Carpal and Digital Muscles - Horse Anatomy

Introduction

The horse, as well as other domestic mammals, have a limited range of motion in the carpal joints compared with humans; therefore these muscles act as either extensors or flexors.

Extensors

The extensors are located at the craniolateral position on the forearm, almost all originate from the lateral epicondyle of the humerus. All of these muscles are innervated by the radial n. from the brachial plexus.

Extensor carpi radialis

The most medial

Insertion: Metacarpal tuberosity of the proximal third metacarpal (cannon) bone. Uniquely in the horse, the lacertus fibrosus (detachment of the biceps brachii) joins this muscle prior to its insertion.

Action: Extends and fixes the carpus, flexes the elbow

Extensor carpi obliquus (aka abductor pollicis longus)

Origin: Cranial radius

Insertion: Head of second metacarpal (medial splint) bone

Common Digital Extensor

Deep to the extensor carpi radialis

Insertion: Extensor process of the distal phalanx via the common digital extensor tendon.

Action: Extends carpus and digit

Lateral Digital Extensor

Caudal to the common digital extensor muscle on the lateral antebrachium

Insertion: Dorsal proximal phalanx via the lateral digital extensor tendon.

Action: Extends the carpus and metacarpophalangeal (fetlock) joint

Flexors

These muscles are at the caudal position on the forearm. They originate from the caudal medial epicondyle of the humerus and are innervated by the median or ulnar n. of the brachial plexus.

Flexor carpi radialis

The most medial, inserts on the proximal part of the second metacarpal (medial splint)bone

Flexor carpi ulnaris

The most lateral, inserts on the accessory carpal bone.

Ulnaris lateralis

The most caudolateral

Insertion: accessory carpal bone and the fourth metacarpal (lateral splint) bone.

Action: Flexes the carpus rather than extends it, due to its insertion caudal to the carpal joint axis.

Superficial Digital Flexor

Overlies, and is partially fused with, the deep digital flexor muscle.

Insertion: Proximal extremity of middle phalanx and the distal extremity of proximal phalanx via the superficial digital flexor tendon.

Deep Digital Flexor

Deep to the superficial digital flexor muscle and flexor muscles of the carpus. Passes through the carpal canal before branching and continues to insert on the palmar distal phalanx via the deep digital flexor tendon.

Interosseus muscle/Suspensory ligament

This is covered in more detail in the Equine Tendons section.

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