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Show 1894.] MYOLOGY OF RODENTS. 285 and E. epixanthus1 the same arrangement is found. This corresponds to the arrangement that Mivart and Murie2 have described in the Agouti, but presents a clearer illustration of it than is found in that animal. In Sciurus and Pteromys the superficial head is small, and instead of rising from the caudal vertebrae comes from the deep surface of the gluteus maximus. Xerus, Spermophilus, Arctomys, and Castor as well as all the other rodents have the typical arrangement. In Myopotamus a strong tendon runs to the head of the fibula. Semitendinosus.-The semitendinosus rises by two heads, the most superficial of which springs from the sacral and anterior caudal spines and slightly from the lumbar fascia. The deep head rises from the tuber ischii deep to the biceps. These two parts unite in the upper part of the thigh, and are inserted into the cnemial crest of the tibia and the fascia of the leg below this. This arrangement applies to all the animals dissected, except the Hystricidae and Pteromys. In the Hystricidae the muscle rises only from the sacral and caudal spines, but in Hystrix cristata and Erethizon dorsatus 3 a small slip is given to reinforce the biceps from this. In Sphingurus no slip goes to the biceps. In Pteromys the muscle rose from the tuber ischii; but I am inclined to regard this as an individual variation, because all the other Sciuromorpha, including Sciurus, have both heads. Semimembranosus.-This muscle consists of two parts, which are sometimes distinct, at others blended. The main part of the muscle rises from the tuberosity and adjacent part of the ramus of the ischium, and is inserted by a rounded tendon into the internal tuberosity of the tibia. It is supplied by the great sciatic nerve. The second portion is often included in tbe description of the adductors, with which it is frequently closely blended; its insertion is always into just above the internal condyle of the femur where, in Man, the adductor tubercle is situated ; this insertion is separated from that of the adjacent adductors by the femoral artery. The origin of this part of the muscle is not constant-sometimes it rises separately from the sides of the caudal vertebrae, sometimes from the tuberosity of the ischium in common with the other head of the muscle, and sometimes from the ramus of the ischium, as part of the adjacent adductor magnus. Whatever its origin it is always supplied by the great sciatic nerve and never by the obturator which supplies the adductors. Iu the Dipodidae the muscle rises from the tuberosity of the ischium, and is inserted into the lower part of the back of the femur and the internal tuberosity of the tibia; the oblique condylar slip is separate and also rises from the tuber (Dipus cegyptius). In the Octodontidae the origin is from the tuber and ramus of 1 Journ. Anat. 1888, p. 126. 2 P. Z. S. 1866, p. 383. 3 P. Z. S. 1882, p. 271. |
