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Show 32 DR. J. MURIE AND MR. A. D. BARTLETT [Jan. 9, within the apex of the foremost pair of incisors of the upper jaw, two sharp inner edges of the lower incisors coming so close together that the line of separation (fig. 1, a) is hardly distinguishable; but when the angles of the inferior maxilla are slightly approximated, as, for instance, by a gentle pressure of the thumb and fore finger, then the two teeth open like the blades of a pair of scissors at their points, as much as one-eighth of an inch, and, according to what Mr. Waterhouse says, their outer cutting-edges are brought more closely in contact with the inner edge of the anterior upper incisors ; but at the same time it must be remarked they can also overlap them (fig. 2, a). But whether this overlapping takes place ordinarily, when the creature crops its food, we are not prepared to say, as it is not readily distinguishable, from the rapidity of the act. In the second specimen of Kangaroo (Petrogale brachyotis), an adult female, the tips of the lower incisors could be separated almost one-fourth of an inch, and the structure was in nearly all respects similar to that of Bennett's Kangaroo-with this difference, that in Petrogale brachyotis there was an interspace at the base of the lower incisors, even when the points of the teeth were brought in contact; while in Halmaturus bennettii the whole of* the inner edges were applied closely to each other (fig. I, a). It is possible that this slight hollow may have been caused by a wearing away of the substance ; that it existed in the adult animal gives feasibility to this belief. The next point of interest connected with this remarkable movement is the consideration of what muscular apparatus or set of muscles produces it. The great breadth and increased size of the inner hollow of the ramus and angle of the mandible in the Marsupials at once suggests that the pterygoid muscles, from their increased purchase and position, would entirely effect this, as it does, to a great extent, in the bovine race the process of rumination. No doubt these muscles are concerned in the motive act of the one half of the mandible upon the other in the Kangaroos ; but certain other muscular fibres seem also to be called strongly into play. The thin layer of the platysma myoides on either side appears to have a slight influence in the production of the opening of the incisors, by gently aiding the approximation of the angles of the lower jaw. The digastric muscles, moderately strong in the specimens dissected, have their usual origin and median tendon slightly in advance of the angles. Their anterior fleshy bellies are inserted half an inch or so behind the posterior junction of the symphysis, so that on contraction of their fibres they serve to pull together the posterior rami, and also produce the aforesaid separation of the lower incisors. The two mylo-hyoidei are not extraordinarily large, although broad; but their position and nearly transverse direction give them even a more direct and important action in the opening movement than the last. The genio-hyoidei are, on the contrary, strong and well developed. Their point of traction from the hyoid bone, and apparent tenseness |
