ligament of the Elbow JointCollateral ligaments are tough, thick, ligamentous structures which help stabilize a joint and restrict its movement, usually in the saggital plane for limb joints. The lateral collateral ligament of the elbow joint connects the lateral epicondyle of the humerus with the lateral tuberosity of the radius. Many collateral ligaments are palpable and are important landmarks for locating joint capsules for injection.

Structure 10Medial Collateral Ligament of the Elbow JointCollateral ligaments are tough, thick, ligamentous structures which help stabilize a joint and restrict its movement, usually in the saggital plane for limb joints. The medial collateral ligament of the elbow joint has long and short parts. The long part ends on the medial border of the radius near the interosseus space. The short part ends the medial tuberosity of the radius. Many collateral ligaments are palpable and are important landmarks for locating joint capsules for injection.

Structure 11Origin of Flexor Muscles on the Medial EpicondyleThe medial epicondyle of the humerus is the site of origin for three important muscles of the forelimb. These are: flexor carpi radialis, superficial digital flexor and the deep digital flexor.

Structure 12Tensor Fascia Antebrachii MuscleThe tensor fascia antibrachii muscle derives its name from its action of stretching (tensing) the fascia of the forearm. It can also extend the elbow joint. The muscle is very thin and flat and covers the medial aspect of the long head of the triceps. It originates from the caudal border of the scapula and also from the tendon of insertion of the latissimus dorsi. It inserts on the deep fascia of the forearm and also on the olecranon, which gives it its ability to extend the elbow. It is innervated by the radial nerve.

Structure 13Ulnaris Lateralis MuscleThe ulnaris lateralis muscle originates from the lateral epicondyle of the humerus. A short tendon of insertion attaches to the accessory carpal bone and a longer tendon of insertion crosses the lateral aspect of the accessory carpal bone to attach on the lateral splint (fourth metacarpal) bone. Although it is included in the extensor group of forearm muscles and is innervated by the radial nerve which is typically associated with the forearm extensors, it is actually a flexor of the carpal joint.

Structure 14Ulnar Head of the Deep Digital Flexor MuscleThe deep digital flexor muscle of the equine thoracic limb is the largest of the forelimb flexors. It has three heads originating from the humerus, radius, and the ulna. Of these, the humeral head is the largest. The deep and superficial digital flexor muscles of the forelimb share a common synovial sheath as they pass through the carpal canal. The deep digital flexor inserts on the distinctive semilunar crest of the third phalanx and is innervated by both median and ulnar nerves. The deep digital flexor can flex the digit and the carpus and extend the elbow but it also plays an important role in the forelimb passive stay-apparatus where it acts to keep the fetlock joint from overextension when the animal is passively standing.

Structure 15Medial head of the Triceps MuscleAll members of this muscle group insert on the olecranon tuber. As its name implies this head of the triceps brachii muscle has the most medial origin. It is the smallest of the tricipital heads and originates from a distinctive rough line on the caudal surface of the humerus near the teres major tuberosity. All parts of the triceps brachii muscle insert upon the olecranon tuber, are innervated by the radial nerve, and are the most important elbow extensors. Without elbow extension the forelimb would collapse under the weight of the standing horse. A synovial bursa is commonly found between the tendon of insertion of the triceps brachii and the expanded part of the olecranon tuber. Inflammation of this bursa may result in a condition known as capped elbow.

Transverse Section of the Horse Forearm

Structure 1Extensor Carpi Radialis muscleThe extensor carpi radialis muscle is a conspicuous feature of the craniolateral aspect of the forearm in the living horse. It has two heads; one originates from the lateral epicondyloid crest of the humerus and the other from a roughened area proximal to the humeral capitulum. It inserts on the metacarpal tuberosity of McIII along with the lacertus fibrosus. The extensor carpi radialis is an important part of the thoracic limb passive stay-apparatus because it passively prevents flexion of the carpal joint when weight is on the limb. It is innervated by the radial nerve.

Structure 2Common Digital Extensor muscleThe bulk of the mass of the common digital extensor muscle is provided by its large humeral head which originates from the lateral epicondyloid crest of the humerus and a roughened area proximal the humeral capitulum. The relatively insignificant radial head is separate from the humeral head and originates from the lateral tuberosity of that bone. The main tendon of this muscle is conspicuous on the dorsal surface of the cannon bone; and just proximal to its main insertion on the on the extensor process of the third phalanx, it is joined by branches of the suspensory ligament. The common digital extensor extends the carpus, the joints of the digits, and can flex the elbow. Innervation, like all forearm extensors is provided by the radial nerve.

Structure 3Lateral Digital Extensor muscleThe lateral digital extensor originates on the lateral tuberosity of the radius, the lateral border of the radius and the lateral aspect of the body of the ulna. It is easy to identify on transverse sections because It is the smallest and most lateral of the craniolateral group of forearm muscles. Its tendon of insertion passes through a conspicuous groove on the lateral styloid process of the radius and then beneath the lateral collateral ligament of the carpus where it is protected by a synovial sheath. Below the carpus it is joined by a small slip from the common digital extensor. The lateral digital extensor inserts on an eminence on the dorsal aspect of the proximal end of the proximal phalanges. Its action extends the digit and the carpus. Like all forearm extensors it is innervated by the radial nerve.

Structure 4Ulnaris Lateralis muscleThe ulnaris lateralis muscle originates from the lateral epicondyle of the humerus. A short tendon of insertion attaches to the accessory carpal bone and a longer tendon of insertion crosses the lateral aspect of the accessory carpal bone to attach on the lateral splint (fourth metacarpal) bone. Although it is included in the extensor group of forearm muscles and is innervated by the radial nerve which is typically associated with the forearm extensors, it is actually a flexor of the carpal joint.

