SKULL OF A 43-MM. HUMAN FETUS. 



93 



the mental tubercle. It is situated some dis- 

 tance below the mental foramen and forms a pro- 

 jection upon the lower border of the bone. Upon 

 the corresponding inner surface there is a small 

 depression. The lateral surface, in the region 

 of the mental foramen and below it, is rather 

 rough. 



Turning now to a consideration of the medial 

 wall, we note as its most prominent characteristic 

 the plate of bone forming the medial wall of the 

 inferior alveolar groove. This, the so-called 

 splenial element, is highest in the region of the 

 mental foramen. In front of this it becomes much 

 thicker, and progressively lower, passing forward 

 and inward to terminate as a thin spur of bone, 

 with an extremity turned a little upward. Behind 

 the foramen, too, the ridge decreases gradually 

 in height and extends backwards rather more 

 than two-thirds of the distance from the anterior 

 tip of the bone to the angle, as shown in figures 

 33 and 35. The upper margin is much serrated 

 and lies about on the same level as the lateral wall 

 of the inferior alveolar groove, which also shows 

 a very jagged edge. A little in front of the mental 

 foramen there is a narrow bridge of bone joining the 

 medial and lateral alveolar walls, which covers the 

 incisive nerve, as shown in figure 34. The upper 

 border of the medial alveolar wall, barring the 

 irregularities mentioned, has a direction in general 

 straight, with the exception of its caudal extremity, 

 which turns upward a little. On the other hand, 

 the attachment of the plate to the main portion of 

 the mandible follows a curved line, with con- 

 cavity upward and forward. This line crosses 

 almost the entire bone, beginning in front not 

 far from the anterior extremity of the mandible, 

 and continuing backward with an increasingly 

 strong inclination upward, almost to the posterior 

 border of the bone. This accounts for the vary- 

 ing depth of the inferior alveolar sulcus — deep 

 in the region of the mental foramen, and decreas- 

 ing in front and behind — following the variation 

 in height of the inner wall. Where the wall ends 

 dorsocaudally the alveolar groove is continued 

 as the groove formed by the inturning of the base 

 of the bone, as far as the condyloid process 

 (figs. 33 and 35). 



Below the medial alveolar wall there is a region 

 presenting a marked inward concavity. This 

 is open behind, due to the fact that the lower bor- 

 der does not turn inward here, but rather pro- 

 jects outward to form a salient upon the lateral 

 aspect. In this concavity rests the lower end of 

 Meckel's cartilage (figs. 31 and 34). In the 

 depths of this concavity there is an area, shown 

 in figures 33 and 35, where the bone and Meckel's 



cartilage are in contact; indeed, the appearance is 

 as though the perichondrium were becoming 

 ossified here. The cartilage, too, in this region 

 (fig. 32) shows evidence of calcification and is 

 evidently undergoing the change preliminary to 

 ossification. Similar findings were described by 

 Low and were also reported in my former paper. 

 The area marked involves the region of confluence 

 of the medial alveolar wall with the main plate 

 and extends a short distance downward from this. 



In connection with the medial surface, the 

 relation of Meckel's cartilage should be described . 

 We have noted that there is a short portion of the 

 rod which lies between the uppermost extremity 

 of the mandible and the head of the malleus. 

 Opposite the mandible it lies, as figures 31 to 35 

 show, in the groove which has been described. 

 As it descends it comes to lie medial to the upper 

 end of the medial alveolar wall and in close con- 

 tact with this. It crosses the upper border of this 

 wall at an angle, its cranial border falling some- 

 what behind and below the edge of the bone, as 

 figures 31 and 34 show. From the angle of the 

 mandible to its lower extremity the posterior 

 edge is turned inward partially to envelop the 

 cartilage, except in the region already described. 



The position of the inferior alveolar nerve, as 

 shown in figure 34, is interesting, as marking the 

 position of the future canal. A short distance 

 behind the coronoid process and below the man- 

 dibular notch, as shown in the figure, the nerve 

 comes to he quite close to the bone. At this 

 point it lies just behind the ridge which has been 

 mentioned and which occurs as a groove upon 

 the outer surface. This region doubtless repre- 

 sents the site of the future mandibular foramen. 

 The nerve, from this point, courses fairly close to 

 the bone, crossing the ridge obliquely from 

 behind forward, to divide, a little before the men- 

 tal foramen is reached, into the mental and in- 

 cisive branches. These nerves appear in figure 

 34, the former passing through the upper extrem- 

 ity of the mental foramen, the latter continuing 

 downward, in the depths of the alveolar groove, 

 to the tip of Meckel's cartilage. 



The remainder of the mandibular nerve, in 

 so far as it relates to the mandible, may be 

 briefly described. The stout trunk seen in 

 figures 31 and 34 shows, springing from almost the 

 same region, the auriculo-temporal, masseter, 

 lingual, and inferior alveolar branches. The 

 auriculo-temporal, showing two strands, winds 

 backward and outward, around Meckel's car- 

 tilage, and between it and the condyloid process, 

 to emerge upon the face. The masseter has 

 been referred to as passing over the representative 



