94 



SKULL OF A 43-MM. HUMAN FETUS. 



of the mandibular notch, some distance below and 

 medial to the zygomatic process of the squama 

 temporalis. The lingual is shown in the figures 

 descending to the medial side of Meckel's car- 

 tilage, and being joined by the chorda tympani. 

 The borders of the mandible may be briefly 

 summarized. Beginning above at the sharply 

 marked condyloid process, and passing forward 

 and downward, we traverse the mandibular 

 notch — shallow when compared with that of the 

 adult condition. From this the thin cranial edge 

 of the bone is passed over. It is fairly smooth. 

 Reaching the region of the mental foramen, the 

 border becomes much more irregular, showing 

 deviations from side to side and from above down- 

 ward. There is a large notch just in front of the 

 lower part of the foramen. This border termi- 

 nates upon a sharp point projecting medially. 

 Beginning again at the condyloid process, but 

 proceeding backward, we pass downward over the 

 dorsal border of the ramus. Between the con- 

 dyloid process and the angle there is a strong 

 outward convexity. From the angle downward 

 the border becomes thicker and is somewhat 

 roughened. As has been mentioned, it is turned 

 inward, so as to present a salient edge when 

 viewed from w T ithin (figs. 33 and 35). In la 

 there was a very thin fissure visible upon the 

 lateral surface which partially cut off this edge 

 from the main plate. This fissure is only sug- 

 gested in this specimen, as a very faint incutting 

 of the bone, parallel to the caudal border and 

 visible from without. 



The turning inward of the base of the mandible 

 does not occur below the level of the upper border 

 of the mental foramen; on the contrary, when the 

 bone is regarded from behind, it is seen that there 

 is a sweeping outward of the border at and below 

 this level, to form an outward projection. The 

 curvature thus described marks the caudal limit 

 of the concave area below the medial alveolar wall, 

 already described. The border, continued down- 

 ward and forward, passes over upon the symphy- 

 sial portion, which shows fine serrations and 

 gradually approaches its partner of the opposite 

 side. This border ends, like the cranial, upon the 

 ventrocranial tip of the bone. 



The condensed mesenchyme of the mandible 

 follows, in general, the outlines of the bone. It is 

 confluent with its partner of the opposite side at 

 the symphysis by a wide area, as shown in figure 

 36. The groove occupied by Meckel's cartilage is 

 rather more conspicuous and, of course, there is a 

 general increase in thickness of the mass. 



The developing teeth, seen in figure 36, occupy 

 the tooth gutter but do not extend as far back as 

 the medial alveolar wall. As the figure shows, 

 there are two main masses, connected by a narrow 

 isthmus. This narrow point is just in front of the 

 upper extremity of the mental foramen, at a 

 point where we have noted the occurrence of a 

 rod of bone joining the medial and lateral alveolar 

 walls. The mesenchyme here is, of course, of 

 greater volume than the inclosed bridge. 



The nasal bone has not yet formed. 



