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Show 1894.] MYOLOGY OF RODENTS. 287 the upper third to half of the linea aspera of the femur. The only exceptions to this that I have met with are the Caviidae, in which the muscle rises by a tendon from the ilio-pectineal eminence only (Cavia cobaya, Ceredon rupestris), and Castor, in which it is very strongly developed and is inserted into the whole length of the shaft of the femur. Quadriceps Extensor Cruris.-The four muscles composing the quadriceps have the same origin and insertion that they have in Man, and are practically the same in all the Hystricomorpha and Sciuromorpha. The rectus rises by two heads, which are sometimes quite distinct, but at others so short as to be almost indistinguishable. They are perhaps most distinct in the Hystricidae, least so in the Sciuridae. The crureus rises from the whole of the anterior surface of the femur by a series of about 30 fleshy . arches. The two vasti can usually be separated easily from the crureus; as a rule the vastus externus is the larger. Adductors.-It is extremely difficult in dissecting a Eodent to say which part of the adductor mass corresponds to the adductor longus, brevis, or maguus of human anatomy. In Dipus cegyptius the adductor longus rises from the front of the pubes under cover of the gracilis and runs to the inner side of tbe pateUa. The adductor maguus and brevis come from the whole subpubic arch aud are inserted into the upper two-thirds of the back of the femur. A good deal of the adductor mass, however, in this animal seems to be blended with the semimembranosus. In the Octodontidae and Chinchillidae the adductors longus and brevis seem to be fused, although in Aulacodus and Octodon a division was readily made out. These coalesced muscles rise from the inner part of the pectineal line and ventral surface of the symphysis, and are inserted into the upper half of the linea aspera. In the Hystricidae, as Meckell observes, the three parts of the muscle can be seen ; this is especially the case in Sphingurus, but it is doubtful whether the three parts correspond morphologically with the three adductors in Man. Among the Dasyproctidae, Dasyprocta has very much the same arrangement as the Chinchillidae, but in Ccelogenys paca the adductor brevis has a distinct insertion by a ribbon-like tendon into the upper part of the linea aspera. It will be noticed that nothing has been said here about fibres passing to just above the internal condyle; these have already been described with the semimembranosus. Among the Sciuromorpha the adductor mass is much more broken up. In Sciurus, which serves as a good type, there are five portions inclusive of the slip already described with the semimembranosus. The following is the arrangement in Sciurus :-(1) Most anterior portion from the ilio-pectineal line to the middle of the posterior border of the femur; this part is distinct from the pectineus. (2) A slip from the posterior part of the pubic symphysis to above 1 Traite gen6ral d'Anatomie comparee, vol. vi. p. 378. |