Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/726

and the super-occipital behind. But the latter takes in all the breadth behind, whilst below the prootic are two, the upper the smaller; the lesser bone is the opisthotic, the larger, rounding the foramen magnum at its sides, is the ex- occipital (op., e.o.) These are infero-lateral elements. Be low the whole are the basi-sphenoid and the basi-occipital, both of them underfloored by the basi-temporal (b.o.,b.l.) So much tilted backwards is the auditory mass that the crown of the anterior canal (a.s.c.) is imbedded in the super- occipital. In a Lizard, Snake, or Turtle that part would be first enclosed in a separate epiotic bone, which would be soon confluent with the super-occipital. But in these high Smtropsida the epiotic is a small, late centre, formed behind the commencement of the anterior canal in the front part of the recess in which the " flocculus " lies. Also the opisthotic is small, but is distinct for three or four months ; it is a wedge of bone, flat-faced within, forming a straight suture with the hind edge of the prootic, and externally runs as a fine thread of bone between the two fenestrse of the labyrinth. We do not see this bone behind until afterwards, and it soon coalesces with the exoccipital, first with it and afterwards with the prootic, as in Lizards and Snakes.^[1] After the elements of the chondrifying cranium have run into each other, the enclosed ear organs, by their copious growth, and also by their having many divertioula, such as the cochlea and canals, trespass on neighbouring territories, so that whilst the cochlere burrow into the parachordal region, the semi

circular canals find room in the occipital arch.

In the Osseous Fish (" Salmon s Skull," Phil. Trans., 1873, plate 5, fig. 8, sp.o.) there is a large bone called the " post-frontal " by Cuvier ; in the Bird it often occurs, and looks like a secondary wing on the great sphenoidal wing (alisphenoid). In the Fish it covers the ampulla of the anterior canal ; in the Bird it is in front of it and of the whole labyrinth. This bone, the " sphenotic," is ossified at the time of hatching.

The anterior sphenoidal region is all soft as yet (figs. 15 .and 16, p.s.); and the great mesethmoidal wall (p.e.} covers only a third of its own proper territory. It now reaches to the notch; nearly to the roof also, but not to the para .sphenoid. The cartilage it is ossifying is continued as an isthmus connecting the parts behind and in front of the .notch (cranio-facial hinge). Through this notch (fig. 16, n.tb.) we see the swollen upper turbinal ; and the nasal canal and bridge for the fore-part of the trabecular nerve is seen near the hind margin of the steep and well-formed septum nasi (s.n.), which has projecting from it the lessen ing rostrum (p.n.) Bridging over the notch, and let into the fore-edge of the frontal, are the never-coalesced nasal processes of the premaxillaries. Outside these, on each side (never in the middle in a Bird), are the nasals (fig. 15, n.) They are curious twisted bones, two-bladed in front to bind round the alinasal cartilages (al.n.) and outer nostril (e.n.) ; behind, they twist a little downwards the inner edge of their flat end. Tied by fibres to the side of the narrowed end of the frontal, and to that part of the .nasal which is imbedded into it, is the lachrymal (fig. 15, I.) Its main part is the super-orbital, and this sends down wards a facial process, narrow and sigmoid. Within the lachrymal is the pars plana (p.p.), a subquadrate curtain, which is persistently cartilaginous in the Fowl, whose nasal labyrinth, unlike that of many birds, scarcely ossifies at all, except the main dividing wall, the perpendicular ethmoid, which always early becomes solid.

The free mandibular bars are now continuous at their fore or lower end; the long and strong dentaries (figs. 15 and 16, d.) early coalesce. In front they cover the attenuat ing Meckel s cartilage (mk.) ; this, however, grows on behind, and its inner process (i.a.p.) is ossifying as the " articulare," the only endo-skeletal bone in the mandible. Behind, outside, and a little within, we see an upper and a lower splint, the surangular and the angular (su., a.), and on the inner side, further forwards, the oblong splenial .); but in this bird there has not been found a "coronoid," common in certain groups of Birds, besides Snakes, Lizards, and Crocodiles. In this and in other things the Fowl is often found wanting as to special elcmei.ts.

The changes in the hyoid arch can be left until we come to the adult stage.

FIG. 17. End view of skull of a Chicken three weeks old, sixth stage, X 3

diameters. Here the opisthotic bone appears in the occipital region, as in the adult Chelonian. Letters as above; s.c., the opening of the sinus-canal.

Cranium of Fowl—Sixth Stage.—This stage, which is that of chickens less than a month old, is introduced to show the occipital region from behind (fig. 17). This end view shows much that is ornithically characteristic. The sub ject was a somewhat starved chicken, whose retardation of growth caused a lingering of the ankylosis, which so soon removes all landmarks. Even now the great fontanelle, or membranous roof of the basin-like chondro-cranium, is barely covered by the still scant frontals and parietals (/., p.) ; their flanking by the big squamosals (sq.), and the projec tion beyond these of the sphenotics, are well shown. A wholly cartilaginous epiotic region is still seen ; it runs also inwards to the foramen magnum (f.m.), and still skirts the tympanic ala (e.o.) But on the upper and outer edge of the exoccipital a small plot is taken from the great ex- occipital. This is the appearance through the cartilage of the opisthotic (op.) ; and this represents the permanent condition of the occipital arch in the Chelonia, which shows a free posterior face of the opisthotic above and outside the exoccipital. This view also shows how the skull is double- floored by the addition of the basi-temporal slab to the ossifying chondro-cranium (b.t., b.o.)

Cranium of Fowl—Seventh Stage.—In chickens two

months old, a section of the skull shows all the sutures except those lost by early fusion of the three para-sphe- noidal elements with the compound basi-sphenoidal ossify ing cartilage. The periotic elements are all distinct, not only from each other, but also from their surroundings (Phil. Trans. 1869, plate 85, fig. 1.) The bony orbito-nasal wall (perpendicular ethmoid) has grown by metamorphosis of the cartilage up to, and somewhat over and under, the inter-orbital fenestra; thus half of this large septum is bony. There is no osseous presphenoid, but instead of the

true orbito-sphenoids two osseous centres have appeared on

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1. Tha nomenclature of these parts is wrong in Mr Parker s paper on the " Fowl s Skull," Phil. Trans., 1869 ; but he named their ele ments correctly in his former paper on the " Struthious Skull," ibid., 1866. Researches into the growth of the Reptilian skull have helped to corr?:t the error. He has found a true "pterotic" in the Sparrow- Hawk (Monthly Micros. Jour., Feb. 1, 1873) ; that was the name given by him to the fowl s epiotic, whilst the latter name was applied to the posterior face of the opisthotic.