224 THE ARTICULATIONS OF THE HORSE 



THE FETLOCK JOINT 



This, the metacarpo-phalangeal articulation (Articulatio metacarpo-phalangea), 

 is a ginglymus formed by the junction of the distal end of the large (third) meta- 

 carpal bone, the proximal end of the first phalanx, and the proximal sesamoid 

 bones. 



Articular Surfaces. — The surface on the large metacarpal bone is approxi- 

 mately cylindrical in curvature, but is divided into two slightly unequal parts by a 

 sagittal ridge. This is received into a socket formed by the first phalanx below and 

 the two sesamoids together with the intersesamoid ligament behind. The latter 

 is a mass of fibro-cartilage in which the sesamoid bones are largely embedded. It 

 extends above the level of the sesamoids, and is grooved to receive the ridge on the 

 metacarpal bone; its volar surface forms a smooth groove for the deep flexor ten- 

 don. 



The joint capsule is attached around the margin of the articular surfaces. It 

 is thick and ample in front; here a bursa is interposed between it and the extensor 

 tendons, but the tendons are also attached to the capsule. Posteriorly it forms a 

 thin-walled pouch which extends upward between the metacarpal bone and the 

 suspensory ligament about as high as the point of bifurcation of the latter.^ The 

 capsule is reinforced by two collateral ligaments. 



The collateral ligaments, medial and lateral (Lig. coUaterale ulnare, radiale), 

 are partially divided into two layers; the superficial layer arises from the eminence 

 on the side of the distal end of the large metacarpal bone, and passes straight to 

 the rough area below the margin of the articular surface of the first phalanx; the 

 deep layer, shorter and much stronger, arises in the depression on the side of the 

 distal end of the metacarpal bone, and passes obliquely downward and backward 

 to be inserted into the abaxial surface of the sesamoid and the proximal end of the 

 first phalanx. 



The capsule is further strengthened by a layer of oblique fibers which pass over the collateral 

 ligament on either side and end on the extensor tendon and the proximal extremity of the first 

 phalanx. It may properly be regarded as fascia rather than ligament. 



Movements. — These are of the nature of flexion and extension, the axis of 

 motion passing through the proximal attachments of the collateral ligaments. In 

 the ordinary standing position the joint is in a state of partial dorsal flexion, the 

 articular angle (in front) being about 140° to 150°. (In the hind limb it is about 

 5° greater.) Diminution of this angle (sometimes termed "overextension") is nor- 

 mally very limited on account of the resistance offered by the sesamoidean appa- 

 ratus, but it varies considerably in amount in different subjects. Volar flexion is 

 limited only by contact of the heels with the metacarpus. During volar flexion 

 a small amount of transverse movement is possible. 



THE SESAMOIDEAN LIGAMENTS 



Under this head will be described a number of important ligaments which are 

 connected with the sesamoid bones and form a sort of stay apparatus or brace. 



The intersesamoidean ligament (Lig. intersesamoideum) not only fills the 

 space between and unites the sesamoid bones, but also extends above them, enter- 

 ing into the formation of the articular surface of the fetlock joint. Other facts 

 in regard to it have been given above. 



The collateral sesamoidean ligaments, lateral and medial (Ligg. sesamoidea 



' This pouch is in part bound down by a layer of elastic tissue which arises by two branches 

 from the distal part of the volar surface of the shaft of the large metacarpal bone and ends on the 

 intersesamoid hgament. It was first described by Skoda who terms it the lig. metacarpo-inter- 

 sesamoideum. 



