Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/799

Rh AMPHIBIA 755 cartilaginous stapes (St.) The anterior face of this pre sents a concave facet, for articulation with, a corresponding surface occupying the posterior half of the inner end of the columdla auris (C. a.), the anterior half of which fits into a fossa in the pro-otic bone. The columella aiiris itself consists of three portions a middle elongated osseous rod, an inner swollen and enlarged cartilaginous part, which articulates partly with the pro-otic and partly with the stapes, and an outer portion, which is elongated at right angles to the rest, fixed into the tympanic membrane, and attached by its dorsal end to the tegmen tympani. The mandible presents one cartilaginous and three osseous constituents on each side. Of the latter, one, the &quot; Mento-Meckelian &quot; (Parker), is a short curved rod of bone, which unites with its fellow in the symphysis, and is, in fact, the ossified symphysial end of Meckel s cartilage, which extends thence through the length of the ramus, becoming thicker posteriorly, and furnishing the articular surface for the quadrate. The second, and largest, bony constituent of the mandible is a long membrane bone, which ensheaths the inner and under region of the outer surface of Meckel s cartilage, rising at one part into a low coronoid process. It obviously represents the angular, coronary, and splenial elements, and may be termed the angulo-splcnial (An.) A small dentary element, which bears no teeth, lies over the outer face of the anterior half of Meckel s cartilage. The hyoidean apparatus of the adult frog (Fig. 8) pre sents a body and two slender cornua. The body consists of a broad and thin squarish plate of cartilage, produced on each side into three processes, which may be called anterior, lateral, and posterior. The anterior process (a)-is slender, curves outwards, and very soon divides in to two processes, one short, anterior, forming a loop by its ligamentary connection with the second, or pos terior, branch, which passes into the long and slender cornu of the hyoid. The lateral process (1) passes FIG. 8. Ventral view of the outwards and slightly dorsad ex- byoidof Jtow wcfe&amp;lt;a, a, .j 7 anterior, 6 lateral, c, pos- pandmg into a broad, hatchet-shaped tenor processes; d, thyro- free extremity. The posterior pro- hyal8 cess (c) is a mere prolongation of the postcro-lateral angles of the body of the hyoid. Finally, from the middle of the posterior margin of the body of the hyoid there project two strong bony rods, wider at the ends than in the middle, which embrace the larynx, and have been termed the thyro-hyals (d). The parieto-frontals, nasals, premaxillaj, maxillae, squa- mosals, palatines, pterygoids, and parasphenoid, the dentary and angulo-opercular bones, may be removed from the frog s skull without injury to the chondrocranium, the structure of which then becomes apparent (Fig. 9). It furnishes a floor, side walls, and roof to the brain case, interrupted only by a large space (fontanelle), covered in by membrane, which lies in the interorbital region under the parieto-frontals ; and by the foramina for the exit of the cranial nerves. It consists entirely of cartilage, except where the exoccipitals, the pro-otics, and the sphen-eth- inoid invade its substance. In front of the septum of the anterior cavity of the sphen-ethmoid, it is continued for ward, between the two nasal sacs, as the cartilaginous septum narium, from which are given off, dorsally and ven- trally, transverse alae of cartilage, which furnish a roof and a floor, respectively, to the nasal chamber. Of these, the floor is the wider. The dorsal and ventral ala3 pass into one another where the chondrocranium ends anteriorly, and give rise to a truncated terminal face, which is wide AJ3, QU..J. IG. 9. Chondrocranium of Rana esculenta ventral aspect, r.p. the rhinal process ; p.n.l. the prsenasal pro cesses; a.n. the alinasal processes, shown by the removal of part of the floor of the left nasal chamber; A.O. the antorbital process; pd. the pedicle of the suspensorium continued into c.v. the ventral crus of the suspen sorium; e.d. its dorsal crus; t.t. the tegmen tympani; (S.^.the sphen-ethmoid ; E.O. the exoccipitals; Qu.J. the quadratojugal. II. V. VI. foramina by which tlie optic, triKeminal and portio dura, and abduccns nerves leave the skull. from side to side, narrow from above downwards, and convex in the latter direction. The lateral angles of this truncated face are produced outwards and forwards into two flattened pros-nasal processes (p. nl.); these widen ex ternally, and end by free edges, which support the adjacent por tions of the pre- maxillae and maxilla3. From the ventral face, just behind the truncated ante rior end of the chondrocranium, spring two slen der cartilages (r.p., r.p.), which do not seem to have been no ticed hitherto. Each of these inclines towards the middle line, and ends against the middle of the posterior face of the ascending process of the premaxilla by a vertically elongated extremity. These may be termed the rhinal processes. An oval nodule of cartilage is attached to the posterior face of the dorsal end of the ascending process of the premaxilla, and serves to connect it with the rhinal process. On the dorsal face of the chondrocranium, just above the point of attachment of the rhinal processes, the external nasal aper tures are situated, and the outer and posterior margins of each of these apertures is surrounded and supported by a curious curved process of the cartilaginous ala the alinasal process (a.n.) Where the sphenoidal and the ethmoidal portions of the sphen-ethmoid meet, a stout, trans verse, partly osseous and partly cartilaginous, bar (A.O.) is given off, which is perforated, at its origin, by the canal for the orbito-nasal nerve. It then narrows, but becoming flattened from above downwards, rapidly widens again, and its axe-head-like extremity abuts against the inner face of the maxilla. The anterior angle of the axe-head is free ; the posterior angle is continued back into a slender cartilagin ous rod, which bifurcates posteriorly; the outer division passes into the ventral crus (c.v.) of the suspensorium. The inner (pd.) is the pedicle of the suspensorium already described. Meckel s cartilage, articulated to the free end of the suspensorium, is unossified throughout the greater part of its extent, no osseous articular e being developed ; but, at its symphysial end, each cartilage becomes ossified, and forma the &quot;mento-Meckelian&quot; element of the mandible. The slender cornu of the hyoid passes directly into the periotic cartilage immediately in front of, though below, the fenestra ovalis. It is unossified throughout its whole length. 1 With many variations in detail, the skulls of the Anura, in general are readily reducible to the type of that of the frog. In the Aylossa, which differ so widely in many respects from the other Anura, the cranium presents some notable peculiarities. In Dactylethra? the skull is similar to that of the ordi nary Anura in general form, but the nasal region is small in proportion to the orbito-temporal space. The fronto- parietals are aukylosed together, and extend forwards as far as a line drawn along the anterior edge of the antorbital 1 Compare Mr Parker s full account of the structure of the skull of Rana temporaria, Philosophical Transactions, 1871. 2 This description applies especially to D. Iccvis.