SKULL OF A 43-MM. HUMAN FETUS. 



69 



cavity), is best appreciated from a study of the 

 figures. Its walls, except at certain regions to be 

 described, are thin. 



Cranial surface. — The entire cranial surface of 

 the capsule is seen in frank view in figure 17. The 

 pars cochlearis is here quite smooth and flattened, 

 and presents the wide internal acoustic meatus. 

 The more irregular pars canalicularis shows dis- 

 tinct rounded eminences for the superior and 

 posterior canal spaces, above which is the groove 

 for the transverse sinus. The relation of this 

 surface to the inner cavity is ascertained by a com- 

 parison of figures 17 and 15. The capsule was 

 drawn from the same viewpoint in the two cases, 

 but in figure 15 most of the medial wall has been 

 removed. Above the space for the superior canal 

 is the cut edge of the capsulo-parietal commissure, 

 interrupted by the long capsulo-parietal foramen. 

 The posterior slender limb of this commissure is 

 joined to the angular mass of the capsule. Pro- 

 ceeding downward, we note the connection with the 

 squama occipitalis, broken by the large capsulo- 

 occipital foramen. The cut edges of these unions 

 are seen in figure 14. Their line of attachment is 

 along a rounded border separating the medial and 

 lateral surfaces of the pars canalicularis. The 

 subarcuate fossa, overarched by the superior 

 semicircular canal, is very deep. It contains only 

 loose connective tissue and small vessels. Just 

 behind it, in the prominence for the crus com- 

 mune, is the endolymphatic foramen. It is wide 

 and elongated and has very thin borders of young 

 cartilage. The ductus endolymphaticus, travers- 

 ing the foramen very obliquely, fills but a small 

 fraction of it, the remainder being completed by 

 membrane. The relationship of the transverse 

 sinus to the endolymphatic duct and sac of this 

 embryo may be seen in plate 5 of an article by 

 Streeter (1918). In la the endolymphatic for- 

 amen was narrower and had thicker walls; there 

 was a short process projecting dorsally from the 

 upper lip, which is not present in No. 886. 



Behind the endolymphatic foramen is a sharp 

 spur of cartilage which projects inward and back- 

 ward medial to the transverse sinus. It is a short 

 distance below and behind the endolymphatic sac. 

 Underlying it the wall is quite thick, so that the 

 posterior canal is removed some distance from the 

 surface here. The inferior ampullary prominence 

 is conspicuous below. It terminates in the 

 posterior intraperilymphatic process, which pro- 

 jects downward into the perilymphatic foramen. 



Lateral surface. — The lateral surface can not be 

 satisfactorily seen from one standpoint. Figure 

 20 presents the most comprehensive view. From 



below upward may be distinguished the cochlear 

 vestibular, and canalicular areas. 



The cochlear area, looking principally down- 

 ward, is very convex and bulging. It shows a 

 very shallow furrow, the septal sulcus, which has 

 the form of a helix and is very indefinite towards 

 the pole (fig. 20). It corresponds to the line of 

 attachment of the spiral septum upon the in- 

 terior of the capsule (fig. 15). The internal 

 carotid artery lies in a short length of this groove, 

 as seen in figure 2. Turning upward around the 

 cranial pole of the cochlea, it traverses the lateral 

 side of the carotid foramen, keeping close to the 

 alicochlear commissure, and thus gains the 

 cranial cavity. Just lateral to the ventral basi- 

 cochlear groove is a long prominent rounded ridge, 

 formed by the space for the first turn of the 

 cochlear duct. This terminates posteriorly in the 

 promontory; anteriorly it turns upward to end at 

 the rounded ventral pole of the cochlea. Some- 

 what below the apex of the pole (figs. 1, 6, 20, 23) 

 is the slender alicochlear commissure. 



There is, at this stage, no evidence of the small 

 supracochlear cartilage, which I described in la 

 just above the cranial pole of the cochlea and below 

 the semilunar ganglion. Its position in la is 

 somewhat above that of the union of the ali- 

 cochlear commissure with the cochlea, found in 

 No. 886. 



The vestibular area is sharply marked off 

 medially by the rounded contour of the cochlea 

 and is laterally blended with the inferior canali- 

 cular surface. It shows, adjoining the promon- 

 tory, the large vestibular window. Above is 

 seen the facial foramen, surmounted by the 

 suprafacial commissure. Across this area the 

 facial nerve courses (fig. 16), lying close to the 

 cartilage. The great superficial petrosal nerve is 

 seen leaving the geniculate ganglion. 



The canalicular area is crossed by a ridge which, 

 beginning above at the capsulo-parietal com- 

 missure, runs downward over the tegmen tym- 

 pani and then backward and downward over the 

 parotic crest to end at the dorsalmost extremity 

 of the jugular foramen. Medial to this ridge the 

 surface looks principally downward and extends 

 inward as far as the inferior ampullary promi- 

 nence. The major portion of the canalicular 

 surface, lying behind and above the ridge just 

 outlined, looks directly outward (figs. 5, 6). It is 

 somewhat convex, especially from before back- 

 ward. All of the canals make prominences, but 

 these are all very low and rather indefinite. The 

 central and superior, almost plane, portion of ttie 

 surface belongs to the angular mass, a large lump 



