Fig. 121.—Side view of the cranium of a Salmon (Salmo salar). Most of the membrane bones and the jaws have been removed. The cartilage is dotted. al.s, Alisphenoid; bo, basioccipital; bs, basisphenoid; eo, exoccipital; ep, epiotic; l.eth, lateral ethmoid; ol, olfactory capsula; op, opisthotic; o.s, orbito-sphenoid; pr.o, prootic; ps, parasphenoid; pt.o, pterotic; so, supraoccipital; sp.o, sphenotic; t.c, trabecular cornu; u.l.c, u.l.c2, first and second upper labial cartilages; v, vomer; II, foramen for the optic nerve. (From W. K. Parker.)
The roof and floor of the primary cranium is completed by certain investing dermal bones (Fig. 123, A). A pair of large frontal bones form the cranial roof, and also help to roof in the orbital cavities. Behind the frontals, and separated from each other by the supraoccipital, there is a pair of small parietals, and anterior to the frontals a median dermal mesethmoid. A small nasal bone overlies each olfactory recess. Ventrally, the base of the cranium, from the basi-occipital to the prenasal region, is strengthened by a large parasphenoid behind, and a much smaller vomer in front, both of which lie in the roof of the mouth. The vomer is a tooth-bone, and probably the parasphenoid also.
Fig. 122.—Vertical and longitudinal section of the cranium of Salmo salar, showing the right half of the cranial cavity. Cartilage is dotted. f, Frontal; v′, fat-containing cavity in the mesethmoid cartilage; V, VII, IX, X, foramina for the fifth, seventh, ninth, and tenth cranial nerves. Remaining reference-letters as in Fig. 121. (From W. K. Parker.)
The mandibular arch (Fig. 123, B) is more modified than that of the Dog-Fish. The palato-pterygo-quadrate bars, or primitive upper jaw, no longer meet in front beneath the cranial floor, but each separately articulates in front with the lateral ethmoid of its side. Although still partly cartilaginous each bar is largely replaced either by cartilage bones, or by bones which begin as membrane bones or as tooth-bones and complete their growth by invading the cartilage and becoming in part cartilage bones. Its anterior portion is formed by a palatine bone which articulates with the lateral ethmoid, and the middle portion by a pterygoid and a mesopterygoid bone, while the hinder part is ossified above as a metapterygoid and below as a quadrate. The latter articulates with the lower jaw. Functionally, however, the primitive upper jaw is now replaced by a secondary upper jaw, formed on each side by a series of tooth-bones, situated external to the former, and meeting in front of the prenasal region of the primary cranium (Fig. 123, A). The series includes a dentigerous premaxilla and maxilla, and a small toothless, scale-like jugal bone. Each half of the lower jaw (Fig. 123, A, B) consists of a rod-like Meckel's cartilage or primary lower jaw.
Fig. 123.—A, view of the left side of the skull of a Salmon; B, the left half of the primary upper and lower jaws, and the hyoid arch. The cartilage is dotted. an, Angular; ar, articular; b.hy, basi-hyal; br.r, branchiostegal rays; c, cranium; c.h, cerato-hyal; c.or, circum-orbital bones; d, dentary; d.eth, dermal mesethmoid; ep.h, epihyal; ep.o, epiotic; eth.p, ethmo-palatine process; f, frontal; h.hy, hypo-hyal; hym, hyomandibular; i.op, inter-operculum; j, jugal; mks, Meckel's cartilage; mpg, mesopterygoid; mt.pg, metapterygoid; mx, maxilla; n, nasal; op, operculum; op′, condyle on the hyomandibular for the operculum; orb, orbit; p, parietal; pa, palatine; p.mx, premaxilla; p.op, pre-operculum; pt, pterygoid; pt.o, pterotic; q, quadrate; so, supra-occipital; s.op, suboperculum; sp.o, sphenotic; s.t, supra-temporal (or squamosal); st.hy, stylo-hyal; sy, symplectic; u.l.c, u.l.c′, upper labial cartilages; u.l.c2, second upper labial. (From W. K. Parker.)
The hinder part of this is ossified to form the articular, which has a deeply concave surface for articulation with the quadrate; and below this there is a small membrane bone, the angular. The rest of the cartilage is partially ensheathed on its outer side by a large tooth-bone, the dentigerous dentary. The hyoid arch is similar to that of the Dog-Fish, except that its primitively cartilaginous segments are almost completely ossified (Fig. 123, B). The large upper segment or hyomandibular bone articulates mainly with the pterotic, but partly also with the sphenotic element of the periotic capsule; below it is connected with a slender symplectic bone, and from the cartilage connecting the two depends the rest of the hyoid arch, consisting in succession of stylo-hyal, epi-hyal, cerato-hyal, and hypo-hyal bones, with a median teeth-bearing basi-hyal. The palato-pterygo-quadrate bar has no direct connexion with the skull, except anteriorly where its palatine element articulates with the lateral ethmoid. The real suspensorium is formed by the hyomandibular and symplectic bones, to which the hinder margins of the quadrate and metapterygoid bones are rigidly attached by suture, hence, as in the Dog-Fish, the skull is hyostylic. Behind the hyoid arch there are five branchial arches, which generally resemble those of the Dog-Fish, except that their component segments are ossified as cartilage bones.
Connected with the hyomandibular and cerato-hyal elements of the hyoid arch there is, on each side, a series of membrane bones for the support of the movable operculum or gill-cover. These consist of an operculum above, which articulates with a backwardly projecting process from the hyomandibular, followed in succession below by a sub-operculum and an inter-operculum, the latter being connected by ligament with the angle of the lower jaw. The series is completed by ten sabre-shaped branchio-stegal rays, which are attached to the cerato-hyal and support the lower margin of the gill-cover.
Sensory canal bones are represented in the Salmon by a ring of small bony plates which encircle the orbit (Fig. 123, A), and by one or two small bones situated above and on the outer side of each periotic capsule (squamosals). To these may be added the pre-operculum situated external to the hinder margins of the hyomandibular and quadrate bones, firmly clamping these bones together, and also the post-temporals, by which the secondary pectoral girdle is attached to the skull. The nasal bones may also be regarded as pertaining to the same series.