3. Mandibular or tympano-mandibular arch, composed of epi-, meso-, pre-, and hypo-tympanic, and the bones of the lower jaw; its appendage consists of the præoperculum and the other opercles.
4. Maxillary or palato-maxillary arch, composed of palatine, maxillary, and premaxillary; its appendage consists of the pterygoid and entopterygoid.
Parts of the splanchnoskeleton are held to be the ear-capsule or petrosal and the otolite, the eye-capsule or sclerotic, the nose-capsule or “ethmoid” and turbinal; the branchial arches.
The bones of the dermoskeleton are the supratemporals, supraorbitals, suborbitals, and labials.
B. In the second method of classifying the bones of the skull prominence is given to the facts of their different origin as ascertained by a study of their development. The parts developed from the primordial skull, or the cartilaginous case protecting the nervous centre are distinguished from those which enclose and support the commencement of the alimentary canal and the respiratory apparatus, and which, consisting of several arches, are comprised under the common name of visceral skeleton of the skull. Further, a distinction is made between the bones preformed in cartilage and those originating in tegumentary or membranous tissue. It is admitted that the primordial cranium is a coalition of several segments, the number of which is determined by that of the visceral arches, these representing the hæmal arches of the vertebral column; but the membrane-bones are excluded from a consideration of the vertebral division of the primordial skull, as elements originally independent of it, although these additions have entered into special relations to the cartilage-bones.
With these views the bones of the Teleosteous skull are classified thus:—
1. Cartilage-bones of the primordial skull.—The basi-occipital (5 in Figs. [23–26]) has retained the form of a vertebral centrum; it is generally concave behind, the concavity containing remains of the notochord; rarely a rounded articulary head of the first vertebra fits into it, as in Symbranchus, and still more rarely it is provided with such an articulary head (Fistularia); frequently it shows two excavations on its inner surface for the reception of the saccus vestibuli. The exoccipitals (10) are situated on the side of the basi-occipital, and contribute the greater portion of the periphery of the foramen magnum; frequently they articulate with the first vertebra, or meet in the upper median line, so as to exclude the supraoccipital from the foramen magnum. The supraoccipital (8) is intercalated between the exoccipitals, and forms a most prominent part by the median crest, which sometimes extends far forwards on the upper side of the skull, and offers attachment to the dorsal portion of the large lateral muscle of the trunk. When the interior portions of this bone remain cartilaginous, some part of the semicircular canals may be lodged in it.
The region of the skull which succeeds the bones described encloses at least the greater portion of the labyrinth, and its component parts have been named with reference to it by some anatomists.[8] The alisphenoids (11) (Prooticum) form sutures posteriorly with the basi- and exoccipitals, 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 it forms another hollow for the reception of the vestibulum; generally it is perforated by the Trigeminal and Facial nerves. The paroccipitals (9) (Epioticum) lodge a portion of the posterior vertical semicircular canal, and form a projection of the skull on each side of the occipital crest, to which a terminal branch of the scapular arch is attached. The Mastoid (12 + 13) (Opisthoticum) occupies the postero-external projection of the head; it encloses a part of the external semicircular canal; is generally coalesced with a membrane-bone, the superficial squamosal, which emits a 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 the cerebral cavity; if the latter is advanced far forwards, the lateral walls of the primordial cranium are protected by more developed ossifications than 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 generally reduced in extent, often replaced by membranes; especially the interorbital septum may be reduced to membrane. The most constant ossifications of this part of the skull are the orbitosphenoids (14), which join the upper anterior margin of the alisphenoids. 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) is as frequently absent as present; it forms the anterior margin of the fossa for the hypophysis. Finally, the postfrontal (4) belongs also to this group of cartilage-bones.
The centre of the foremost part of the skull is occupied by the ethmoid (3), which shows great variations as regards its extent and the degree of ossification; it may extend backwards into the interorbital septum, and reach the orbitosphenoids, or may be confined to the extremity of the skull; it may remain entirely cartilaginous, or 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 fore-part of the ethmoid is the pre-frontals (2), which form the base of the nasal fossa.