92 



SKULL OF A 43-MM. HUMAN FETUS. 



the upper, so that the two form a V-shaped trough 

 which, however, is as yet open below. The 

 upper edges are thin and suggest the edge of a 

 knife. In la a narrow fusion between the lower 

 borders has taken place near the anterior end, 

 and it is well known that such a fusion soon occurs 

 along the entire length of the bone, as the Hertwig 

 model shows, the plates growing upward and 

 hedging in the nasal septum. The two strips of 

 bone are enveloped by a common sheath of 

 mesenchyme, which is shown in part in figure 8. 

 In this figure the anterior tips of the bones appear 

 in their correct relation to the septum (which is 

 shown in negative manner as a groove) and to 

 the mesenchyme. The anterior tip comes into 

 close relationship with the posterior end of the 

 paraseptal cartilage, but lies closer to the mid- 

 sagittal plane than this. 



MANDIBLE. 



The mandible is represented at this stage by 

 paired plates of bone which lie just lateral 

 to Meckel's cartilages. The right half alone 

 has been modeled and is shown from without 

 in figure 5. The extremities of the plate are 

 wider than the intermediate portion, which shows 

 somewhat below the middle a constriction. Here 

 the lower end of the bone gives the appearance of 

 having been bent forward upon the upper, form- 

 ing a wide angle open cranially. The caudal 

 border, in contrast to the cranial, shows scarcely 

 any change in direction. Just below this con- 

 striction the large mental foramen appears and 

 from this region the bone turns inward, its ventral 

 extremity coming to lie very close to its partner 

 of the opposite side, without, however, actually 

 touching it. This inturning is represented in 

 figure 3. 



The upper end of the bone is shaped somewhat 

 like a dagger, the point, which is notched, pro- 

 jecting toward the root of the zygomatic process 

 of the squama temporalis — the region of the future 

 mandibular fossa — from which, however, it is 

 separated by a wide interval. This region, 

 obviously, represents the future condyle. From 

 it the boundaries rapidly diverge to projections of 

 bone upon the caudal and cranial borders, which 

 are almost opposite each other and which mark 

 the extremities of the widest part of the mandible. 

 The caudal projection, which also points medially, 

 is the upper extremity of the inturned caudal edge 

 or base of the mandible and represents the angle 

 and the limit of the ramus here. The ventral 

 delimitation of the ramus is not definite. The 

 ventral projection, which shows a thin spur of 



bone directed upward and forward, represents 

 the future coronoid process. Upon the border of 

 the bone joining the coronoid and condyloid proc- 

 esses, there is a gentle upward concavity which 

 is crossed, a little above, by the nerve to the 

 masscter muscle. It represents the mandibular 

 notch. 



The lateral surface is fairly smooth, somewhat 

 more so above than below. Due to the turning 

 inward of the base, there appears on the lateral 

 surface a rounded vertical ridge, which is best 

 marked above and which, as it descends, flattens 

 out and approaches the caudal border, which it 

 meets about the level of the upper border of the 

 mental foramen. Upon the medial surface there 

 is a corresponding groove, seen in figures 33 and 35, 

 as the upward continuation of the inferior alveolar 

 groove. In it, as seen in figures 31 and 34, a 

 portion of Meckel's cartilage lies, though the 

 cartilage and bone are separated by a substantial 

 interval, most marked above, where the groove is 

 deepest; indeed, in the space between the condyle 

 and Meckel's cartilage the auriculo-temporal 

 nerve is found and it occupies only a small part 

 of the available space. In front of the ridge upon 

 the lateral surface there is a vertical groove, best 

 marked above, whose cranial side passes forward 

 upon the cranial border of the bone. A corre- 

 sponding ridge occurs upon the inner surface. 

 This bending of the plate gives to cross-sections 

 of it an s-shaped appearance; indeed, such a 

 shape is perceived by looking directly at the upper 

 end of the bone. 



The mental foramen is much larger relatively 

 than that of the adult bone. It is roughly 

 quadrilateral with the long axis dorsoventral. 

 Its anterior, posterior, and upper boundaries are 

 free and made of narrow, bony plates; its ventral 

 border, on the contrary, is formed by the angle 

 made by the junction of the medial alveolar 

 wall with the lateral surface. It is a low ridge 

 showing a sharp spike of bone at its cranial end. 

 When the foramen is looked at directly from the 

 side the medial alveolar wall obstructs the view 

 completely. The plate of bone forming the dorso- 

 cranial boundary of this foramen is thin and nar- 

 row. In front and behind, the lower part of the 

 foramen opens into the V-shaped lower -art of the 

 inferior alveolar groove. 



The surface of the bone immediately below the 

 foramen is somewhat depressed, and there is a 

 distinct groove passing downward from the low- 

 est part of the opening to the ventral margin of 

 the bone. This groove serves as the anterior 

 delimitation of a small eminence of bone whose 

 position justifies the conclusion that it represents 



