OCR Text |
Show 1896.] MYOLOGY OF RODENTS. 165 paroccipital process, by fleshy fibres. The sterno-mastoid has the usual origin and is inserted by tendon into the base of the par-occipital process in front of the last muscle ; it is the larger muscle of the two. In Cricetomgs the cleido-mastoid is not covered by the trapezius at its origin, it is, however, overlapped by the sterno-mastoid at its insertion. In the (rerbille the two muscles are continuous at their origin, the cleido-mastoid being the larger and rising from the inner half of the clavicle. Rhizomys resembles Cricetus, Myoxus, and the Murinae in the origin of the cleido-mastoid being overlapped by the trapezius, and Cricetomys in, the insertion being overlapped by the sterno-mastoid. In Hydromys l the cleido-mastoid is the smaller muscle and is overlapped at its insertion by the sterno-mastoid. In Georychus and Bathyergus the cleido-mastoid is not overlapped by the trapezius ; in the latter the two muscles tend to fuse as in the Gerbille, but to a greater extent; at their insertion they are completely fused and are attached by a narrow tendon to the base of the paroccipital process. Stemo-hyoid and thyroid.-In most cases these muscles have the human attachments. In Bathyergus, however, the sterno-hyoid misses the hyoid bone and continues on to the symphysis menti, in this respect somewhat resembling the arrangement found in Myopotamus2. In Hydromys the arrangement seems identical with that of Myopotamus 3. Omo-hyoid.-As in the Sciuromorpha the omo-hyoid is always present in the Myomorpha; it has the same attachments, never, as far as I have seen, being attached to the clavicle. There is no central tendon. In Bathyergus the muscle is not attached to the hyoid bone, but is continued forwards with the sterno-hyoid to the symphysis menti. Levator Claviculce (Acromio-trachelian).-In all the animals examined this muscle rose from the anterior arch and transverse process of the atlas. Its usual insertion is into the acromial process, but sometimes it extends to the spine of the scapula. This description applies also to Hydromys 3. In Bathyergus the muscle is very large and extends from the acromion on to the outer half of the clavicle at its insertion, in this respect resembling the Dipodidae. Rectus Capitis Anticus Major and Minor and Longus Colli.- These muscles have the Hystricomorphine attachments. Scalene Muscles.-A scalenus anticus, that is a muscle passing to the first rib in front of the subclavian artery and brachial plexus, was only seen in the following animals-Gerbillus, Georychus, and Bathyergus. In these it resembled the same muscle among the Hystricomorpha, in rising from the basioccipital bone. The scalenus medius and posticus of human anatomy are represented by one mass, which rises from the transverse processes of all the cervical vertebrae, passes behind the brachial plexus, and is inserted into 1 P. Z. S. 1887, p. 55. 2 P. Z. S. 1894. p. 256. 1 P. Z. S. 1887, p. 55. |