Hi M?.. H. (;. F. SPURTJKLL OX TUE [Fc'l). G. 



7. The Articulation of the Vertebrate Jaw. 

 By IL George F. Spureell, 



[Received Februav}- 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 

 ai-rangement would be that the lines of the teeth in the upper and 

 lower jaws would be thrown less out of the parallel when the 

 rnouth 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 me 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 t}'pes I separated two : — 



I. The type in which the molar teeth ai-e 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 backwai-ds 

 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 i-ight 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 consideiably. 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 

 liecomes almost tul)ular. All lateral movement of the jaw is thus 



