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Show 114 ME. H. G. F. SPURRELL ON THE [Feb. 6, 7. The Articulation of the Vertebrate Jaw. By H. G E O R G E F. SPURRELL. [Received February 1, 1906.] (Text-figures 34-47.) Consideration of the human skull led me to the belief that the angle of the jaw is contrived to place the temporo-mandibular joint above the level of the teeth. The advantage of this arrangement would be that the lines of the teeth in the upper and lower jaws would be thrown less out of the parallel when the mouth opened and that the teeth would meet simultaneously when the mouth shut, and would all press on food between them with more nearly equal force. Further, it seemed to m e that this arrangement favoured, if it was not absolutely necessary to, the antero-posterior and lateral movements of the opposed surfaces of the molars over one another in mastication. To test the probability of this supposition, I examined the skulls of other animals. From the numerous mammalian types I separated two:- I. The type in which the molar teeth are laterally compressed in the long axis of the jaw so as to give it a sharp cutting-edge. In this type the jaw has a very slight angle, if any. A line drawn through the teeth and produced backwards almost cuts the temporo-mandibular joint. Example, Wolf (text-fig. 34). II. That in which the molar teeth have broad flat tops, for grinding vegetable food. In this type the jaw is bent, in some cases almost to a right angle, and the temporomandibular joint thus raised well above the level of the teeth. Example, Hare (text-fig. 35). Type 1 is the carnivorous type. The molars are required to cut soft stringy flesh and to crack large and very hard objects; therefore the presence of the tuberculated posterior molars and the blade-like carnassial teeth. Roughly speaking, the jaws of a carnivore resemble a combination of nut-crackers and shears: shears because the hinder teeth overlap considerably. Then as the fulcrum is in a straight line behind them, and the mouth is closed by approximating the points A and B, the edges of the back teeth must play on one another successively along their length, like the edges of shears (text fig. 36). In accordance with this type of dentition and conformation of jaw, a peculiar form of joint is required. Hence the condyle is shaped like a long transverse cylinder (text-fig. 37). It fits closely into a long groove, so deepened by a process of bone behind that it becomes almost tubular. All lateral movement of the jaw is thus |