34 THE ANATOMY OP THE HOBSE. 



a synovial membrane, which lines it, and is reflected over the flexor 

 tendons to facilitate their gliding. If the fibrous band just described be 

 cut, and a probe be passed upwards and downwards within the sheath, 

 an idea of the extent of the synovial sac will be gained. It will be 

 found to extend upwards for two or three inches above the carpus, 

 and downwards as far as the middle of the metacarpus. 



Directions. — The fibrous band should be entirely removed in order to 

 permit the examination of the tendons, and of the nerves and bloodvessels 

 which accompany these within the sheath. 



The Mbtacaepo-phalangeal or Great Sesamoid Sheath (Fig. 1). This 

 is a second synovial apparatus developed in connection with the flexor 

 tendons. If a vertical incision be made through the superficial flexor just 

 above the fetlock, and a probe passed into the incision, it will enter the 

 synovial cavity, and may be pushed upwards for two or three inches 

 above the fetlock, and downwards as far as the middle of the second 

 phalanx. The synovial membrane lubricates the pulley-like surface 

 formed by the sesamoid bones and the inter-sesamoid ligament, and is 

 reflected on to the tendons. It is supported laterally by a fibrous 

 expansion which, adhering to the superficial flexor behind, is inserted 

 in front by three slijDS on each side, the highest insertion being into the 

 sesamoid, and the other two into the first phalanx. At its lower extremity 

 this synovial membrane meets that of the navicular sheath, and in front 

 of the same point it is separated from the synovial capsule of the coflan- 

 joint by a kind of partition of yellow fibrous tissue connecting the front 

 of the perforans tendon to the back of the os coronte (Plate 10, fig. 2). 



The Superficial, Flexor tendon (Plates 5, 9, 10, and 11). The 

 tendon succeeds the fleshy portion of the muscle at the lower part of the 

 fore-arm, and it is there reinforced by a fibrous band which springs from 

 the back of the radius and is sometimes termed the superior carpal ligament, 

 in contradistinction to the band which reinforces the tendon of the deep 

 flexor below the carpus. The tendon passes through the carpal sheath 

 in company with and behind the deep flexor, and then descends behind 

 the metacarpus. Having arrived at the fetlock, there is formed in it 

 a remarkable ring, through which the tendon of the deep flexor plays. 

 It is in consequence of this arrangement that the superficial muscle is 

 termed perforatus, and the deep one perforaiis. As already seen, the 

 tendons are here enveloped by the synovial membrane of the meta- 

 carpo-phalangeal sheath. At its extremity the tendon is bifid and 

 each slip is inserted into the upper extremity of the second phalanx on 

 its lateral aspect. 



Action. — The muscle flexes successively the pastern, fetlock, and 

 carpal joints. 



The Deep Flexor tendon (Plates 5, 9, 10, and 11) is, through- 

 out its course, closely related to the preceding, in front of which it lies. 



