Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/798

Rh 754 A.MPHIBIA i in man (#..) This is largely ossified in continuity with the exoccipital 1 posteriorly, and the pro-otic (to which in all probability it properly belongs) in front. The outer extremity of the tegmen, however, remains car tilaginous, and, in front, it passes into a curved band of cartilage, which, as it is continued backwards into the suspensorium, may be termed the &quot; dorsal crus&quot; of the suspensorium (Fig. 9, c.d.} The T-shaped squamosal bone (Sq. ) sends a broad, flat process inwards, which rests upon the tegmen tympani, while its long descending process lies external to the cartilaginous suspensorium, and the pos terior half of its cross-piece, or proper squamoso-zygomatic part, has the same relation to the dorsal crus of the suspen sorium. The suspensorium has a second attachment to the skull, by a &quot;ventral crus&quot; (Fig. 9, c.v.), which diverges from the dorsal crus at the anterior extremity of the sus pensorium, and is continued into two branches. One of these, passing outwards and forwards, becomes the ptery- goid cartilage. The other (Fig. 9, pd. ), directed backwards and inwards, may be termed the &quot; pedicle of the suspen sorium ; &quot; it becomes thickened at its inner extremity, and articulates with a facet in front of the fenestra ovalis, and close to the attachment of the hyoidean cornu. A thin fibrous band extends from this inferior crus to the side walls of the skull, passing between the first division of the fifth nerve in front, and the second and third divisions behind. The space between the dorsal crus of the suspen sorium and the pedicle is filled, in the fresh state, with fibrous tissue, which constitutes the anterior boundary of the tympanum. It is traversed (as Duges long since pointed out) by the posterior division of the seventh nerve, which therefore lies above the pedicle. The pro-otic ossification (Pr. 0.) not only walls in the anterior part of the otic -capsule, but extends for a short distance forwards in the side walls of the skull. Hence, the foramen of exit for the trigeminal and portio dura (V.) is pierced in this por tion of the pro-otic ; and the foramen for the sixth nerve is seen at its lower margin. In front of the pro-otic, the lateral walls of the skull remain cartilaginous for some distance, and are perforated by the large optic foramen (II.) Anteriorly to the exit of the optic nerves, the side walls of the skull are formed by elongated plates of bone, which are parts of an extensive ossification of the anterior .moiety of the brain-case and the posterior part of the nasal capsules, constituting the complex structure termed by Cuvier &quot; os en ceinture,&quot; and by Duges &quot; ethmoide.&quot; As it takes the place of the ethmoid, presphenoid, and orbito- sphenoids, it may be termed the sphen-ethmoid (S.e.) It may be compared to a dice-box, one-half of which is divided by a longitudinal partition. This half is anterior, the longitudinal partition being represented by the ossified inesethmoid ; while the posterior, undivided, half lodges the anterior portion of the cerebral hemispheres and the olfactory lobes. The front wall of this posterior cavity is perforated by the olfactory foramina ; while the outer and posterior wall of each anterior, or nasal, chamber presents, where it forms the anterior and inner boundary of the orbit, a small aperture (V 1 ) through which the orbito-nasal nerve passes. The exoccipitals, pro-otics, and sphen-ethmoid are ossifications which involve the chondrocranium, though they largely consist of secondary bone. The supra- occipital is represented, if at all, by a mere calcification of the cartilage, and the like is true of the quadrate, which is an ossification of the distal end of the suspensorium. The quadrate, however, very early becomes continuous with a slender style of membrane bone, the proper jugal, which applies itself to the inner face of the posterior end of the 1 Duges (p. 37) states that the pro-otic and the exoccipital always remain distinct in Rana esculenta ; but.it is common to find them extensively ankylosed and inseparable in old frogs of this species. maxilla, and thus gives rise to the quadrato-jugal (Q.&amp;lt;7.) Ligamentous fibres also connect the anterior end of the zygomatic process of the squamosal directly with the ptery- goid, and indirectly with the maxilla and jugal, and pass from the same process to the fronto-parietal bone, forming a fascia over the levators of the mandible, and encircling the orbit. A strong band is continued forwards, over the ascending process of the maxilla, to the alinasal cartilage of the chondrocranium. The short premaxillae (Pmx.) are united suturally in the middle line, and have stout ascending processes, which become closely connected with the &quot; rhinal processes&quot; by means of oval nodules of cartilage adherent to their poste rior surfaces. The long maxillae unite with the premaxillce in front, and with the jugals behind : each sends up a short anterior ascending process towards the alinasal cartilage, with which it is united by ligament, 2 and further back, gives off a longer ascending process which becomes con nected with the nasal bones. The palatine bones (PL) are straight, slender, and flattened. They lie transversely to the axis of the skull, behind the posterior nares, closely applied to the ventral surface of the sphen-ethmoid and of the antorbital processes of the chondrocranium. Externally they come into contact with the pterygoids and maxillae ; internally and anteriorly with the vomers. Each pterygoid (Pt.) is a triradiate bone, with an anterior, an inner, and a posterior, or outer, ray. The first, or anterior, process of the pterygoid (Pt 1 ) is the longest, and lies, for a considerable distance, in contact with the maxilla, reaching forward to the outer extremity of the palatine. The second (Pf 2 ) underlies the pedicle of the suspensorium, and comes into contact with the end of the transversely elongated, sub-auditory, portion of the para- sphenoid. The third, or posterior, process of the pterygoid (Pt 3 ) is prolonged, in correspondence with the backward elongation of the suspensorium, along the inner side of which it lies, as far as the articulation of the mandible. The vomers ( Fo.) are broad triangular plates of bone, with irregularly-notched outer edges, which are closely applied to the ventral surface of the sphen-ethmoid. Their inner edges are separated by a narrow interval, and each bears numerous teeth, set along a line which is not quite trans verse to the axis of the skull. On the dorsal aspect of the skull two elongated flattened bones, united in a median suture, represent the coalesced frontal and parietals (Fr., Pa.), which are separate in the young frog. In front of these, also meeting in the middle line, are two triangular bones, the apices of which extend outwards to the ascend ing processes of the maxillae, and which roof over the nasal capsules (JVa.) These correspond in position and relations with the nasal and so-called &quot;prefrontaP bones of Saurop- sida, and perhaps with the lachrymals : for brevity s sake, they may be termed nasals. The parasphenoid has the form of a dagger with a very wide guard and short handle. The latter lies beneath the ventral junction of the exoccipitals, while the blade extends forwards, and its point underlies the posterior moiety of the sphen-ethmoid, but does not reach the vomers. The &quot;guard&quot; passes outwards on each side beneath the auditory capsules, and ends by an abruptly-truncated extremity, its anterior and external angle coming into relation with the inner process of the pterygoid. The slender, permanently cartilaginous, hyoidean cornn (//y.) passes into the cartilage of the auditory capsule on the ventral side, between the fenestra ovalu and the articu lar surface for the inferior crus of the suspensorium. The fenestra ovalis lies in a cartilaginous interspace between the exoccipital and the pro-otic, and is filled by the oval 2 The small ossifications in this region, termed &quot; cornets &quot; by Duges, were absent in the skull figured.