OCR Text |
Show 192 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, closely adherent, into an otic process (o, Plate XXXI. fig. 4). The posterior division of the seventh nerve (which answers to what is commonly called the facial nerve, and may be termed the hyo-man-dibular division of the seventh) runs directly in front of the auditory capsule, and beneath the otic process of the suspensorium. The anterior division (palatine or Vidian division) of the seventh, on the contrary, passes directly forwards, close to the pedicle of the suspensorium, parallel with the orbito-nasal, and below but external to it. Meckel's cartilage (Mck.) is very thick at its articular end, but rapidly tapers off to a point beyond the coronoid enlargement, to which the elevators of the jaw are attached. The termination of Meckel's cartilage lies at a considerable distance from the symphysial end of the ramus of the mandible (Plate X X I X . fig. 1 and Plate X X X I . fig. 7). / _ ,, The hyoidean arch has already been well described and figured by Fischer. It is entirely cartilaginous and fibrous. It consists (Plate X X X . fig. 2) of a long and stout ceratohyal cartilage (C.h), and a small hypohyal (H.h). The two hypohyals are united with one another by fibrous tissue, which represents the basihyal. Fibrous tissue connects the proximal half of the ceratohyal with the suspensorium and with the otic region of the skull ; and a strong ligamentous band, the hyo-suspensorial Mgamerd (h.s.l, Plate X X I X . fig. 1), passes from the ceratohyal, at about the junction of its middle and upper thirds, to the middle of the posterior edge of the suspensorium. From this point another strong fibrous bundle, the suspen-sorio- stapedialligament (s.s.l), is continued upwards and backwards to the stapes. The hyo-mandibular branch of the seventh nerve (VII.js) passes above this ligament to its distribution, just as it passes above the columella auris in the Frog. Rather above the attachment of the hyo-suspensorial ligament, another strong band of fibrous tissue arises from the ceratohyal, and, passing down on the inner side of the former, is inserted into the angle of the mandible. This may be termed the mandibulo-hyoid ligament, and answers to the interopercular element of the osseous fishes' skull (Plate X X I X . fig. 1 and Plate X X X I . fig. 7,m.h.l). The branchial apparatus is composed of two median pieces, the first and second basibranchials (Bb\ Bb2) ; of which the former is cartilaginous, pointed in front, where it is connected with the fibrous representative of the basihyal, broad behind, where it unites with the two ceratobranchials (Cb1). The second basibranchial is the only portion of the branchial apparatus which is ossified, and has been described above. The stout anterior ceratobranchials (Cb1)* meet in the middle line, between the first and second basibranchials. The broad, dorsal end of each articulates with the correspondingly broad, ventral end of the first epibranchial (Ep.b1), which is curved, and tapers to a point at its dorsal extremity. * Fischer (I. c. p. 19) considers that these represent the anterior and posterior ceratobranchials coalesced ; but I see no evidence that such a process has taken place. |