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Show 1896.] MYOLOGY OF RODENTS. 179 Semitendinosus.-The normal arrangement seems to be, as in the other Rodents, that one head should rise from the posterior sacral and anterior caudal vertebrae, while the other comes from the tuber ischii. Either of these heads is often wanting. In Georychus, Bathyergus, and Mus barbarus the head from the tuberosity alone was found, while in Microtus and Heteromys only the spinal head was present. With regard to the arrangement in Mus barbarus, it is interesting to note that Mus rattus has both beads. The insertion in all cases is the same as that described in the other Rodents. Semimembranosus.-The two parts of this muscle are always present, and correspond to the description already given in other Rodents. The semimembranosus proper is quite constant, always rising from the tuber ischh and being inserted into the back of the internal tuberosity of the tibia. The supracondylar slip, as in the Hystricomorpha, is variable both in size and in origin ; its insertion, however, above the internal condyle of the femur is quite constant. In Gerbillus, Mus barbarus, and Mus rattus the supra-condylar portion rises from the anterior caudal vertebrae. In Cricetus it is small and comes from the caudal vertebrae and the tuber. In the other animals examined it rises from the tuber ischii only in common with the rest of the semimembranosus except in the case of Bathyergus, where its origin is more in common with that of the semitendinosus from the tuber. This supra-condylar slip is largest in Georychus, where it is greater than the rest of the muscle; in Bathyergus it is not so large; in Rhizomys it is only half the size of the rest of the muscle; while in Cricetus it is quite small. In Hydromys, Windle says l that the semimembranosus is inserted into the condyle of the femur alone, that is to say, the main part of the muscle is wanting. His dissection must of course be repeated, to see whether he had chanced upon an individual variation or whether this arrangement is constant in Hydromys. The nerve-supply of the two parts of the semimembranosus is as in other Rodents. Gracilis.-This muscle is usually double and the two parts have approximately the attachments described in the Hystricomorphine Rodents; the anterior muscle usually overlaps the posterior a good deal. In Myoxus, Rhizomys, and Heteromys no separation was seen. In Georychus and Bathyergus the separation was very marked, there being quite an interval near the insertion. In Myodes the interval is greatest at the origin. Hydromys according to Windle agrees with Myoxus and Rhizomys. Pectineus.-This muscle corresponds to the account of it in the other Rodents. In Gerbillus, Cricetus, Myoxus, aud Rhizomys the muscle is double, but I a m inclined to regard the inner part as belonging to the adductor mass, possibly representing the adductor longus. Quadriceps Extensor.-In all the animals dissected, the two heads of the rectus could be made out, though in Rhizomys, Georychus, 1 P.Z.S. 1887, p. 57 12* |
