1122 



DIGESTIVE SYSTEM 



three to five masticating cusps (hence sometimes called 'multicuspids'). The 

 crowns are massive, somewhat resembling rounded cubes, and the lingual and buc- 

 cal surfaces present vertical grooves continuous with the fissures separating the 

 cusps. The pulp cavity (fig. 872) has slight extensions corresponding to the cusps, 

 and also communicates with the canals of the roots, which are usually two or three 

 in number, and more or less curved. 



Fig. 877. — Diagram Showing the Articulation of the Teeth. (Poirier-Charpy.) 



labial 



I>uccal 



^"««3l buoci 



A 



Incisors 



B 



Premolars 



The upper molars are most easily distinguished from the lower by the presence of a triple 

 root. The masticating surface is nearly square with rounded angles. They each have typically 

 four cusps, separated by grooves resembling a diagonally placed H (fig. 852). The crowns of 

 the upper molars are obliquely placed so as to slant downward and slightly lateralward. 



Each upper molar has three roots, two buccal and one lingual or palatal. They are all 

 (especially the buccal) in more or less close relation with the floor of the maxillary antrum (of 

 Highmore) (fig. 876). The buccal roots are flattened antero-posteriorly, and longitudinally 

 grooved, and bent backward. The palatal root is more rounded, with a groove on the lingual 

 surface, and usually bent medialward. Either of the buccal roots may fuse with the palatal, 

 or there may be an extra fourth root. 



As to the individual upper molars, the first has almost invariably four typical cusps (rarely 

 only three, or with an additional fifth rudimentary). The second upper molar has only three 

 cusps in about half of the cases (in Europeans), and four in the remainder. The third, or wisdom 

 tooth [dens serotinus] is exceedingly variable in size and form (fig. 875). It has three cusps 

 much more frequently than four, and its three roots are often more or less fused into a conical 

 mass. It is usually much smaller than the other molars, and is absent in nearly one-fifth of all 

 cases. 



Fig. 878. — Diagrams Showing the Early Development of Three Teeth, One of which 

 is Shown in Vertical Section. (Lewis and Stohr.) 



Epithelium of the margin 

 of the jaw 



Enamel 

 organs 



Dental 



groove 



Dental ridge IZ^""^^ 



Papillae 



A B 



Enamel organs 



c 



Necks of enamel organs 



D 



The lower molars have usually four or five cusps (two hngual, and two or three buccal) 

 the fi.ssures .scj)aratiiig thcin being cro.s.s-.shaj)od or stellate (fig. 864). The crowns incline 

 upward and slightly iiiedialward. They have each two roots, anterior and posterior, flattened 

 antero-posteriorly, and usually .somewhat curved backward. The roots, especially the anterior, 

 may be longitudinally grooved. The anterior has two root-canals, the posterior usually only 

 one. The api(;<'s of tli(! roots of the lower molars, especially of the third, approach the man- 

 dibular (inferior d<;ntal) canal (fig. S7()). 



Of the indiridiial lower riiolars, the first is usually slightly the largest, and has five cusps 

 in the great majority of ca.ses (variously estimated at from 60 to 95 per cent.), otherwise four, 

 The four main cusps (two buccal and two lingual) are separated by a cruciform fissure, which 

 bifurcates posteriorly to embrace the small fifth cusp (which is placed slightly to the buccal 



