nerve descends rather steeply at first, sloping slightly anteriorly until it reaches the body of the mandible. Here it turns forward to pass toward the anterior portion of the bone and comes within the range of implant procedures. There are two basic positions of the canal to consider: its location in relation to the residual alveolar crest infero- superiorly, and its location laterally.

When normally healthy teeth in good occlusion are present, the position of the canal varies in relation to them. Most typically the canal lies immediately below the roots of the molars (5), with the distance between the canal and roots increasing as the canal progresses anteriorally. Where the canal bifurcates, usually in the region of the first premolar but sometimes slightly distally, it runs outward, upward, and sometimes backward to open at the mental foramen (6). Because this segment of the canal (7) distinctly departs from the horizontal pathway and conducts a branch of the nerve, some authors prefer naming it the mental canal.

Because of the mental canal's oblique course, the mental foramen is often projected upon the apex of the second or, atypically the first premolar. The oblique course also gives the canal an almost U-shaped backward and upward sweep on some periapical projections.

In relation to the teeth, the canal in a few cases may lie well below their roots, closer to the inferior margin of the jaw than to the apicies of the roots. This occurs in persons who have a relatively high mandibular body combined with roots of moderate length. When the individual has a low mandibular body and long roots the canal appears to contact all molars and the second pre-molar. This situation is more common in children, whose mandibular body height has yet to increase by appositional growth at the alveolar crest accompanied by normal tooth eruption, than in a fully mature adult.

In some individuals the canal appears to be situated above the level of the molar apices (8) or, rarely, the bicuspid apicies (9). This is due to superimposition of the canal over the roots. Anatomically the canal (10) lies slightly buccal to the apices of the roots of the teeth (11). Thus a canal positioned high (12) on the mandibular body can appear to contact the teeth. Only very rarely are the roots of an abnormally inclined third molar in an atypically lingually offset alveolar process buccal to the canal.

No matter where it lies in relation to the roots of the teeth, when teeth are lost the canal is higher on the mandibular body, closer to the alveolar crest. Tooth loss is always associated with

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