OCR Text |
Show 194 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, converge and coalesce into the internasal prolongations, which give rise to the mesethmoid of the adult Frog. But, in the tadpole, at this stage of its development, the " parachordal cartilages," which have been developed at the sides of the notochord, have united with one another and with the trabeculse, and thus the pituitary space is much shorter than in Menobranchus. The cartilaginous skull of a tadpole of this age, in fact, has already obtained a higher development than it ever reaches in Menobranchus. The auditory capsules are rounded behind, in the tadpole, and do not extend backwards as pointed processes beyond the level of the exoccipitals ; in which respect the tadpole's skull is more frog-like, and less fish-like, than that of the adult Menobranchus. In the tadpole's skull, the suspensorium is attached to the trabecula of its side, close to the point at which the latter passes into the parachordal cartilage. The cartilaginous band (m, Plate X X X I . fig. 3), in fact, which passes into the trabecula, is the dorsal end of the mandibular arch, and corresponds with the pedicle of the suspensorium in Menobranchus, having the same relations to the ganglia and branches of the fifth and seventh nerves. In the adult Frog, the pedicle of the suspensorium has been carried outwards by the lateral growth of the auditory region of the skull, and is articulated by a joint* with the cartilage of this region, close to the outer extremity of the transverse arm of the parasphenoid. The inner process of the pterygoid lies on its ventral side, closely applied to it. The elbow (o) by which the suspensorium of the tadpole abuts against the anterior and external face of the auditory capsule evidently corresponds with the otic process of the suspensorium of Menobranchus. In the adult Frog, the suspensorium, which is ossified only at its mandibular end, forks, at its cranial end, into two branches or crura, the interspace between which is filled by fibrous tissue (Plate X X X I . fig. 6 ). These crura, and the fibrous tissue which connects them, form the front wall of the tympanic cavity: the dorsal crus, which answers to the otic process, passes into the tegmen tympani, or roof of the tympanum, which is furnished by the outgrowth of the auditory capsule ; the ventral crus is the pedicle of the suspensorium just mentioned. Passing between the two crura (as Duges long since pointed out) the seventh nerve enters the tympanum, closely applied to the inner wall of which (but not included in any Fallopian canal) it passes, above the level of the fenestra ovalis, over the columella auris. It takes, in fact, exactly the same course as in a mammal, except that it runs round the auditory capsule, instead of being included in a canal by the growth of the latter round it. Some remarkable consequences appear to flow from the observed metamorphoses of the cranial end of the mandibular arch in the Frog. If the ossification which has already set in in the mandibular * My friend Mr. Parker, F.E.S., in his remarkable memoir on "The Structure and Development of the Skull of the Common Frog " (Philosophical Transactions, 1871), has givon a different account of the origin of this singular articulation ; but I believe I may say that he now agrees with me. |