114 MR. H. G. F. SPUREELL ON THE [Feb. 6, 



7. The Articulation of the Vertebrate Jaw. 

 By H. George 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 tem^Doro-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 simultaneo;isly 

 when the mouth shut, and Avould all press on food between them 

 with more nearly equal force. Further, it seemed to me that this 

 arrangement favoured, if it was not absolu.tely 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 temporo- 

 mandibular 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 



