234 THE TEETH. 



nal root of the upper molar is seen sometimes to be joined 

 by a " broad plate" to the anterior external, and then 

 again all three have been found united in one mass. The 

 roots of the lower molars are seen occasionally to be sim- 

 ilarly connected. And, indeed, so great is this irregularity 

 in the roots of the molars, as to length, figure and direc- 

 tion, that, according to Mr. Nasmyth, it is impossible to 

 tell beforehand the amount of resistance to be encountered 

 in their removal. The average length of a molar he esti- 

 mates at from eight to thirteen lines. 



In reference to the function of the molars, this same 

 gentleman uses the following language: "The mechanical 

 disposition of the molar teeth is beautifully fitted to the 

 purposes which these organs have to fulfil; for example, 

 the first act of mastication, consisting in the closure of the 

 lower against the upper jaw, while it secures the food, 

 makes its greatest force of pressure against the outer limb 

 of the crown of the superior molars, that limb which we 

 know to be supported by two out of the three roots of the 

 tooth. 



"Again, when trituration ensues, the ramus of the jaw is 

 drawn inwards, and the chief amount of pressure is trans- 

 ferred to the outer limb of the lower molars, where the 

 greatest strength of fang exists." 



Structure. The structure of each tooth consists essential- 

 ly of three parts, the pulp, dentine or ivory, and enamel, to 

 which the cementum or crusta-petrosa is added, (Fig. 61, A.) 

 Each tooth contains within itself a cavity for lodging the 

 pulp, called the pulp cavity, (Fig. 61, B, C.) The shape of the 

 cavity corresponds to that of the tooth to which it belongs. 

 The dental pulp has the same form as the cavity, and is de- 

 scribed by Mr. Thos. Bell, as " very soft, gelatinous, and 

 semi-transparent, and having its surface covered by an ex- 

 tremely delicate, thin, vascular membrane, closely attached 

 to it by vessels." 



The vessels (Fig. 62, from a drawing by Mr. Nasmyth) 

 which supply the pulp, enter the tooth at the apex of its 

 root, forming a capillary net-work on the pulp, and show 



