SKULL OF A 43-MM. HUMAN FETUS. 



63 



posterius of the reptiles. In the lizard the pro- 

 cessus ascendens has been described by Gaupp 

 (1900) and by Rice (1920). Rice states that in 

 the lizard it gives protection to the endolym- 

 phatic sacs, which lie upon either side of it. Such 

 a function is, of course, out of the question in the 

 skull of homo, since the endolymphatic sacs are 

 very remote from this position. 



In the skull of la I described a small free nodule, 

 with a very minute cartilaginous fragment besi le 

 it. These I termed the posterior cranial carti- 

 lages and suggested that they might represent tne 

 unpaired, elongated, transverse, free nodule of 

 cartilage described by Bolk in human chondro- 

 crania, which lay in the membrane in the midline 

 some distance above the tectum posterius (and 

 ascending process) in the position of the future 

 interparietal bone and which seemed to be under- 

 going regression in his second stage, shown in his 

 figure 2. On account of the fact that the sections 

 of la were missing just behind these nodules I was 

 unable to ascertain their exact relationship to the 

 tectum. In No. 886 there is no cartilage corre- 

 sponding to this single nodule of Bolk. 



BASIOCCIPITAL CARTILAGE. 



The basioccipital cartilage has been described 

 with the basal plate. Its ossification center 

 (figs. 1 and 2) is already indicated in a single area 

 of cartilage undergoing the change preliminary to 

 ossification. This area occupies practically the 

 entire thickness of the plate, as shown in figure 11, 

 and extends for about 700 micra antero-pos- 

 teriorly. Its extremities, however, are indefinite 

 and the cartilage for some distance in front of and 

 behind it is of an older type than that toward the 

 sides of the plate. Its innermost portion shows 

 the greatest degree of change in the cartilage cells 

 and there is a gradual transition toward the normal 

 condition peripherally. Its anterior end (fig. 11) 

 is traversed by the notochord, which is here much 

 attenuated. 



EXOCCIPITAL CARTILAGE. 



Traced backward, the exoccipital cartilage 

 undergoes a progressive widening together with 

 a twisting and bending, the outer edge be- 

 coming tilted upward and the upper surface 

 looking more and more directly inward and for- 

 ward. The hypoglossal foramen shows no parti- 

 tion on either side. Its large and stout medial 

 border is directly continuous behind with the 

 thickened margin of the foramen occipitale 

 magnum. This bar is bent downward to form the 

 rounded occipital condyle, best seen in side views 

 of the skull. The lateral border of the foramen 



is comparatively slender and is placed at a con- 

 siderably higher level. It joins the anterior end 

 of the jugular tubercle. 



The jugular tubercle (fig. 1), which separates the 

 hypoglossal foramen from the jugular cave lying 

 laterally, is best marked anteriorly, where it is 

 thin and high. As Terry (1917) remarks for the 

 cat skull, it "presents much more the form of a 

 ridge than of a tubercle." Passing almost directly 

 backward, it mounts upon the rising surface of the 

 exoccipital cartilage, where it becomes lower and 

 broader. It terminates by curving inward and 

 is lost upon the rounded margin of the foramen 

 occipitale magnum at the posterior condylar 

 notch. Later, in the occipital bone, the jugular 

 tubercle comes to overlie the hypoglossal canal, 

 and even reaches forward beyond it. Through- 

 out its extent the ridge exhibits characteristic 

 preossification changes, and we have here the 

 beginning ossification center of the exoccipital 

 fundament. 



This center is shown in figure 1. Its posterior 

 half is somewhat wider than the anterior and 

 evidences the greatest amount of change in the 

 cartilage. Here, too, in contrast to the anterior 

 part of the center, the process involves the entire 

 thickness of the plate, the area in the posterior 

 condylar notch in figure 2, and better still in 

 figure 4, showing it upon the ectal surface. Here 

 the process has extended to the border of the fora- 

 men occipitale magnum, at the posterior condylar 

 notch (figs. 5 and 6), thus implicating the neural 

 arch of the occipital vertebra. In la the preossi- 

 fication change was farther advanced. 



Medial to the jugular tubercle, the surface 

 slopes down to the foramen magnum — more 

 steeply behind than in front — and passes over 

 upon the condylar surface, which here projects 

 medially into the contour of the foramen (fig. 1). 

 Just behind the hypoglossal foramen there is a 

 hollow for the hypoglossal nerve. 



Lamina alaris. — Lateral to the jugular tubercle 

 is the lamina alaris, deeply grooved to form the 

 sigmoid sulcus. Its outer edge, confluent above 

 with the neighboring otic capsule, does not extend 

 so far forward as the jugular tubercle. Ventrally 

 the plate terminates as a thin concave lip, over 

 which the sigmoid sinus empties into the jugular 

 foramen. The edge of this lip, slightly convex 

 anteriorly, shows a down-turned tip (fig. 10) which 

 strikingly resembles the corresponding formation 

 in the osseous condition. The ventral part of 

 the lamina alaris is formed by the paracondyloid 

 process. 



The floor of the sigmoid sulcus rises steeply 

 behind, thus bounding the jugular recess posteri- 



