SKULL OF A HUMAN FETUS OF 40 MM. 365 



If the suprafacial commissure were removed (fig. 7) it would 

 be seen that the superior cochlear prominence is continued down- 

 ward and backward as the first and widest part of the spiral 

 lamina, which here separates the upper coiled portion of the coch- 

 lear duct, in front, from the internal acoustic meatus behind. 

 In front of the upper part of the ventro-medial edge of the lamina 

 spiralis, at a point marked by the widened cranio-ventral extrem- 

 ity of the foramen for the cochlear root, the medial downward 

 continuation of the superior cochlear prominence passes over 

 upon the medial surface of the pars cochlearis. When the 

 medial wall of the pars cochlearis is removed it is seen that the 

 lamina spiralis is attached to the lateral wall of the pars coch- 

 learis, the line of attachment being in the form of a helix, which 

 makes but little more than one turn. If the lower edge of the 

 internal acoustic meatus (figs. 5 and 7) be followed forward and 

 inward it passes over the upper edge of the foramen for the coch- 

 lear root to reach the medial edge of the lamina spiralis; thenpe 

 it may be followed along the edge of the narrowing lamina, 

 whose curvature becomes progressively sharper, ending on the 

 lateral wall in a downward turn. In this way there is formed a 

 commodious recess for the upper coiled part of the cochlear duct 

 and its surrounding space. In all the cochlear duct makes about 

 two turns (fig. 8). 



The internal acoustic meatus (figs. 5 and 7) presents a rounded 

 border, although its edges are somewhat straightened below 

 and behind. The dorsal portion of the upper border is s'harp, 

 and represents the medial edge of the foramen faciale. Passing 

 caudally the edge becomes less sharply marked on the dorsal 

 side, the cartilaginous surfaces which form it here meeting at a 

 right angle. Ventrally we come upon the caudal edge, which is 

 very sharp indeed, and represents the upper edge of the base of 

 the massa pyramidalis. The dorsal and caudal edges form a 

 rounded angle, and about 1 cm. below this point the entrance 

 of the foramen singulare appears. Passing upward from the 

 ventral end of the lower edge we come upon a crescentic and 

 illdefined border which delimits the meatus cranio-ventrally, 



