Fig. 120.—Side view of the skull of the common Dog-Fish (Scyllium canicula). aud.cp, Auditory capsule; br.a 1, 5, branchial arches; br.r, br.r′, cartilaginous rays attached to the hyoid arch and the first four branchial arches; Cr, cranium; ex.br, extra-branchial cartilages; hy.cn, cerato-hyal; hy.m, hyomandibular; lb, labial cartilages; lg, ligaments passing from the jaws to the cranium and to the distal end of the hyomandibular; lg′, ethmo-palatine ligament; l.j, lower jaw or Meckel's cartilage; Nv. 2, optic foramen; Nv. 5, foramen for the Vth and part of the VIIth cranial nerves; olf.cp, olfactory capsule; or, orbit; up.j, upper jaw or palato-quadrate cartilage. (From Wiedersheim, after W. K. Parker.)
In front of the occipital region two lateral bulgings indicate the periotic capsules, and more anteriorly still, in the trabecular region, the sides of the cranium are modified to form two spacious lateral recesses, the orbits, each of which is bounded above and below by supra-orbital and infra-orbital ridges respectively, behind by an outgrowth from the periotic capsule (post-orbital process), and in front by a similar projection from the hinder wall of the olfactory capsule (lateral ethmoidal process). In front of the cranial cavity and the orbits may be seen the laterally-placed dome-like olfactory capsules, which are open below, where the nasal sacs communicate with the exterior. Between the two capsules an anterior extension of the cranial floor forms a flattened mesethmoidal plate, behind which is the large, membrane-closed, anterior cranial fontanelle. The lateral walls of the cranium are perforated by numerous apertures, some of which serve for the entrance or exit of blood-vessels, and others, mostly pertaining to the inner walls of the orbits, for the transmission of the different cranial nerves from the brain to various parts of the head. In many Elasmobranchs the roots of certain of the anterior spinal nerves perforate the side-walls of the occipital region, and indicate the fusion of vertebral components with the cranium. In the cranial roof between the two periotic capsules there are two small apertures at the bottom of a common median depression: through each aperture the ductus endolymphaticus (aqueductus vestibuli) passes from the vestibular part of the auditory organ to the exterior of the skull.
Three cartilaginous rods, one from the roof of each olfactory capsule, and one, the prenasal or rostral process, from the ethmoid cartilage, converge and meet, or nearly meet, in front to form the rostrum or support for the preoral or "cut-water" portion of the head.
The visceral arches are seven in number. The first or mandibular arch consists on each side of an upper portion, the palato-pterygo-quadrate or palato-quadrate cartilage, which passes forwards in the side-wall of the oral cavity, along the upper margin of the mouth, its anterior or palatine part curving inwards to a ligamentous connexion with its fellow beneath the cranial floor. Each cartilage has an upwardly directed process (ethmo-palatine process) which is connected by a suspensory ethmo-palatine ligament with the lateral wall of the cranium behind the lateral ethmoid process. The lower or ventral half of the mandibular arch (Meckel's cartilage) is similar in shape to the upper; it articulates behind with the quadrate portion of the latter by a movable joint, and is thence prolonged forwards and downwards in relation with the lower margin of the mouth to a median ligamentous union with its fellow of the opposite side. The palato-pterygo-quadrate and Meckel's cartilages together form the primitive upper and lower jaws, and support the teeth. The hyoid arch also consists of a dorsal and a ventral half on each side. The dorsal half or hyomandibular element articulates above with the periotic capsule. The ventral portion, or cerato-hyal, passes downwards and is connected with its fellow by a median copula or basi-hyal cartilage situated in the floor of the oral cavity. A series of simple cartilaginous rays (branchial rays) are attached to the hinder margins of the hyomandibular and cerato-hyal elements. The distal end of the hyomandibular is connected by strong ligaments with the hinder portions of both the palato-pterygo-quadrate cartilage and Meckel's cartilage; in fact, the hyomandibular is the effective suspensorium by which the upper and lower jaws are connected with the skull, and all Fishes in which this arrangement exists are said to be hyostylic.[[189]] Behind the hyoid arch follow five branchial arches. Each of these is segmented into a dorsal or pharyngo-branchial element, followed by an epi-, a cerato-, and a hypo-branchial piece, but the later element is absent in the fifth arch. The lateral halves of the last three arches are connected ventrally by a large median basi-branchial cartilage, but in the first and second arches by the median apposition of their respective hypo-branchial elements. Like the hyomandibular and cerato-hyal segments of the hyoid arch, the epi- and cerato-branchial elements of all the branchial arches except the fifth are fringed along their outer convex margins by a series of branchial rays, and, in addition, there are three pairs of slender, curved, cartilaginous rods, or extra-branchials, in relation with the distal extremities of the branchial rays of the second, third, and fourth branchial arches. The function of the branchial arches, and their branchial rays, and extra-branchial cartilages, is to support the inter-branchial septa which separate the gill-clefts and carry the vascular gill lamellae. All the arches lie near the inner margins of the septa, close to the hypoblastic epithelium of the pharynx, while the outer portions of the septa are supported by the branchial rays and the extra-branchials, the latter lying directly beneath the external skin. The segments of the arches are movably connected with one another by ligaments; and by the contraction of the branchial muscles the arches may be separated or approximated so as to enlarge or diminish the size of the intervening clefts.
