DEVELOPMENT OE THE SKULL IN STURGEONS. 
169 
the pituitary body as lying in a separate and very solid box, for the rest of the skull 
is quite distinct from it, above. This arises from the fact that the foramina ovalia are 
here, with the trigeminal nerves (V.); the alisphenoidal wall, the roof, and the 
thickened sphenotics are seen in the upper part of the section. The parasphenoid 
( pct.s .) curls round the flatter base, the parietal (pg. by mistake for p'.) and post- 
orbtial scutes were cut through. Below, the adductor mandibukc muscles ( ad.m.), the 
pterygoid cartilages and bones (pg.q., pg. q., read pg.) are severed, and the latter are 
in one piece, for this is behind their great notch. The smaller azygous metapterygoid 
(' mt.pg' .) is seen; the extreme angle of the quadrate (q.c.) ; and the hinge and part 
of the shaft of the mandible (mk) through the dentary and the pre-opercular bones. 
Section 14.-—Here (Plate 16, fig. 6) the breach in the lower part of the walls is 
repaired, for this is behind the foramina ovalia, and through the fore part of the auditory 
capsules (cm) ; the ampulla and front part of the arch of the anterior canal (a.s.c.) are 
laid open. The roof is thicker and flatter; the sphenotic wings (sp.o.) are still large 
and thick; the cavity of the skull is single, narrower, and is half a long ellipse in 
shape, the angles of the broad upper end being rounded. Here the notochord is not 
seen, it has retreated too far backwards, but the parachordal mass (iv.) is hugely thick, 
and is wider; it is close behind the post-clinoid wall. The fore part of the hind-brain 
(C 3 .) is cut across, where it is giving off the facial nerves (VII.), close behind the roots of 
the trigeminals (fig. 5, V.). The last section was cut through the interspace between 
the pterygoid band and the orbitar process (fig. 5, pg.q., q.c, ; and see also Plate 16, 
figs. 1 and 7); here the back of the suspensorium (q.c.) is cut through, at the hind part 
of the maxillary bone and the orbitar process (wise., or.p). A notch seen in the hind 
margin of the pterygo-quadrate cartilage (Plate 16, fig. 5, pg.q.c) is cut across here, 
and the median part of the cartilage crops up, again, right and left; between these 
the main azygous metapterygoid (mt.pg'.) is seen. The angle of the mandible (mk.) 
and the end of the dentary (d.) are also cut across. 
Section 15. In this section (Plate 17, fig. 7) the chondrocranium attains its greatest 
solidity, and here the small cranial notochord (nc.) has its point cut through. The 
sphenotic has given place to the “pterotic” region, the arch of the anterior and the 
ampulla of the horizontal canals (a.s.c., h.s.c.) are exposed, and so also is the auditory 
nerve (VIII.), as it arises in the hind-brain (C 3 .) and runs through the meatus internus 
into the vestibule (vb.). Here the tegmen cranii is thick and concave, and the pterotic 
expansions (pt.o.) are very rough and lobulate. Under the bulging sinuous para¬ 
chordal mass the parasphenoid (pa.s.) has become very wide, and now sends down its 
free edges. Under the capsules the “protractor hyomandibularis” muscles (pt.hrn.) 
are seen as huge triangular masses, and the “ adductor mandibulse ” muscles (cid.m.) 
are cut through in their hinder part. A little of the pterygoid region (pg.q.) and the 
hind angle of the quadrate (q.c.) come into view, and between the former the median 
and main lateral metapterygoids (mt.pg'., mt.pg".). The deepest part of the pterygoid 
bone (pg.) is seen flanking the inner face of the suspensorium. Under the quadrate 
MDCCCLXXXII. Z 
