SKULL OF A HUMAN FETUS OF 40 MM. 363 



projection directed forward from the inferior utriculoampul- 

 lary prominence. The cochlear fenestra is apparently closed off 

 on the left side of the Hertwig model, and almost so on the right. 



The perilymphatic fenestra (fig. 2) is sharply concave, from 

 before backward, the direction of the concavity being downward. 

 Its inner wall is formed by the lower edge of the massa pyramidalis 

 of the median wall of the vestibular portion. When regarded 

 from below the circumference of the perilymphatic fenestra 

 appears to have been formed by the bifurcation of the inferior 

 cochlear prominence at the promontory, the lateral limb forming 

 the promontorium; the medial the lower border of the massa 

 pyramidalis; the two limbs uniting dorsally in the inter-perilym- 

 phatic process. 



The boundaries of the medial surface have already been noted 

 (fig. 7). It is quite smooth, and is more flattened than the 

 lateral. Ventrally the elongated, narrow, crescentic line of union 

 with the basal plate may be seen; immediately ventral to and 

 parallel with this, the everted, narrow, extra-cranial surface, 

 formed by the medial aspect of the inferior cochlear prominence, 

 makes up the outer wall of the ventral basicochlear groove 

 (fig. 2) as a strip 1.5 mm. wide. 



Dorsal to the basal lamina the medial surface is intracranial, 

 the strip immediately bordering the lamina being concerned in 

 the formation of the outer wall of the dorsal basicochlear groove 

 (fig. 5), which is sharply marked throughout, but more so above 

 than below. In the dorso-cranial area of the medial surface 

 the large, deep, meatus acusticus internus appears (fig. 7); 

 below, the surface passes into the caudal surface of the pars 

 cochlearis and behind into the medial surface of the pars 

 canalicularis. 



If we now consider, briefly, the passageway of the ductus coch- 

 learis (figs. 8 and 9) we find the first, or uncoiled part, outwardly 

 deflected at and for a short distance beyond its entrance from the 

 perilymphatic fenestra. The lateral wall is here quite thin, but the 

 opposite medial wall presents a pronounced conical thickening, to 

 which reference has been made as the massa pyramidalis (fig. 2). 

 The apex of the pyramid projects laterally into the first portion 



