The bones which enter into the construction of the fetlock-joint are four in number, the large metacarpal or canon-bone, the os suffraginis or long pastern, and, behind these, two small sesamoid bones.
The lower extremity of the canon-bone rests upon the superior extremity of the large pastern, the convexities of the one fitting into corresponding concavities presented by the other. The two sesamoid bones are closely united together by a thick, short, strong ligament [intersesamoid ligament), and articulate with the back part of the lower extremity of the canon-bone. The importance of this joint as a spring and a means of breaking and dispersing jar or concussion has rendered necessary a number of ligaments over and above those ordinarily present in a diarthrodial or free-moving joint.
Fig. 356. - Ligaments of the Fetlock-Joint.
A, A, Suspensory ligament. B, B, Outer and inner branches of same.
C, C, Outer and inner sesamoid bones.
D, Superficial or long sesamoid ligament. E, E, Deep or short sesamoid ligament. F, F, Lateral phalangial ligament. G, Crucial sesamoid ligament. H, Intersesamoid ligament. I, I, Posterior inter-phalangial ligaments.
In addition to a capsular ligament, common to all the bones, there are also two lateral ligaments uniting the canon with the large pastern, and two lateral sesamoid ligaments, each having two branches extending from the bones of that name forward, one to the lower and outer part of the large metacarpal bone, and the other to the upper and outer part of the long pastern bone (fig. 357).
As the sesamoid bones have no support from below, it is necessary they should have it from above, to prevent their undue descent. This is provided by the suspensory ligament (fig. 356), the lower extremity of which, after dividing into two thick strands, is inserted into these bones. A similar provision is made to prevent too great upward displacement. For this purpose three ligaments (inferior sesamoid), distinguished respectively as the long, short, and crucial, connect the sesamoid bones with the posterior border of the os coronae, and with the posterior surface of the long pastern.
The movements of this joint are hinge-like and of great range both in flexion and extension.