1118 THE DIGESTIVE SYSTEM. 



above and below and are distinguished as first, second, and third molars. The 

 last in each jaw is also known as the dens serotinus. All the molars are charac- 

 terised by the large size of the crown and the possession of three or more trihedral 

 tubercles on the masticating surface (Figs. 883 and 884). They are the largest 

 of all the teeth, but they diminish in size, as a rule, from the first to the third. 

 In shape the crown is more or less quadrangular, with convex labial and lingual 

 surfaces. The roots are either two or three in number, but frequently in the last 

 molars they are united to a varying degree. 



The molars of the maxilla and mandible differ so considerably in their further 

 details that they must be considered separately. They may be most readily dis- 

 tinguished from one another by the fact that normally the upper molars possess 

 three roots (Figs. 881 and 882), whilst the lower molars have two at most. The 

 number -of tubercles, though not so reliable a guide as the form of the root, is also 

 generally sufficient to distinguish them. In the upper molars there are either 

 three or four tubercles, whilst in the lower the number is most commonly five 

 (see, however, below). 



In the upper molars, the crown, viewed from the masticating surface (Fig. 883), 

 is rhomboidal in shape (i.e. quadrangular with the angles not right angles). The 

 labial and the lingual surfaces are convex. The number of tubercles is either four 

 or three. On the first there are invariably four two on the labial and two on 

 the lingual side the anterior lingual of these being connected with the posterior 

 labial by an oblique ridge (Fig. 883), which is also found on the second and third 

 molars when these bear four distinct cusps. The second upper molar has either 

 four or three tubercles in about an equal proportion of European skulls, whilst in 

 the third the number is much more frequently three than four. The roots in the 

 upper molars are three in number (except, occasionally, when the three roots of 

 the third are confluent), two being labial, and the third lingual (Figs. 881, 882, 

 and 885). 



In the lower molars, the crown, viewed from above (Fig. 884), is somewhat 

 cubical. The labial and lingual surfaces are convex, as in the upper molars. The 

 first, as a rule, bears five tubercles, two being on the labial side, two on the lingual, 

 and the fifth behind and lateral, that is, between the two posterior tubercles and 

 somewhat to the labial side. The second has usually only four tubercles ; a fifth, 

 however, is sometimes present. The third has either four or five, the former 

 number more frequently than the latter. 



The roots of the lower molars are two in number, each wide, grooved, and 

 flattened antero-posteriorly. One root is anterior, the other posterior, and both 

 are usually recurved in their lower portions (Fig. 885). As in the corresponding 

 teeth of the maxilla, the roots of the lower last molars are often more or lese 

 united into a single mass. 



The chief characters of the upper and lower molars may be summarised thus : 



The first molar is usually larger than the second, and the second than the third. Th( 

 upper molars are directed downwards and laterally ; whilst the lower molars, which the forme: , 

 partly overlap, slope upwards and medially, with the result that the labial tubercles of the lowe 

 molars lie in the groove separating the lingual from the labial tubercles of the upper teetl 

 (Fig. 886, p. 1120). As a result of this overlapping, the labial edge of the crown is sharp am 

 the lingual edge rounded in the upper molars ; whilst the lingual edge is sharp and the labia 

 edge rounded in the lower set. 



The fissures which separate the cusps on the grinding surfaces of the molar teeth are generall; 

 continued as faint grooves on the labial and lingual surfaces. 



Upper Molars. The crowns, as already stated, are rhomboidal in shape, and when vie win, 

 their masticating surfaces, as in Fig. 883, if the planes of separation between them be prolongec 

 they would strike the median plane near the posterior part of the hard palate ; in other word; 

 their anterior and posterior surfaces are not in transverse but in oblique planes, slopin 

 strongly postero -medially, and converging in that direction. A knowledge of this is useful i 

 determining the side to which an upper molar belongs, as is the fact that the anterior labial ro( 

 is broader than the posterior (Fig. 882). 



As regards the number of tubercles (Fig. 883) : The first upper molar has four tubercles i 

 practically all skulls (99 per cent) ; occasionally, indeed, another, but very rudimentary, tubercle 

 present on the lingual side of the antero-lingual tubercle. The second molar has either three 

 four in an almost equal proportion of Europeans, but more frequently four, taking the teeth 

 all nations together. (According to Topinard, four are present in 66 per cent, of all race 



