SKULL OF A 43-MM. HUMAN FETUS. 



87 



1914, p. 416). The densest bone (surrounded by 

 a dotted line in fig. 1) forms a crescentic plate of 

 homogeneous osseous tissue, from which gradually 

 narrowing spicules radiate to the periphery. The 

 relation of the bone to the cerebral hemisphere is 

 seen in figure 7. 



The orbital portion of the bone forms an antero- 

 lateral extension of the surface of the orbito- 

 sphenoid and spheno-ethmoidal cartilages. Its 

 lower surface is very slightly concave, being 

 molded for the orbital cavity, and its upper sur- 

 face shows a corresponding gentle convexity, 

 where it forms the peripheral portion of the floor 

 of the anterior cranial fossa. Its tissue, although 

 reticular throughout, is rather denser than that 

 of the frontal part above the region of the supra- 

 orbital ridge. The inner margin of the plate lies 

 but a short distance from the edge of the ala 

 orbitalis and spheno-ethmoidal cartilage (fig. 1), 

 though never in actual contact. In la this edge 

 had grown under the cartilages mentioned. 



It is apparent from a comparison of the mature 

 bone with the present specimen that the spheno- 

 ethmoidal cartilage must undergo resorption, the 

 orbital plate growing backward and inward, to 

 join the ethmoid and the lesser wing of the sphe- 

 noid. The result of this process is to form prac- 

 tically a right angle by the approximation of 

 the margins articulating with the sphenoid and 

 with the ethmoid. Already there is an indication 

 of the beginning of such an angle in a spur of 

 bone jutting inward and backward from the mar- 

 gin in question toward the spheno-ethmoidal 

 cartilage. 



The frontal portion forms by far the largest part 

 of the bone and makes with the orbital portion a 

 very obtuse angle. It presents a concavity look- 

 ing inward and upward ; in antero-posterior planes, 

 however, this concavity does not appear, the 

 bone in such sections being almost straight. Its 

 margin describes rather more than half a circle. 

 Upon the exterior no frontal tuberosities are yet 

 apparent. 



MAXILLA. 



The maxilla is cancellous in structure. In it 

 the main features of the bone as at term can 

 be made out. The facial surface looks ventro- 

 laterally— much more laterally than in the adult. 

 It shows a gentle outward convexity and, although 

 presenting roughenings and small holes, is never- 

 theless smooth in comparison with the medial 

 or palatine surface, which is very rough on account 

 of numerous bony excrescences, and is very slightly 

 concave. The palatine surface is seen from within 

 in figure 26, and from the front and a little above 



in figure 27, but neither of these figures gives a 

 frank view of this surface. The facial surface is 

 shown from below in figure 3 and from the side in 

 figure 5, but here, again, its full area does not 

 appear. If the bony processes are removed, these 

 surfaces become very small. It is hardly possible 

 to speak of the orbital and infratemporal surfaces, 

 for the area which represents them embraces 

 merely the groove for the infraorbital nerve and a 

 very insignificant region behind this, which comes 

 to an end at the caudal extremity of the alveolar 

 process. 



The palatine surface presents a very interesting 

 condition. There is a distinct wide groove con- 

 taining mesenchyme, which begins above just in 

 front of the apex of the frontal process and tra- 

 verses the bone to end below 0.2 mm. from the 

 medial extremity. The lower end is wider than 

 the upper and is straight, while the upper turns 

 forward a little, as shown in figure 26. This 

 groove represents the incisive suture, marking the 

 line of division between the maxillary and pre- 

 maxillary elements, and agrees in position with the 

 figures of Felber (1919), who has recently investi- 

 gated the development of the maxilla (see abstract 

 by Schultz, 1920). The borders of the groove 

 are sharply marked and conspicuous, the upper 

 being the longer and reaching to the apex of the 

 frontal process. These borders are somewhat 

 nodular and at the lower end they project medially 

 as processes, the posterior being the longer. 

 Behind and lateral to this is the alveolus for the 

 canine tooth, while medial to the lower end of the 

 anterior border of the groove, in the territory 

 of the premaxilla, are found the alveoli of the 

 lateral and medial incisors (fig. 29). The floor 

 of the groove is fairly smooth. Above, it is 

 incomplete, there being a cleft, 0.1 mm. long, 

 which extends through to the facial surface just 

 behind the anterior border of the groove. A little 

 behind this there is a much shorter fissure. 



Felber has shown that during growth of the 

 maxilla the premaxillary and maxillary elements 

 preserve a certain independence. They arise 

 from separate centers. The frontal process 

 grows upward as two spicules, a maxillary and a 

 premaxillary, separated by a cleft, and the union 

 of the two elements occurs from the alveolar 

 process upward. Thus it is that the elements 

 unite last at the apex of the frontal process. 

 Their fusion, too, is completed earlier on the 

 facial than on the palatal aspect. 



The body of the maxilla may be defined as all 

 that is left after the processes are removed, and 

 since the processes are the most prominent parts 

 they will be described first. 



