OCR Text |
Show 1866.] ON THE LOWER JAW OF THE KANGAROOS. 33 while the incisors are asunder, leads to the inference that they likewise modify and aid the opening movement, although mere position and the parallelism of their fibres to the median line of divergence (at least as they appear on dissection) makes one hesitate to attribute too much power to them ; but they certainly act along with, and greatly strengthen the force of, the mylo-hyoid'ei. By the conjoined simultaneous action of the whole of the muscles mentioned, the movement of the symphysis and separation of the incisors seem to be effected; while the return to the state of closure follows relaxation of these, with possibly contraction of one of the pterygoid muscles. The chief agent, however, in the approximation of the anterior portions of the symphysis and the internal edges of the incisors is no doubt the transverse fibres of the orbicularis oris (here situated at the anterior portion of the bony symphysis and the root of the lower incisors), which, although delicate medially, is nevertheless well developed laterally (see figs. 1 and 2, b). In proof that the portion of the symphysis in close juxtaposition are the pivot or point of leverage in the movement spoken of, and that the muscles stated are those concerned in the action, a partially dissected specimen need only be experimented on, when very gentle inward pushing at the angle will be seen to produce the separation of the incisors ; and a like force applied in advance of the pivot (e. g. where the orbicular muscle is placed) immediately and easily causes closure. Hence, as to the point at issue, we have tried to show from our observations the analogy of the movements and use of the mandible and incisors of the Kangaroo to those of a pair of cutting-forceps or scissors, the posterior part of the symphysis being the pivot, the angles the handles, and the incisors the blades, the inner edge of which is the cutting-edge. \ The manner of use, at least in confinement, we have attempted to describe; but whether the teeth are put to the same use when the animal is in a state of nature is a fact unknown to us. Mr. Gould, whose opportunities of observation of their native habits was at one time great, informs us that the food of the Petrogale brachyotis is often dry and tough vegetables obtained among rocky places. It is possible therefore that the cutting-edges in that case might well I serve to sever dry or fibrous material. P.S. Since this paper was read, our attention has been called by t Dr. J. E. Gray to a paragraph in ' The Book of Nature,' by John Mason Good (vol. i. p. 254), where the author says, "The Mus ma-ritimus, or African Rat, the largest species of this genus which has t hitherto been discovered, and seldom less than a full-sized rabbit, has the singular property of separating at pleasure to a considerable distance the two front teeth of the lower jaw, which are not less than an inch and a quarter long. That elegant and extraordinary creature the Kangaroo, which we may soon hope to see naturalized in our own country, is possessed of a similar faculty." But this statement does not seem to forestall the facts which we have observed regarding the use or manner of action of the lower i PROC. Z O O L . Soc-1866, No. III. |