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Show 258 MR. F. G. PARSONS ON T H E [Mar. 20, Castor it is entirely covered by the trapezius, and is inserted into the upper border of the acromial process and the outer part of the spine of the scapula. The above observations show that the levator claviculae is a muscle of very little use for classificatory purposes among the Hystricomorpha; in this group it seems to vary even in different individuals of the same species; it is possible that it is in process of shifting its attachment from the basioccipital to the front of the atlas. The number of observations, however, are not sufficient for generalization. Rectus Capitis Anticus Major and Minor.-The rectus capitis anticus major rises in the Hystricomorpha from the transverse processes of two or three cervical vertebrae below the atlas, and is inserted into the basioccipital bone in front and internal to the scalenus anticus; except in the Caviidae it is difficult to separate from the longus colli. In the Sciuromorpha the muscle usually rises from a greater number of transverse processes. The rectus capitis anticus minor and lateralis have the human attachments; the latter is large and closely connected to the superior oblique. Longus Colli.-The longus colli closely resembles the same muscle in M a n ; it consists of two oblique and one straight part. The posterior oblique part rises from the bodies of the anterior three or four thoracic vertebrae, and is inserted into the transverse processes of the posterior cervical vertebrae. The anterior obhque portion runs from the insertion of the last part to the longus colli tubercle on the ventral arch of the atlas. The straight part runs from the bodies of the anterior thoracic vertebrae to those of the anterior cervical. In Castor it extends a long way into the thorax. Scalenus Anticus.-As there is a good deal of difficulty in identifying the scalene muscles of Bodents with the three scalenes of human anatomy, I have given the name of scalenus anticus only to a muscle inserted into the first rib between the subclavian artery and vein. This muscle when present rises by a tendon from the basioccipital in front and internal to the levator claviculae ; in Ccelogenys it also derives a few fibres from one or two cervical transverse processes. It is absent in the Hystricidae (Hystrix, Sphingurus) and in all the Sciuromorpha, but present in the other animals examined. Scalenus Medius and Posticus.-These two muscles are most conveniently described together, as it is often impossible to say where one ends and the other begins. In Aulacodus, which is a good type of the arrangement in the Octodontidae, one muscle, which I take to represent the scalenus medius, rises from the transverse processes of the first four cervical vertebrae and is inserted into the sides of the 4th and 5th ribs between the serrations of the serratus magnus. Another muscle, probably the scalenus posticus, rises from the posterior three cervical transverse processes and is inserted into the first and second ribs. In Chinchilla the arrangement is almost identical. |
