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Show 1903.] THE MUSCLES OF THE UNGULATA. 271 Elephant (81, 85), however, an expansion from this insertion is found running down to the inner side of the ankle. According to some authors the semimembranosus may occasionally be absent altogether: this was the case in Kinberg's Chevrotain (20), Chatin's Water-Chevrotain (24) *, and Lesbre's Horse (65). The presemimembranosus is usually larger than the semimembranosus, and is inserted by fleshy fibres into the lower part of the femur just above the internal condyle; besides its fusion with the semimembranosus it is very intimately connected with the adductor mass and so has escaped notice by some observers. The nerve-supply in the Pig, Antelope, Sheep, and Hyrax is from the nerve to the hamstrings, a large branch which comes off the internal popliteal element of the great sciatic. Semitendinosus.-This muscle, like the last, may rise by one or two heads, of which the one from the tuber ischii is always present. A caudal head is rarely found in the Artiodactyla, Chatin's Water- Chevrotain (24) and the Pig (4, 6) are the only exceptions we know. In the other suborders, the Horse (63, 64, 65), Tapir (58, 62), and Hyrax (74, 75, 78) have two heads (see text-figs. 24 & 26, S.T.). The insertion is into the second quarter of the internal surface of the shaft of the tibia as a rule, and from this a small expansion is sent off to help the biceps and gracilis in reinforcing and providing a sheath for the tendo Achillis, and so acting on the calcaneal tuberosity. These expansions from the inner and outer hamstrings are seen at their best in the Ungulata; but they are so common among the Mammalia generally t, that they probably have a phylogenetic history to account for their presence, as well as a physiological advantage to account for their special development in Ungulates. In all probability their past history is that they are remnants of the longitudinal muscle-fibres passing uninterruj^tedly from the pelvic girdle to the foot, such as are found among the Reptilia, while the explanation of the fact that they are so well developed in Ungulates must be looked for in the mode of life of these animals. Among all the orders of mammals there is not one which is more distinctly cursorial in the habits of its species; with the exception of the Tree-Hyrax there are no arboreal types; the Hippopotamus and Water-Chevrotain are the only ones which are partially aquatic, while there is no example of a fossorial Ungulate. For these reasons, any modification which we meet with in the limb-muscles of Ungulates as a group, will probably be explained by its being an advantage to them in running. The expansions of the hamstrings to the calcaneum act as plantar flexors of the ankle-joint, while the hamstrings themselves are flexors of the knee. In watching the sequence of movements of * In our specimens of Chevrotain (21, 22) the muscle was undoubtedly present, though so closely connected with the presemimembranosus that it might easily be overlooked without careful dissection of the insertion. f The fascial expansions from the semitendinosus aud gracilis in M a n will at once occur to the human anatomist. |
