Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/801

Rh AMPHIBIA 757 Instead of there being a well-developed cartilaginous brain-case, interrupted only by a dorsal fontanelle, as in the frog, both the floor and roof of the cranial cavity are formed by merely fibrous tissue, which underlies the frontal and parietal bones, and overlies the parasphenoid ; and only its sides and its anterior end are bounded by cartilage. The occipital region remains membranous in the middle line, both dorsally and ventrally, and exhibits, in the latter aspect, the remains of the notochord. The cartilaginous rods (Tr.), which bound the cranial cavity laterally, and represent the trabeculce of the embryonic vertebrate skull, are separated by a wide oval space, which occupies the whole length of the floor of the cranial chamber. An teriorly, they converge, and, just before they do so, give attachment to the slender antorbital processes (A. 0.) which lie behind the posterior nares. They then unite, and, be coming applied together, coalesce into a flattened narrow mesethmoid, to the anterior extremity of which the pre- maxillary bones are applied. They give off neither alinasal nor subnasal processes, and therefore furnish neither roof iisr floor to the nasal chamber. Posteriorly, they become flattened from above downwards, and coalesce with the auditory capsules, and with the cartilage which extends be neath these, and gives rise to the occipital condyles (E. 0. ) Each auditory capsule has a generally oval form, but is produced posteriorly, so as to give rise to a conical epiotic process (Ep. 0.), which projects beyond the level of the occipital condyle. Fitted into the outer wall of each is the relatively large, conical, stapes, whence strong ligament- ous fibres proceed to the posterior face of the suspensorium. Immediately in front of the auditory capsule, the suspen sorium passes by a strong pedicle (shown, but not lettered in Fig. 12) into the trabecula, and then, directed outwards, downwards, and forwards, ends in the quadrate process (&amp;lt;?), with which the dorsal end of Meckel s cartilage arti culates. A large process (o) ascends from the posterior face of the suspensorium, and applies itself to the outer and anterior face of the auditory capsule. A small and liardly perceptible elevation (p) is seen near the quadrate process of the suspensorium. Finally, a flat pi ocess (a, Fig. 11) ascends above the pedicle, and applies itself to the dorsal face of the trabecula. On comparing this with the suspensorium of the frog, it is clear that the rudimentary process (p) answers to the pterygoid cartilage ; and that the process o (the otic process) answers to the dorsal crus of the suspensorium. In fact, the posterior, or hyo-mandibular, branch of the seventh nerve passes back beneath this, and above the stapedial ligament, to its distribution. The pedicle answers to the part so named (including the ventral crus of the suspensorium) in the frog, though it retains the embryonic relations to the trabecula, such as exist in the tadpole. The ascending process (a) lies be tween the orbito-nasal and the other branches of the trige- minal, the orbito-nasal passing between it and the trabecula. A similar process is very generally found in the Urodela (being particularly large, for example, in Menopoma), but appears to be represented only by fibrous tissue in the Anura. Meckel s cartilage (McL, Fig. 12) is thick and deep at its articular end, but, after furnishing a surface of attach ment for the elevator muscles of the jaw, it rapidly narrows, and ends in a point, at some distance from the symphysis of the dentary bones. The hyoidean apparatus (Fig. 1 3) is represented, on each side, by a cartilaginous rod, subdivided into a short hypo- liyal (II. h.) and long cerato-hyal (C. h.) A strong ligament extends from the front face of the latter, below its free summit, to the suspensorium, reaching this at the same Hk. place as the stapedial ligament, into which it is continued. From a point a little above the attachment of this ligament, another ligamentous band arises, and, crossing the former, on the inner side of which it passes, becomes attached to the angle of the mandible. It answers to the inter- operculum of a fish, and has nearly the same relations as the stylo-maxillary ligament of the higher Vertebrata. In the ventral median line, the hypo-hyals are connected only by fibrous tissue. Firmly united with this, however, there is a median first basibranchial cartilage (Bb 1 }, suc ceeded by a second basibranchial (fib 2 ), which is ossified, and is the only bony con stituent of the hyo- branchial apparatus. There are only three branchial arches. The first consists of a stout and long cerato-branchial (Ci 1 ), bearing an equally well - deve loped epibranchial (Epb 1 }. The second is represented by a Fie. 13. Hyoid and branchial apparatus of Meno- mere nodule of car- l S an. chu *[ atf1 &quot; 1 *-, m - hypo-iiyai:Cik. cerate- //~nv i } ; ^ rst basibranchial; Jil&amp;gt; 2, ossified tllagC (Cb&quot;), With second basibranchinl ; Ep. ft 1, Ep. I 2, Ep.b 3 , first, which the expanded 8econd and third f P ibra &quot; chials = Gl - # * end of the second epibranchial (Ep. i 2 ) articulates. The third and smallest epibranchial (Ep. 6 3 ) is articulated with the step-like broad end of the second. There is no trace of any fourth branchial arch, such as exists in tadpoles and in young salamanders ; and in Siredon, Siren, Amphi uma, and Menopoma. The most curious feature in the bony skull, orosteo-cranium, of Menobranchus is the presence of the prominent conical ossifications which lie external to the exoccipitals,and occupy the place of the epiotic and opisthotic bones. In possessing these elements of the skull, in so large and distinct a form, Menobranchus differs from all Amphibia, save Proteus and the extinct Labyrinthodonts. The parietal bones are sepa rate from the frontals, and send, as is usual in the Urodela, a long process forwards on each side of the latter. In Menobranchus, this process is extremely long, reaching the olfactory foramen, the posterior margin of which it bounds. There are no nasal nor prefrontal bones, nor any distinct ali- sphenoidal, orbito-sphenoidal, or ethmoidal ossifications; of the maxilla, nothing but a rudiment appears, and this is sometimes absent. There are no jiigal or quadrato-jugal ossifications. A palato-pterygoid plate, bearing teeth on its expanded palatine portion, extends from the antorbital process to the inner face of the suspensorium, which, as stated above, has a mere tubercle in the place of a pterygoid process. The vorners are long, bear teeth along their outer edges, and diverge backwards so as to leave an interspace between their inner edges. Posteriorly, they articulate with the anterior ends of the palato-pterygoids. The squamosal is a long, slender, curved bone, devoid of any zygomatic process, which extends from the articular end of the suspensorium, along its outer edge, to the outer side of the epiotic. From its posterior margin it sends down a short process over the stapedial ligament. The parasphenoid is a broad thin plate of bone, which extends from near the junction of the vomers to the occipital foramen. The distal end of the suspensorium is incompletely ossified, as a quadrate bone ; and a dentary and a splenial element, both dentigerous, lie, the former