passing the distal portion of the extra-columella, part of the hyoid fuses with it, often forming thereby a little hole, the remnant of imperfect fusion.
In the crocodiles the arrangement is at first complete and diagrammatically clear, not obscured by vagaries of the hyoid, which is free and much reduced. In the embryo the large extra-columellar cartilage, abutting against the tympanic membrane, and with another process against the quadrate, sends its third, downward, process as a thick rod of cartilage to the posterior inner angle of the mandible with which it is directly in cartilaginous continuity. It was W. K. Parker's mistake to call this cartilage the cerato-hyal. In young embryos it looks like an upward continuation of Meckel's cartilage, much resembling mammalian conditions. But in nearly ripe embryos this cartilage is already reduced to a string of connective tissue, cartilage remaining only at the upper end, and where this string enters the mandible lies the siphonium, the tube which connects the air cavities of the mandible with the Eustachian passages, the long connecting channel becoming—side by side with the extracolumellar-mandibular ligament—embedded into a canal of the quadrate, so that in older stages, and above all in the adult, the proper display of the whole arrangement requires a little anatomical skill. The whole string, whether cartilaginous or ligamentous, which connects the downward extracolumellar process with the articulate, is of course homologous with the continuation of Meckel's cartilage into the malleus of foetal and young mammals; and the chain of bones and cartilages between the auditory capsule, fenestra ovalis, and the proximal part of the mandible is also homologous wherever such a chain occurs; lastly, fenestra ovalis and membrana tympani are fixed points. Consequently columella = stapes, extracolumella of Sauropsida = lentiform + incus + malleus of Mammalia.
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Fig. 37.—Diagram showing Evolution of the Ossicular Chain of the Ear. 1. Hyostylic Elasmobranch. H, hyoid; Hm, hyomandible; M, mandible; P Q, palatoquadrate. 2. Lacertilian. Co, columella or stapes; and E, extra-columella with supra-, extra- and infra- “stapedial” processes. 3. Hypothetic stage between 2 and 4, Sphenodon. Par = parotic bone. 5. Lacertilian. Parotic process with a piece of cartilage at its end, remnant of piece of the hyoid; connexion of intra-stapedial process with mandible vanishing. 6. Embryo of Crocodile. Continuous cartilaginous connexion of extra-columella with Meckel's cartilage. 7. Embryonic Mammal; for comparison. Cd, the new condyle, articulating with Sq, squamosal; Cor, coronoid process; quadrate transforming into tympanic ring. |
The inner ear has been studied minutely and well by C. Hasse, E. Clason and G. Retzius. It is enclosed by the periotic bones. The fenestra rotunda is surmounted by the opisthotic, the fenestra ovalis by the same and by the pro-otic, and this protects also the anterior vertical semicircular canal. The posterior canal is opisthotic, the horizontal is pro- and opisthotic. The anterior canal is the largest of the three, a feature characteristic of the Sauropsida. The lagena, with its own acoustic papilla, begins to show a basilar membrane with papilla, at the expense of that in the sacculus. In Sphenodon and lizards a slight curving of the lagena indicates the beginning of a cochlea, and a scala is developed in crocodiles, but neither cochlea nor scala is specially twisted. The endo-lymphatic ducts end as closed sacs, in lizards and snakes, in the roof of the skull, between the occipital and parietal bones. They reach an enormous development in many geckos, where they form large twisted sacs beneath the skin, covering the sides of the neck, which then assumes a much swollen appearance. They contain white otolithic masses, with lymph. It is remarkable that the extent of these sacs varies not only in allied species, but even individually, independent of sex and age, although they are naturally liable to increase with age.
5. Eyes are present in all reptiles, although in many of the burrowing snakes and lizards they may be so completely covered by the skin as to have lost their function. Most reptiles have upper and lower lids, moved by palpebral muscles, and a third lid, the nictitating membrane, which can be drawn over the front of the cornea from the inner angle obliquely up and backwards. Its mechanism is simplest in lizards. A muscle, a split from the retractor muscle of the eyeball, arises from the posterior part of the orbit, is attached to the posterior wall of the eyeball, and there forms a pulley for the long tendon which arises from the median side of the orbit and passes over the back of the ball forwards into the nictitating membrane. Contraction of this muscle draws the membrane backwards and over the eye. In crocodiles and tortoises the tendon of the nictitating membrane broadens out into a muscle (M. pyramidalis), which arises from the median side of the posterior portion of the ball; above, the optic nerve it crosses over the broad insertion of the retractor of the ball, without being much guided by it, although this muscle by its contraction slightly prevents the nictitating tendon and muscle from touching the optic nerve.
It is easy to recognize the mechanism of birds as a combination of the two types just described; their musc. quadrates s. bursalis is of course the single muscle of the lizards, but now restricted.to, and broadened out upon, the eyeball.
Special Modifications of the Lids.—In the snakes the upper and lower lids are reduced to the rim, and the nictitating membrane has become the permanent cover, which protects the eye like a watch-glass, leaving between itself and the cornea a space, drained by the naso-lacrymal duct, and behind this space the eyeball moves as freely as in other animals. A similar arrangement exists in the true geckos, not in the Eublepharidae, which still possess the outer lids. In some lizards, especially such as live in deserts, the middle of the lower lid has a transparent disk, and it is always the lower lid which is drawn over the eye, the upper in nearly all Sauropsida being much smaller and less movable; for instance, some specimens of the Lacertine genus Eremias in Africa and India. In the Indian genus Cabrita, and in Ophiops of Africa and India, the lower lid is permanently fused with the rim of the shrunken upper lid and forms a transparent window superficially looking like that of the snakes. Exactly the same arrangement has been developed by Ablepharus, one of the Scincidae.
The eyeball is provided with the usual rectus and obliquus muscles, in addition to a retractor oculi. Apparently all reptiles possess a pair of Harderian or nictitating glands, which open in front, in the nasal, inner corner, and lacrymal glands which open likewise into the conjunctiva sac, but near the outer or temporal corner. The secretion of both is drained off through the lacrymal canals, which in lizards open below in the outer wall of the posterior nares; in snakes they open into the mouth by a narrow aperture on the inner side of the palatine bone.
The walls of the anterior half of the sclerotic of lizards, tortoises and Sphenodon contain numerous cartilaginous or osseous plates, which imbricate in ring shape; they are absent in snakes and crocodiles. Internally the eye of most reptiles possesses at least traces of a pecten; very small indeed in tortoises, or in crocodiles where it is represented by only a few mosslike, pigmented vessels. In many lizards these vessels, arising from near the optic nerve, form a network which extends right up to the posterior side of the lens; in others, especially in Iguanidae, is developed a typical, large pecten, deeply pigmented with black, fan-shaped or umbrella-shaped, sometimes folded. In chameleons it is a short cone; apparently
