Fig. 136.—Example of a worn-down tooth.
Fig. 137.—Handle-shaped ears.
Sometimes the number of teeth is less than 32, in which case it is necessary to distinguish two cases of very different significance: First, the last molars ("wisdom teeth") may be wanting; secondly, some of the other teeth may be wanting (incisors, canines, or premolars). The last molar is of no use whatever to man, because it does not enter into the service of mastication, and it is tending to disappear. We may even predict that the day is coming when mankind will no longer have wisdom teeth, and the human dental formula will be as follows:
| 2—2 | 1—1 | 2—2 | 2—2 | = 28 teeth |
| –—– | –—– | –—– | –—– | |
| 2—2 | 1—1 | 2—2 | 2—2 | |
| incisors | canines | premolar | molars |
The absence of useful teeth, on the contrary, is a grave sign of degeneration, and one which leaves wide spaces between two adjacent teeth (wide diastemata).
The diastema, or space left between adjacent teeth, is of great importance.
There are various causes for this stigma. Besides the one already mentioned, due to congenital absence of a tooth (broad diastema), another recognized cause is an anomalous placing of the teeth (narrow diastema). The significance of this is not always the same: for example, the diastema between two upper incisors indicates a very slight anomaly of embryonal development, and, some people think, gives a sympathetic charm to the smile. On the contrary, a diastema occurring at the side of a canine tooth signifies a congenital malformation.
At other times such anomalous spaces may be due to the fact that the teeth have remained small, or happen to have worn away laterally and present an almost filiform or thread-like aspect (diastemata due to microdontia resulting from syphilis or various dystrophic conditions).