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Show 1894.] MYOLOGY OF RODENTS. 289 to show how far a femoral origin of this muscle is characteristic of the Caviidae. In the Sciuromorpha the muscle is specially well developed and encroaches on to the head of the fibula. There is no femoral head except in Castor, but here it is not nearly as well developed as in the Dasyproctidae and does not rise as in them by a definite tendon (Sciurus, Pteromys, Xerus, Spermophilus, Arctomys, Castor). Extensor Longus Digitorum.-This muscle in all cases rises by a tendon from the front of the external condyle just outside the patellar surface on that bone. This is its only origin, except in Sphingurus and Dipus, where a few accessory fibres rise from the outer tuberosity of the tibia. The muscle passes clown and becomes tendinous in the lower part of the leg, being bound down by two well-marked annular ligaments, the lower of which is attached to the calcaneum and forms a distinct sling. The tendon divides for the four outer toes, when these are present, being inserted into the middle and terminal phalanges. When there are only three toes the middle one sometimes has a double tendon, as in Lagostomus and Cavia cobaya. The tendons are united by vincula, which in Myopotamus spread out in the web between the toes; they are strongly marked in Castor. Extensor Proprius Hallucis.-This small muscle rises from the middle or lower third of the front of the fibula and is inserted into the terminal phalanx of the hallux, when that toe is present. In Dipus cegyptius it is absent. In the Octodontidae it either unites with or sends a slip to the extensor longus digitorum tendon to the second toe (Aulacodus, Octodon, Myopotamus). In the Hystricidae it rises from the lower part of the shaft of the fibula. In the ChinchiUidae it is absent (Chinchilla, Lagostomus). In the Dasyproctidae it is a large muscle and rises from the upper three-quarters of the fibula; in Ccelogenys it is inserted as in the Octodontidae ; while in Dasyprocta, in which the hallux is wanting, it goes entirely to the second toe, joining the long extensor tendon there. In the Caviidae it is present and runs to the second (internal) toe ; it is bound down to the inner side of the base of the innermost metatarsal by a short fibrous tunnel (Ceredon rupestris, Cavia cobaya, Dolichotis patagonical). Among the Sciuromorpha it rises from the middle of the fibula and is inserted only into the hallux in Sciurus, Spermophilus, Xerus, and Arctomys. In Pteromys it springs from the lower third of the bone and sends a small slip to the second toe. In Castor it rises from a strong oblique fibrous band which runs from the head of the fibula to the lower end of the tibia, so that the muscle has no bony origin; it rises opposite the middle of the fibula, and is inserted into the first two toes. Extensor Brevis Digitorum.-This muscle rises from the upper and anterior part of the calcaneum, and runs to join the tendons of ] Beddard, P. Z. S. 1891, p. 236. |
