SKULL OF A HUMAN FETUS OF 40 MM. 389 



lateral edge of the floor there is seen to project, in a direction 

 downward and outward, the short, rod-like processus alaris. 



When the model is regarded from the side (fig. 3), the floor of 

 the sella tm'cica, which is slightly concave cranially, appears as 

 the upper expanded end of the planum basale, the ventral wall 

 of the sella appearing as a continuation upward of the ventral 

 surface of the planum; indeed if the ventral border of the latter 

 (which we have noted is almost straight) is projected upward it 

 will pass just ventral to, and parallel with, the ventral wall afore- 

 mentioned. From the same position, too, the ventral wall of 

 the sella appears as the dorsal expanded end of the cranio-ventral 

 or horizontal limb of the central stem of the chondrocranium, 

 and the appearance is as if the caudo-dorsal edge of this had 

 been applied to the cranio-ventral edge of the vertical limb, 

 making an angle of 115°, open caudo-ventrally, the flattened 

 extremities forming the ventral wall and floor of the sella turcica 

 respectively. Ventrally (fig. 2) this angle is seen to be quite 

 sharply marked, and to lie on the line between the ventral edges 

 of the roots of the processus alares, or, just below the level of 

 the ventral wall of the sella. Ventral to this angle the cartilage 

 of the central stem gradually becomes narrower from side to side, 

 and expanded caudo-cranially, passing ventrally into the nasal 

 septum (fig. 10). 



If we now turn our attention again to the upper surface of the 

 median stem, we pass forward from the sella turcica, over the 

 rounded, transverse, tuberculum sellae, and come upon the flat- 

 tened lamina hypochiasmatica (fig. 1) (sulcus chiasmaticus of 

 Levi), triangular in shape, with the apex placed ventrally in the 

 midline, and the base formed bj'^ the upper edge of the ventral 

 wall of the sella turcica. This surface underlies the optic chiasma, 

 and is horizontal, thus making almost a right angle with the ven- 

 tral wall of the sella. Ventrally the apex rises upon the dorsal 

 edge of the interorbital septum, forming therewith an angle of 

 120°. Immediately lateral to this junction is a small, slit-like 

 foramen, which may be known as the foramen praechiasmaticum 

 (figs. 1, 10 and 14); it has been shown in the models of the skulls 

 of several mammals, as the ape (Fischer) and the rabbit (Voit), 



