The flexor carpiradialis (flexor metacarpi internus) originates from the medial epicondyle of the humerus.  It is inserted to the proximal end of the medial metacarpal (inner metacarpal or splint) bone.  This muscle is the smaller of the three and is not usually divided in doing carpal tenotomy.

The flexor carpiulnaris (flexor metacarpi medius) has two heads of origin; one, the larger, originates from the epicondyle of the humerus and the other from the posterior surface of the olecranon.  The two heads unite at the upper third of the radius and the muscle, becoming tendinous, as is the case with the other carpal flexors, is attached by one point of insertion to the accessory carpal bone (trapezum).  The other blends with the posterior annular ligament of the carpus.

The ulnaris lateralis (flexor metacarpi externus) has its origin from the lateral epicondyle of the humerus and inserts to the proximal extremity of the fourth metacarpal (outer splint) bone and by another attachment to the accessory carpal bone (trapezium) with the tendon of the flexor carpiulnaris (flexor metacarpi medius).

Acting together, these muscles flex the carpus or extend the elbow and this action is antagonized by the biceps brachii (flexor brachii) and extensors of the carpus and phalanges.

Etiology and Occurrence.—­Inflammation of the muscular or tendinous parts of the carpal flexors, does not occur as frequently as does inflammation of the flexors of the extremity.  They are subject to injury such as is occasioned by hard work and concussion and contract as a result; but, more frequently a congenital malformation of the leg is responsible for undue strain upon these parts.  Horses that are “knee sprung” or that have a congenital condition where in the anterior line, as formed by the radius, carpal and metacarpal bones, is bent forward at the carpus, are subject to inflammation and contraction of the carpal flexors.  When these flexors are contracted, the condition is commonly known among horsemen as “buck knee.”  In itself, inflammation of the carpal flexors is not a condition which is likely to prove troublesome, but because of carpal involvement (which is often present) the cause of the trouble remains, and inflammation of the carpal flexors recurs or becomes chronic and contraction of tendons results.

Symptomatology.—­Inflammation of the carpal flexors, when acute and uncomplicated, is characterized by a painfully swollen condition of the affected tendons.  No weight is borne upon the affected leg and the carpal joint is flexed.  Mixed lameness is present.  There is no difficulty encountered in arriving at a diagnosis because of the very noticeably inflamed parts.

Many fully developed cases of contraction of the tendons of the carpal flexors are observed where the condition has become established gradually and no lameness has resulted from tendinitis or carpitis.  In some of these cases, subjects are stumblers and when they are carelessly handled or kept at fast work over irregular or hard roads, chronic carpitis with hyperplasia of the structures of the anterior carpal region results, owing to frequent bruising from falls.