The labial cartilages are represented by a pair of slender rods in relation with the outer surfaces of the palato-pterygo-quadrate cartilages, and a similar pair in connexion with the Meckelian cartilages. There is also a pair of small cartilages in relation with the nostrils. It is probable that the rods which constitute the lateral elements of the rostrum belong to the same category.
In the Cyclostomes and the Elasmobranchs the skull is entirely cartilaginous, although it may often be superficially calcified in Elasmobranchs, and although there may even be definitely and symmetrically arranged calcified plates in Pleuracanthus, true bone is never present. In many Fishes, and notably in the Teleostomi, the embryonic cartilaginous cranium becomes complicated by the addition of an extensive series of investing membrane bones, formed by the ossification of the connective tissue external to the cartilage, so that a secondary bony cranium is formed external to the primary cranium much in the same way that a secondary pectoral girdle is formed in connexion with the primary girdle. Such bones probably owe their primary origin to the fusion and insinking of exoskeletal structures (scales or dermal spines). To these investing bones there may also be added a series of bones formed by the actual conversion of the cranial cartilage into osseous tissue (cartilage bones), which to a greater or less extent in different Fishes replaces the original cartilage. The bones of the skull may conveniently be classified as follows:—(i.) Dermal or membrane bones. Under this head are included—(a) the ordinary investing bones of the skull. (b) Tooth-bones, that is, bones formed by the fusion of the bases of teeth and developed in relation with the walls of the oral cavity. Probably all tooth-bearing bones are of this nature. (c) Sensory canal bones, that is, tubular bones developed round the sensory canals of the head. Certain of these bones may secondarily acquire the shape and character of investing bones while still retaining protective relations to their sensory canals. (ii.) Cartilage bones.
As an easily obtainable example of a skull which has acquired a fairly complete series of both cartilage- and membrane-bones, while retaining a well-developed primary cranium, the skull of the Salmon (Salmo salar) may be described.[[190]] At an early stage of development, even so late as the second week of hatching, the primary cranium is still entirely cartilaginous, and in this condition the Salmon's skull is comparable with that of an adult Dog-Fish. As development proceeds the primary cranium becomes supplemented by the addition of numerous investing dermal bones which form the secondary cranium, and later cartilage bones appear and, to a considerable extent, replace the original cartilage. The Salmon's skull is interesting in this respect, that the primary cranium grows with the growth of the Fish, so that in the adult the nasal, ethmoidal, and prenasal regions are entirely cartilaginous, and in the hinder part of the cranium cartilage is largely persistent between the cartilage bones.
Dealing first with the cartilage bones of the primary cranium, it may be stated that there are formed in that part of the parachordal cartilage surrounding the foramen magnum a median basioccipital below, which is concave behind where it articulates with the centrum of the first vertebra, a supraoccipital above, and two laterally-placed exoccipital bones (Figs. 121, 122). Each periotic capsule is ossified by the formation of five bones in the primitively cartilaginous mass, the prootic, sphenotic, opisthotic, epiotic, and the pterotic. The inner walls of the capsules have atrophied in the adult, and hence the cavities which contain the auditory organs appear as open lateral recesses of the cranial cavity. In front of the periotic capsules there are various bones which are formed in the cartilage of the trabecular part of the cranium. Thus, in front of the basi-occipital, and developed in the cartilage of the cranial floor, there is a median Y-shaped basisphenoid, and, at some distance above it on each side, an alisphenoid helps to form the lateral wall of the cranial cavity. Between the eyes the side walls of the cranium fuse to form a vertical inter-orbital septum, and, in consequence, two orbito-sphenoid bones, which normally form the lateral cranial walls in this region, become partially confluent in the median line and close the cranial cavity in front. The only cartilage bones found in the massive persistent portion of the primary cranium which forms the pre-orbital region are the projecting lateral ethmoids, forming the posterior boundaries of the recesses for the olfactory organs, and separating the latter from the orbits.