Structure 5Deep Digital Flexor muscleThe deep digital flexor muscle of the equine thoracic limb is the largest of the forelimb flexors. It has three heads originating from the humerus, radius, and the ulna. Of these, the humeral head is the largest. The deep and superficial digital flexor muscles of the forelimb share a common synovial sheath as they pass through the carpal canal. The deep digital flexor inserts on the distinctive semilunar crest of the third phalanx and is innervated by both median and ulnar nerves. The deep digital flexor can flex the digit and the carpus and extend the elbow but it also plays an important role in the forelimb passive stay-apparatus where it acts to keep the fetlock joint from overextension when the animal is passively standing.

Structure 6Flexor Carpi Ulnaris muscleThe flexor carpi ulnaris originates from the medial epicondyle of the humerus and inserts on the accessory carpal bone. It is easy to identify on transverse section because of its distinctive crescent shape. This muscle and the ulnaris lateralis are the only two muscles to insert on the accessory carpal bone of the horse. As its name implies the primary action of the flexor carpi ulnaris is to flex the carpus but it also has the ability to extend the elbow. Innervation is provided by the ulnar nerve.

Structure 7Superficial Digital Flexor muscleThe forelimb superficial digital flexor originates on the medial epicondyle of the humerus. It shares a common synovial sheath with the deep digital flexor as it passes through the carpal canal. As in the dog, its tendon forms a tube at the metacarpophalangeal joint through which the tendon of the deep digital flexor passes. The superficial digital flexor of both thoracic and pelvic limbs of the horse bifurcates to the lateral and medial aspects of the distal end of PI and the lateral and medial aspects of the proximal end of PII. This insertion is unique to equids. The superficial digital flexor muscle of both fore and hind limbs plays a role in the passive stay-apparatus by preventing buckling (flexion) of the pastern joint when weight is on the limb. The superficial digital flexor of the forelimb is innervated by the ulnar nerve. In the forelimb it has a distinctive marbled appearance in transverse sections.

Structure 8Flexor Carpi Radialis muscleThe flexor carpi radialis is the antagonist of the extensor carpi radialis as is evident from its position along the opposite side of the radius. Transecting this muscle at its belly and reflecting the stumps will expose the median artery and nerve. The flexor carpi radialis is protected by a synovial sheath as it passes through the carpal canal. It inserts on the medial splint bone and is innervated by the median nerve. Its action is to flex the carpus and extend the elbow.

Structure 9Ulnar nerveThe ulnar nerve together with the median arises from the brachial plexus and enters the forearm between the axillary artery and vein where it lies in close association with the radial nerve. Its large stump is conspicuous on the medial side of the shoulder during the initial dissection of the forelimb. In transverse sections of the forearm it is easily identified due to its large size and its caudal and very superficial position. The ulnar nerve provides innervation for some important forearm flexors, nemely, the flexor carpi unlaris, and the superficial and deep digital flexors. It also provides cutaneous innervation to the caudolateral and caudomedial aspects of the forelimb.

Structure 10Median nerveThe median nerve is derived from both cervical and thoracic spinal nerve branches from the brachial plexus. It enters the arm medial to the large axillary artery. Below the axillary artery the median nerve joins the musculocutaneous nerve and the two nerves pass distally together for a short distance and are closely adhered. The median nerve has many branches that provide innervation to some important flexors of the forearm and cutaneous innervation to the skin of the distal limb. However, from a clinical standpoint, its most important branch is its continuation into the metacarpus as the medial palmer nerve which is commonly blocked for lameness diagnosis.

Structure 11Median arteryThe median artery is the main arterial axis of the antibrachium and is the continuation of the brachial artery beyond its common interosseus branch. The median artery branches just above the carpus but the largest branch, now called the medial palmar artery passes through the carpal canal along with the digital flexor tendons. Just above the fetlock joint the medial palmar artery branches into the medial and lateral digital arteries which are the main blood supply to the foot.

Structure 12Shaft of the RadiusThe antibrachium or arm of the horse is composed of two components; the radius and the ulna. Of these two the radius is the largest. In the normal standing horse the radius sits almost verticle and is gently curved. It articulates with the humerus at its proximal end and with the carpal bones at its distal end.

Transverse Section of the Horse Proximal Carpus

Structure 1Radial carpal boneThe radial carpal bone is very clinically significant because it is subject to traumatic injuries in athletic horses. Chip fractures are the most commonly occuring type of injury for the radial carpal bone, especially in racehorses. Most of these chip fractures occur on the proximal and distal margins of the dorsal aspect of the bone. The margins of the radial and intermediate carpal bones are not separable on lateromedial radiographic views but the flexed lateromedial view separates the margins of these two bones and makes chip fractures easier to see

DE LA 2 A A LA 16 NO HAY EXPLICACIN.Transverse Section of the Horse Metacarpus

EN ESTA NO HAY EXPLICAICNES

Transverse Section of the Horse Fetlock

ACA NO HAY EXPLICACIONES PARA NINGUN ITEMTransverse Section of the Horse MidThigh

NO TIENE EXPLICACIONESTransverse Section of the Patella

NO TIENE EXPLICACIONESTransverse Section of the Horse StifleNO TIENE EXPLICACIONESTransverse Section of the Horse Middle Tibia

Transverse Section of the Horse Hock

Transverse Section of the Horse Hock