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Show 1896.] MYOLOGY OF RODEXTS. 161 ' Thierreich,' that the size of the temporal varies inversely with that of the eyes, for in them the eye is rudimentary while the muscle is very large (Rhizomys, Georychus, Bathyergus). Masseter.-The four parts of the muscle already described are present in the Myomorpha, but the posterior superficial and posterior deep parts are usually difficult to separate satisfactorily. The auterior superficial portion is constant and rises in front of the infraorbital foramen, from the side of the maxilla, by a narrow tendon. It is very strongly marked in the Spalacidae, in which the lower jaw has a broad pulley-like groove under which the muscle passes to be inserted into the inner surface of the bone; this arrangement is very well seen in Georychus and Bathyergus. The anterior deep part shows an intermediate arrangement between the Sciuromorphine and Hystricomorphine type and throws a Superficial dissection of Hamster's masseter. good deal of light on the morphology of this portion. In the Hystricomorpha there is a large infraorbital foramen through which the anterior deep part of the masseter passes to be inserted by a narrow flat tendon into the mandible opposite, or just in front of, the anterior cheek-teeth. In the Sciuromorpha the part which I have hitherto described as " anterior deep " rises from a vertical groove in front of the zygoma, and passes down to the same insertion without traversing any bony canal, the infraorbital foramen being only large enough to allow the passage of the nerve. As the insertion of a muscle is justly regarded as of more importance than the origin in determining homologies, I have hitherto looked upon these two portions as homologous, but the arrangement in many of the Myomorpha has made m e change this opinion. In the Myomorpha the infraorbital foramen is intermediate between the arrangement found in the other two suborders ; it is usually present as a vertical slit, through which passes a small bundle of muscular fibres corresponding to the anterior deep part of the Hystricomorphine masseter; but in addition to this there is another slip, corresponding to the Sciuromorphine anterior deep part, which rises in front of the zygoma PROC ZOOL. Soc-1896, No. X L 11 |
