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Show 286 MR. F. G. PARSONS ON THE [Mar. 20, the ischium ; the iusertion is normal, but an expansion is continued forwards deep to the internal lateral ligament of the knee. The oblique slip in Myopotamus and Octodon rises from the tuber ischii, but in Aulacodus it comes from the sacral vertebrae as in Hystrix (Aulacodus, Capromys melanurusl, Myopotamus, Octodon). In the Hystricidae the arrangement is not constant: Hystrix and Erethizon entirely resemble Aulacodus in the origin of the two parts, but in Sphingurus the second slip rises from the tuber ischii with the rest of the muscle instead of from the sacral vertebrae; it soon becomes distinct to run to the internal condyle. Among the Chinchillidae, Lagostomus has the same arrangement as Hystrix and Aulacodus, but Chinchilla resembles Sphingurus. In the Dasyproctidae the two parts of the muscle rise together from the tuber and ramus and only separate towards the lower part of the thigh (Dasyprocta, Ccelogenys). In the Sciuromorpha the condylar portion of the muscle is separate from the rest and closely connected to the adductor mass, with which it will for convenience be described ; it is, however, still supplied by the great sciatic nerve (Sciurus, Pteromys, Xerus, Spermophilus, Arctomys, Castor). It will be seen that tbe Eodents illustrate the changes by which part of the semimembranosus becomes part of the adductor magnus. In Ccelogenys, for example, the slip going to the condyle of the femur is part of the semimembranosus : in Sphingurus it rises with that muscle but soon separates from it; in Hystrix it is a perfectly distinct muscle, having a different origin to the semimembranosus or adductors, while in Sciurus it is intimately blended with the adductors, though still preserving its original nerve supply. It is remarkable, too, that, except for its constant arrangement in the Sciuromorpha, it seems to be of very little classificatory value. Gracilis.-The gracilis is very often double. The most anterior portion, which has sometimes been described as the sartorius, rises from the ilio-pectineal line and from the anterior part of the ventral surface of the symphysis pubis ; it is inserted into the inner border of the patella and ligamentum patellae. The posterior gracilis usually rises from the posterior part of the ventral surface of the symphysis and from the subpubic arch; its insertion is into the cnemial crest of the tibiae and the fascia of the leg, which makes it a powerful internal rotator of the tibiae. The two muscles are always supplied by the obturator nerve. Sometimes the gracilis is a single muscle, but it then usually shows signs of a separation. The general rule seems to be that the Hystricomorpha have two graciles and the Sciuromorpha one. The two exceptions that I have met with are Aulacodus, in which the muscle is single, and Castor, in which it is double. Pectineus.-The pectineus rises from the ilio-pectineal eminence and line under cover of the anterior gracilis, and is inserted into 1 P. Z. S. 1884, p. 234. |
