SKULL OF A HUMAN FETUS OF 40 MM. 329 



the otic capsule and the planum basale, the cranialmost extremity 

 of this bridge being the first to appear. 



Encircling the zone of union of the pars cochlearis with the 

 planum is a well-marked groove. The posterior portion of this, 

 which may be known as the dorsal basicochlear groove (fig. 5), 

 is crescentic in outline, and is fairly well defined. It contains 

 the inferior petrosal sinus, and appears more sharply marked 

 than in the models of Levi and Jacoby, judging from the illus- 

 trations of these authors. The ventral portion — much deeper 

 and narrower — is also crescentic, and may be termed the ventral 

 basicochlear groove (fig. 2). These grooves meet, above and 

 below, their confluences being marked by notches, designated 

 the sphenocochlear and occipitocochlear notches (fig. 5) respec- 

 tively. The sphenocochlear notch occupies the interval be- 

 tween that part of the lateral surface of the corpus sphenoidale 

 which lies dorsal to the processus alaris, medially, and the medial 

 aspect of the cranial pole of the pars cochlearis, laterally (fig. 1). 

 It is narrow and deep. The occipitocochlear notch is the ventro- 

 medial extremity of the jugular foramen. 



Viewed from behind (figs. 1 and 5) the planum is seen to be 

 concave from side to side in its lower two-thirds, and caudo- 

 cranially throughout its entire extent. The cranial third of the 

 dorsal surface terminates above in the crista transversa, a trans- 

 verse ridge from which the dorsum sellae springs upwards, and 

 which, according to Voit, may be taken to mark roughly the 

 boundary between the body of the sphenoid and the otic or upper 

 portion of the planum basale. This area of the dorsal surface 

 is convex from side to side, and covers the part of the planum 

 which has been called the clivus of Blumenbach. The lower 

 portion of the planum is quite steep, but the cranial portion is 

 much more so, the inclination thus agreeing with the descrip- 

 tion given by Levi for the 28 mm. stage. 



The ventral surface of the planum (fig. 2) is convex from side 

 to side, and almost straight caudo-cranially. It is thus evident 

 that the caudal and cranial extremities are thicker than the 

 middle portion, the latter being in the position of a primitive gap 

 in the cartilage as shown by Levi in the 13 mm. and 14 mm. 



