72 



SKULL OF A 43-MM. HUMAN FETUS. 



the "pyramidal mass." The spiral septum and 

 this ridge seem to be approaching one another over 

 the cochlear duct and spiral ganglion and it is 

 probable that in this way the first portion of the 

 cochlear canal space is cut off — indeed it would 

 seem that the canal coils are gradually separated 

 in this way. As yet, however, there is a wide 

 interval separating these ridges. 



Internal acoustic meatus. — The upper border of 

 the meatus (fig. 17) is formed by the inner edge of 

 the suprafacial commissure and a posterior con- 

 tinuation of this salience, while the lower border, 

 lying considerably nearer the median plane, 

 represents the upper edge of the medial wall of 

 the cochlea. Both of these borders are sharply 

 marked, while the anterior and posterior, joining 

 them, are more rounded. From the anterior 

 border is continued backward and downward the 

 beginning of the spiral septum; the free edge of 

 this septum may be followed forward and inward 

 as it curves sharply to meet the inner border of the 

 meatus, thus outlining the anterior end of the 

 inferior acoustic foramen. The intermediate 

 portion of the posterior meatal border is formed by 

 the wall of the elliptic recess, which, as we have 

 noted, narrows and descends into the depths of the 

 meatus to join the upper part of the spiral septum 

 and thus to form the transverse crest. 



The superior acoustic foramen, opening into the 

 elliptic recess, is situated lateral to the upper part 

 of the crest, and looks directly forward, so that it 

 can not well be seen in figure 17. The position is 

 indicated by an arrow in figure 15. It represents 

 the future superior vestibular area. Lying medial 

 to the crest, and delimited medially by the sharp 

 inferior border of the meatus, is the elongated 

 inferior acoustic foramen. This is not shown well 

 in figure 17, since it looks directly upward, but 

 figure 15 discloses its medial wall. When viewed 

 directly from above it is seen that the spiral 

 septum approaches the ridge upon the medial 

 cochlear wall (already mentioned) and thus the 

 foramen is incompletely divided. In la there was 

 a narrow cartilaginous union here. The anterior 

 portion, slightly the larger, is the site of the future 

 central canal and spiral foraminous tract; the 

 posterior portion represents the inferior vesti- 

 bular area and the foramen singulare. The latter 

 opens into the cartilaginous canalis singularis 

 (fig. 18) carrying the branch of the vestibular nerve 

 to the ampulla of the posterior canal, and in this 

 respect resembles the condition in la and also in 

 the rabbit (Voit). 



Thus we have, as in the mature bone, a space 

 above the transverse crest, representing the 

 fossula superior, containing the passageways for 



the facial nerve and the superior division of the 

 vestibular nerve, and a space below the crest, 

 representing the fossula inferior, for the inferior 

 division of the vestibular nerve. 



The facial nerve is shown in the internal acous- 

 tic meatus in figures 15 and 16. It passes almost 

 directly outward, and a little downward, and lies 

 in front of the vestibular ganglion. The genicular 

 ganglion is placed a little beyond the lateral edge 

 of the suprafacial commissure. 



The vestibular ganglion is crescent-like in form. 

 Its outer end— a thin point projecting laterally 

 and forward — underlies the commissure, but does 

 not reach as far as its outer border. It lies in 

 front of the superior acoustic foramen. The 

 upper extremity is tucked under the inner margin 

 of the meatus. The ganglion narrows as it 

 curves downward and backward, presenting a 

 medial convexity, and terminates deep in the 

 caudal end of the inferior acoustic meatus. 



Massa angularis. — The angular mass, already 

 referred to, is seen from the front in figure 24 and 

 from below in figure 25. In figure 15 it appears 

 from within, in its relation to the canal spaces and 

 to the remaining cartilage of the capsule. Its 

 lateral surface, quite smooth, has been noted in 

 figures 5, 6, and 20. Thus it extends from the 

 medial to the lateral surface of the pars canali- 

 cularis. 



In direct antero-posterior views it presents a 

 triangular outline, the lateral surface forming the 

 base, and the region of the fossa subarcuata 

 representing the apex. The mass of cartilage 

 projecting from the lower border, in figure 24, is 

 the trabecula inclosed by the lateral semicircular 

 canal. In figure 25 it is seen trimmed more 

 closely to the angular mass proper. A similar, 

 though somewhat less obvious, triangular outline 

 appears when the mass is viewed from above, or 

 from below, as in figure 25, all the borders being 

 more rounded and the apex, still in the same 

 locality, being less marked. Seen from within or 

 from without, the mass is roughly quadrilateral. 

 The relation of the semicircular canals to its 

 anterior, medial, and lower aspects may be seen 

 from the figures. The fossa subarcuata pene- 

 trates deeply into its interior and ends in a very 

 slight dilatation. Underlying the space for the 

 crus commune (fig. 15), there is a ridge which 

 becomes sharpened posteriorly, where it joins the 

 trabecula inclosed by the posterior canal space. 

 The cartilage is lightly staining with scattered 

 nuclei. 



The space within the otic capsule is very large. 

 Figures 15 and 23, of the lateral and medial walls, 

 and figures 18 and 21, which show the external 



