Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/659

641 OSTEOLOGY.] ICHTHYOLOGY port a series of interhasmals (79), to which the anal rays are articulated. There is a great amount of variation as regards th. degree in which the primordial cartilaginous cranium per sists; it is always more or less replaced by bone; fre quently it disappears entirely, but in some fishes like the Salmonidce or Esocidce, the cartilage persists to the same or even to a greater extent than we shall find in the Ganoidei holostei. Besides the bones preformed in cartilage there are a great number of membrane-bones, that is, bones originating in membranous or tegumentary tissue. The different kinds of these membrane-bones occur with greater or less con stancy throughout the subclass Teleostei; they often coalesce with and are no longer separable from the neigh bouring or underlying cartilage-bones. 1. Cartilage -Bones of the Primordial Skull. The basi- occipital (5 in figs. 25-27) has retained the form of vertebral centrum ; it is generally concave behind, the 641 FIG. 25. Skeleton of Perch s Skull. concavity containing remains of the notochord. The ex- occipitals_(10) are situated on the side of the basioccipital, and contribute the greater portion of the periphery of the foramen magnum. The supraoccipital (8) is intercalated between the exoccipitals, and forms a most prominent part by its median crest, which sometimes extends far forward on the upper side of the skull, and offers attachment to the dorsal portion of the large lateral muscle of the trunk. A transverse supraoccipital ridge, coming from each side of the base of this crest, runs outward to the external angles of the bone. When the interior portion of this bone remains cartilaginous, some part of the semicircular canals may be lodged in it. The region of the skull which succeeds these bones en closes at least the greater portion of the labyrinth, and its component parts have been named with reference to it by some anatomists. 1 The alisphenoids (11) (prooticum] form sutures posteriorly with the basi- and ex-occipitals, and meet each other in the median line at the bottom of the cerebral cavity; they contribute to the formation of a hollow in which the hypophysis cerebri and the saccus vasculosus are received ; in conjunction with the exoccipital they form another hollow for the reception of the vestibulum- generally they are perforated by the trigeminal and facial nerves. The paroccipitals (9) (epioticum) lodge a portion pi the posterior vertical semicircular canal, arid form a pro jection of the skull on each side of the occipital crest, to which a terminal branch of the scapular arch is attached, mastoid (12 + 13) (opisthoticum) occupies the postero- external semicircular canal, is generally united to a membrane-bone, the superficial squamosal, which emits a 1 As first proposed by Huxley. process for the suspension of the scapular arch, and is frequently, as in the perch, divided into two separate bones. The anterior portion of the skull varies greatly as regards form, which is chiefly dependent on the extent of thecere- FIG. 26. Hyoid arch, branchial apparatus, and scapulary arch of the Perch. bral cavity; if the latter is advanced far forwards, the lateral walls of the primordial cranium are protected by more developed ossifications than is the case if the cerebral cavity is shortened by the presence of a wide and deep orbit. In the latter case parts which normally form the side of the skull are situated in front of the brain-case, between it and the orbit, and, being generally reduced in extent, are often replaced by membranes; the interorbital septum especially may thus be reduced to a membrane. The most constant ossifications of this part of the skull are the orbitosphenoids (14), which join the upper anterior margin of the alisphenoids; superiorly the olfactory and inferiorly the optic nerves pass between them out of the cranium. They vary much with regard to their development : they are small in Gadoids; larger in the perch, pike, Salmonoids, Macrodon, and the Clupeoids; and very large in Cyprinoids and Siluroids, in which they contribute to the formation of the side of the brain- case. ^The single Y-shaped sphenoideum anterius (15) ^ethmoid of Owen and basisphenoid of Huxley) is as frequently absent as present ; each lateral branch is con nected with an orbitosphenoid, whilst the lower branch rests upon the long basal bone ; it forms the anterior mar gin of the fossa for the hypophysis. Finally, to this jroup of cartilage-bones belongs also the postfrontal (4), a small bone from which the infraorbital ring is suspended. The centre of the foremost part of the skull is occupied )y the ethmoid (3), which shows great variations as regards ts extent and the degree of ossification; it may extend backwards into the interorbital septum, and reach the orbitosphenoids, or it may be confined to the extremity of the skull ; it may remain entirely cartilaginous, or it may ossify into a lamina which separates the two orbits and encloses an anterior prolongation of the brain-case, along which the olfactory nerves pass, modifications occurring again in higher vertebrates. A paired ossification attached to the forepart of the ethmoid is the prefrontals (2), which form the base of the nasal fossa. 2. Membrane-Bones attached to the Primordial Skull. To this group belong the parietals (7) and f rontals ( 1 ). The latter form the upper margin of the orbits, and extend from the nasal cavities to the occipital. They are enlarged at the expense of the parietals, which are of much smaller extent XTT. - ST
 * ernal projection of the head ; it encloses a part of the