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. collaterale 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 ligament. It was first described by Skoda who terms it the lig. metacarpo-inter- 
sesamoideum. 
