1896.] MYOLOfiY OP llODENTS. 179 



Semitendinosus. — The normal arrangement seems to be, as in the 

 other Eodents, that one head should rise from the posterior sacral 

 and anterior caudal vertebroe, while the other comes from the tuber 

 ischii. Either of these heads is often wantiog. In Georychiu, 

 Batliyergus, and Mii^ harharus the head from the tuberosity alone 

 was found, while in Microttts and Heteromys only the spinal head 

 was present. With regard to the arrangement in Mus harharus, 

 it is interesting to note that Mus rattus has both heads. The 

 insertion in all cases is the same as that described in the other 

 Eodents. 



Semimembranosus. — The two parts of this muscle are always 

 present, and correspond to the description already given in other 

 llodents. The semimembranosus proper is quite constant, always 

 rising from the tuber ischii 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 iuteriial condyle of the femur is 

 quite constant. In Oerhillus, Mus harharus, and Mus rattus the 

 supra-condylar portion rises from the anterior caudal vertebrae. 

 In Gricetus it is small and comes from the caudal vertebrse 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 Batliyergus, vs'here 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 lihizomys 

 it is only half the size of the rest of the muscle ; while in Cricetus 

 it is quite small. In nydromys,V^'mA\e says ' that the semimem- 

 branosus 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 

 au individual variation or whether this arrangement is constant 

 in Hydromys. The nerve-supply of the two parts of the semi- 

 membranosus is as in other Eodents. 



Gracilis. — This muscle is usually double and the two parts have 

 approximately the attachments described in the Hystricomorphine 

 Eodents ; the anterior muscle usually overlaps the posterior a good 

 deal. In Myoxus, lihizomys, 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. 



Pecthutts. — This muscle corresponds to the account of it in the 

 other Eodents. In Gerbillus, Gricetus, Myoxus, and lihizomys the 

 muscle is double, but I am 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 Rhizomrfs, Georychus, 

 ' P.Z.S. 1887, p. 57 



12* 



