Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/722

inter-orbital septum, and has grafted itself upon the cartilage of the pituitary ring, causing it also to ossify. On each side, behind these new bony patches, a free tongue of cartilage is seen. These tongues are the apices of the trabecular bars. They are better seen in the partial view (fig. 7). Underflooring the fore-half of the main skull base, close behind the pituitary ossifications, the thick mat of sub-mucous tissue is fast becoming bony. There is a right and left squarish patch, very large and elegant ; in the grooved mid-line a few osseous points are also seer., ready to grow right and left into the two plates, and to make them one. These are the basi-temporals (b.t.) They soon coalesce with the ossifying basi-sphenoid above, aiid they represent the handle of the dagger-shaped para- sphenoid of the Frog.^[1] The super-occipital region (s.o.) is still soft, and the exoccipitals do not reach to the condy- loid foramina (9). They begin at the hole for the vagus, and run to the selvedge at the foramen magnum (8, f.m.) The kidney-shaped transverse condyle (o.c.) has in its substance the diminishing notochord (n.c.), which, however, is better seen in the partial upper view (fig. 7, n.c.) This figure shows that the notochord has tried, as it were, to break up into three fusiform segments. The hinder of these is enclosed in an ectosteal sheath, which will affect with bony growth the neighbouring cartilage, right and left, to form the basi-occipital. The fore-part is narrower, and lies in an open space, the counterpart of the mem branous floor (" posterior basi-cranial fontanelle "- Rathke), which lies behind the pituitary space in the Snake s embryo. On each side of the middle notochordal spindle are seen the elegant mammillary elevations caused by the enclosed cochleae (cl.) The internal carotids (i.e.) are seen creeping through the pituitary hole, and on each side the ossification has been set up, which forms the true endo-skeletal basi-sphenoid. On each side of the posterior pituitary or clinoid wall, the cartilage is pinched in to let the great trigeminal nerves pass over ; but two parts of intensest interest are seen on each side of these shallow notches. These are the apices of the trabeculse (/<?.), free-ended, curved tongues of cartilage, with dark cells in their interior, and fixed to the out-turned anterior ends of the parachordal cartilages, just where the new bony matter is seen. The meaning of these parts that make the pituitary region was not seen at the time by the author of the paper we are quoting (Parker " On the Fowl s Skull"); further research was needed, and his figures are of much

greater value than his descriptions.

FIG. 9. Skull of Chiek, fourth stage, head nearly 1J inch long, beginning of

third week, X 4 diameters. The additional letters are a.s.f., alisptenoifial fenestra; p.b/., posterior basi-cranialj fontanelle; pro., prootic bone; a.i.e., anterior semicircular canal ; t.e.o., tympanic wing of exoccipital ; b.o , basi- occlpital ; 6a, foramen rotundum ; 5, foramen ovale.

Cranium of Fowl—Fourth Stage.—At the beginning of the third week of incubation the chondro-cranium is not only perfect as to form, but it has also so much increased its bony territories as to make it very useful for comparison with chondrosteous skulls of the lower types. When every investing bone has been removed, we get such a structure as is shown (from above) in fig. 9. Here the cranial basin has a short and fenestrate floor, and doubly-winged sides, which are gently scooped above, and form a pair of sym metrical ledges on which the widest part of the brain rests ; whilst the floor dips much lower down, where the medulla oblongata and the pituitary body rest. The scooped fenestrate alisphenoids (a.s.) look inward, but fail to reach the median line by a great space, which space is filled in by no orbital unngs, such as we should see in a Lizard, Turtle, or Mammal. The whole orbital region is, in the skull, a steep wall, having a retral crest on its fore-part, this crest being the rudiment of the large sheet of cartilage (tegmen cranii), which is thrown over the brain in the young Salmon.^[2] This wall (p.s.), this crest, and the elegant cartilaginous awning in front of the crest, containing valves and folded curtains of most cunning construction (see figs. 10 and 11) all these have grown out of that inter-nasal plate formed by the trabeculrc as their commissure (figs. 1 and 2, tr.)

FIG. w^jjjjug

through the external nostrils, X 8 diameters, t.n., septum nasi ; o. .n., alinasal; n.tb., alinasal turbinal, Ln.ic., lateral nasal wall ; n.px., nasal processes of premaxillaries ; d.px., dentary processes of the same ; p.px., palatine processes of the same ; pa., palatine ; mx., maxillary. On each side of the thick base of the septum the nasal nerves are seen.

Morphologically considered, these are added and distinct

elements, but their differentia tion from the trabeculae can not be seen well until now, and even now it is imperfect. The leafy coverings of cartilage are seen to dip down on each side of the prenasal spatula in front, and the cartilage at this part forms a coiled, valvular nose-lid the "alinasal re gion." From this, in its in side, there hangs a curtain, all of cartilage (fig. 10, n.tb.), the "alinasal turbinal." It curves towards the septum, and then turns upwards below to become parallel to the inturned nasal wall (l.n.w.) Behind the ali nasal comes the aliseptal region (al.s.) ; and when cut across at the notch behind it and the hinder part of the roof, it shows a doubly coiled outgrowth, the "inferior turbinal" (fig. 11, i.tb.) The hinder region or " aliethmoid " is the true olfactory region; the roof suddenly turns inward, and is coiled upon itself, so as to form the bagpipe-shaped upper turbinals, whose swollen faces look inwards to the mesethmoid. Behind their inturned part they send down a cartilaginous curtain,

the pars plana, or antorbital plate, the fore-face of which.

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1. See article Amphibia, vol. i. p. 753, fig. 5, t.cr.
2. See " Salmon s Skull," op. cit., plate 4, fig. 2, t.cr. al.n.