THE CAUDAL (HAMSTRING) MUSCLES
In the horse, the three muscles of this group possess well-developed vertebral heads of origin (in addition to the usual pelvic heads); it is these vertebral heads that account for the characteristic filling and rounding of the croup (Figure 24-2/5,6).
The vertebral head of the biceps arises from the sacrum and adjacent part of the sacrosciatic ligament. It descends behind and partly covers the gluteal muscles before it crosses the ischial tuber to be joined by the smaller pelvic head that arises from that process. The muscle inserts by three divisions (Figure 24-2/5'): the first in the fascia lata and on the patella, the second on the lateral patellar ligament and tibial crest, and the third, the tarsal tendon, on the common calcanean tendon. The vertebral head is supplied by the caudal gluteal nerve, and the pelvic head is supplied by the sciatic nerve.The vertebral head of the semitendinosus (Figure 24-2/6) has an origin adjoining that of the biceps. After this merges with the pelvic head, the combination edges medially to insert on the medial aspect of the tibia and the crural fascia. It also detaches a tarsal tendon that joins the common calcanean tendon. The vertebral and pelvic heads are supplied by the caudal gluteal and sciatic nerves, respectively.
The semimembranosus (Figure 24-2/7) is included in the hamstring group, although topographically it is a muscle of the medial aspect of the thigh. The vertebral head is relatively weak, and the pelvic head is more substantial. The combined muscle is largely covered by the gracilis and follows the caudal margin of the adductor to which it is closely bound. It inserts by two divisions. The cranial division inserts on the medial epicondyle of the femur and the medial collateral ligament of the stifle joint; the caudal division proceeds distally to the medial condyle of the tibia.
The principal nerve supply is from the sciatic nerve.The actions and uses of the hamstring muscles are complicated and in certain respects enigmatic. It is clear that all three units are well placed to extend the hip. Rather than looking at the three muscles individually when considering the actions on the stifle, one will find it more useful to divide the hamstring group into two functional units: one that inserts proximal to the axis of rotation of the joint and the other distal to it. The “proximal unit” comprises parts of the muscles that are potentially extensor, as they may straighten the stifle by drawing the femur caudally when the limb bears weight. The “distal unit” will flex the stifle when the hoof is raised from the ground but will extend it when the hoof is firmly planted. The contributions of the biceps and semitendinosus to the common calcanean tendon must not be forgotten; these parts are extensor to the hock.
Some of these actions are clearly incompatible because the movements of the stifle and hock joints are linked in their actions by the reciprocal mechanism (see p. 638). It follows that the entire hamstring group, which includes parts that may flex the stifle, cannot always contract en masse.