bones, the basi-occipital (b.o.), basi-sphenoid (b.sp.), pre-sphenoid
(p.sp.), and in front, the ethmoid (eth.), which sends down a median
plate, not shown, in the figure, to form the nasal septum between right
and left nasal passages. Like extended wings on either side of the
basi-occipital are the ex-occipital (e.o.) (the bone is marked
in Figure 4, but the letters are a little obscured by shading).
Similarly the ali-sphenoids (a.s.), are wings to the basi-, and the
orbito-sphenoids (o.s.), to the pre-sphenoid bone (p.sp.). Between the
ex-occipital and ali-sphenoid there is wedged in a bone, the periotic
(p.o.) containing the internal ear (Section 115). Above the foramen
magnum the median supra-occipital bone completes what is called
the occipital arch. A pair of parietals (pa.) come above the
ali-sphenoids, and a pair of frontals (f.) above the orbito-sphenoids. At
the side the brain case is still incomplete, and here the aquamosal
(sq.) enters into its wall. In the external view (Figure 3) the bulla hides
the periotic bone from without. The student should examine all four
figures for these bones before proceeding.