1. The superficial flexor or perforatus tendon ([Figs. 15, b], and [16, a, b]) lies behind, immediately under the skin, and covers the deep flexor or perforans tendon. At the gliding surface between the sesamoid bones ([Fig. 15, f]) it broadens, and forms a ring or tube ([Fig. 15, b′]) through which the perforans tendon (a‴) passes, while a short distance farther down it bifurcates, or divides into two branches ([Figs. 15, b″], and [16, b]), which terminate, one on either side, partly on the inferior lateral borders of the first phalanx and partly on the fibro-cartilage of the second phalanx. It acts simultaneously on the long and short pasterns.
2. The deep flexor or perforans tendon ([Figs. 15, a], and [16, c]) is cylindrical and stronger than the perforatus tendon; above the fetlock-joint it lies between the perforatus and the suspensory ligament of the fetlock. At the sesamoid bones it passes through the ring formed by the perforatus tendon ([Fig. 15, b′]), then becomes broad and double-edged, passes between the two terminal branches of the perforatus, glides over the fibro-cartilage of the second phalanx and over the inferior surface of the navicular bone, and finally ends on the semilunar crest of the third phalanx. In common with the perforatus tendon it flexes the foot.
Fig. 16.
Right forefoot seen from behind and a little from the external side: a, perforatus tendon; b, terminal branches of the same; c, perforans tendon; d, annular ligament which attaches to the sesamoid bones: d′, the “x” ligament, which attaches by four branches to the os suffraginis; d″, an upper branch of the same (the lower branches are not shown in the figure); e, reinforcing sheath of the perforans tendon, covering the under surface of the latter and attached by its branches at e′ to the lower end of the os suffraginis; f, suspensory ligament of the fetlock.
If at a point a few inches above the fetlock a limb be cut through from behind, the knife will pass successively through the following structures: skin, perforatus tendon, perforans tendon, suspensory ligament, cannon bone, lateral extensor tendon, anterior extensor tendon, and, lastly, the skin on the anterior surface of the limb. The flexor tendons are frequently thickened and shortened by inflammation due to injury, and as a result the foot is pulled backward and the hoof gradually becomes more nearly upright,—i.e., stubby, steep-toed. A knowledge of the normal condition of the tendons is, therefore, absolutely necessary to the horseshoer. Both flexor tendons are embraced and held in place by ligaments and fascia passing out from the phalanges ([Figs. 16, d′], and [24, e, f]). The extensor and flexor tendons essentially contribute to the strong union of the phalangeal bones, and especially to the support and stability of the fetlock-joint. The gliding of the tendons is made easy by the secretion of a lubricating fluid, called synovia, from the inner surface of the sheaths which surround them. In thin-skinned well-bred horses with sound limbs one can not only distinctly feel the tendons through the skin, but can see their outline. When the tendons and bones are free from all inflammatory thickenings, and the tendon sheaths are not visibly distended, we say that the leg is “clean.”
Mucous Bursæ and Tendon Sheaths.
Accessory to the tendons, there are in the foot roundish, membranous sacs (mucous bursæ) and membranous tubes (tendon sheaths). Both contain a liquid resembling synovia (“joint-water”), which facilitates the gliding of the tendons. These bursæ and sheaths are often distended to form soft tumors, known as hygromata (“wind-puffs,” “wind-galls”).
(a) Mucous Bursæ.—They lie beneath tendons at those places where the tendons pass over bony prominences.
1. The mucous bursa of the anterior extensor tendon of the toe is about the size of a walnut, and lies between the tendon and the capsular ligament of the fetlock-joint ([Figs. 17, g], and [18, e]).