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Show 1887.] ANATOMY OF HYDROMYS. 55 the under and inner part of tbe zygoma within the orbit, and partly from the superior maxilla anterior to the orbit; this last portion joining the rest by passing between the superior maxilla and the zygomatic process just mentioned. This 3rd part joins the anterior part of the 2nd. The remaining facial muscles call for no special notice. Sterno-mastoid, which is larger than cleido-mastoid, is inserted by a small round tendon into the mastoid process. Cleido-mastoid takes origin from the inner part of the clavicle, and is attached under the last to the mastoid process. Sterno-hyoid : both muscles are united into a single sheet without trace of median differentiation. The middle part is inserted much higher up than the two lateral, viz. in the angle between the two anterior bellies of the digastrics, the lateral fibres being attached below the tendinous part of the digastric and near the omo-hyoid. The omo-hyoid itself has no central tendon. Digastric has no true tendon, the central position connected with the hyoid being constricted and covered on its superficial surface with a few tendinous fibres. Tbe two anterior bellies are connected with one another. It may perhaps here best be noted that the two halves of the inferior maxilla are extremely movable upon one another, a quantity of fibrous tissue intervening at the symphysis so as to form a kind of fulcrum by means of which a scissors-like action of the extremely long inferior incisors is obtainable (vide fig. 5). The teeth are divaricated from one another partly by the action of the digastrics and partly by a transverse intermandibular muscle (fig. 5, a), which lies above the insertion of the digastrics and quite separate from them. It is placed at the upper part of the angle between the two halves of the maxilla, and is attached to the inferior surface of either. Approximation is produced, at least in part, by the masseter and especially by the part described above as 1. From this it appears that an interval could be produced between the two lower incisors during the opening of the mouth which would disappear with its closure. Murie and Bartlett1, in a paper on the "Movement of the Symphysis of the Lower Jaw in the Kangaroo," give an excellent account of the mechanism of this movement in the Macropodidse, and quote from Good's 'Book of Nature ' a statement to the effect that a similar movement takes place in Mus marilimus, the African rat. In Hydromys there is no such development of the transverse fibres of the orbicularis oris as the above authors describe in Halmaturus bennettii ; whilst the intermandibular muscle above mentioned is quite distinct from any of the other inframaxillary muscles, all of which are present in addition. The amount of divergence possible would be from g to I inch. Muscles of Shoulder-girdle and Upper Extremity.-The two portions of the trapezius are quite distinct, the origin of the lower being partly from the lumbar fascia. There is a large occipital rhomboid, and rhomboidei major and minor form a single sheet without separation. Omo-cervicalis arises from the transverse 1 P. Z. S. 1866, p. 28. |
