This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1989). In ordinary Lizards* the skin forms a kind of veil stretched over the orbit, and pierced by a horizontal fissure, which is closed by a sphincter muscle. The lower eyelid is the most moveable, and encloses a small cartilaginous plate; and there is, besides, generally a fold of the conjunctiva at the inner canthus of the eye, which is the first appearance of a third eyelid or membrana nictitans.
(1990). In the Chelonian Reptiles and in the Crocodiles, the upper and lower eyelids are sufficiently perfect accurately to close the eye; but there are no eyelashes as yet present. Moreover these animals possess an additional eyelid or nictitating membrane, similar to that of Birds, which can be drawn at pleasure over the front of the eye, so as entirely to conceal it. This is effected by a special muscle provided for the purpose, which arises from the posterior part of the globe of the eye, and, after winding round the optic nerve, passes beneath the eyeball, to be inserted into the free margin of the membrana nictitans. In Frogs and Toads the upper and lower eyelids are nearly motionless; but the third is largely developed, and moved in the same way as that of the Crocodile.
(1991). In the higher Reptilia a distinct lacrymal gland and jnrncta lacrymalia are met with, occupying the same positions as those of the human subject.
* Cuvicr, Lecons d'Anat. Comp. tom. ii. p. 433.
(1992). The third, fourth, and sixth pairs of the cerebral nerves have the same distribution in all the Vertebrata, and represent respectively the oculo-muscular, the pathetici, and the abducentes of Man.
(1993). The nerves belonging to the fifth pair likewise correspond both in their distribution and office with the trifacial nerves of mammi-ferous Vertebrata.
(1994). The facial nerve, or portio dura of the seventh pair, is small, in proportion to the limited development of the soft parts of the face, but it is constantly present.
(1995). The auditory nerve, of course, is destined to the ear, and its distribution is almost the same as in Fishes; nevertheless, in the general construction of the organ of hearing, Reptiles present very important and interesting advances towards a higher form of the acoustic apparatus, which we must proceed to notice.
(1996). The ear of Fishes, being only adapted to hear sounds conveyed through a watery medium, was found to consist only of the membranous labyrinth, enclosed in the cavity of the skull, and without any communication with the exterior of the body. Reptiles, on the contrary, living in air, must be enabled to appreciate the sonorous vibrations of the atmosphere, and are consequently provided with an auditory apparatus capable of responding to pulsations of sound of far greater delicacy than those transmitted through the denser element.
(1997). The first great improvement, therefore, which the anatomist notices in the composition of the ear of a Reptile, is the addition of a tympanic cavity, and of a tense and delicate-membranous drum, the vibrations of which are communicated to the labyrinth or internal ear through the intervention of an ossicle that represents the stapes of Mammalia.
(1998). The drum of the ear is situated immediately beneath the skin, the parts composing the external ear of quadrupeds being as yet entirely deficient. The membrana tympani, that now for the first time makes its appearance in the series of animals, is tensely stretched across the tympanic aperture, being covered externally by the integument of the head. In the Turtle (fig. 349) the tympanic membrane is represented by a cartilaginous plate (a.) The ossicle, or columnella as it is here called, is single and trumpet-shaped; it passes quite across the tympanic cavity (b), its external extremity being inserted into the drum, while at its opposite end it expands into a disk (c), which closes an aperture (foramen ovale) that communicates with the membranous vestibule of the internal ear. It is obvious, therefore, that every tremor impressed upon the membrana tympani will be conveyed by the columnella to the foramen ovale, and thus communicated to the fluid contained in the labyrinth, upon which, as in fishes, the auditory nerve is distributed.
(1999). The cavity of the tympanum communicates with the interior of the mouth by a wide opening, that represents the Eustachian tube, a circumstance evidently intended to prevent air or fluid from being pent up in the tympanic chamber and thus interfering with the free vibration of the drum.
(2000). In Serpents, on account of the peculiar disposition of the pieces of the temporal bone, before described (§ 1907), there is no tympanic cavity, and the columnella (fig. 335, v) is absolutely imbedded in the flesh; the arrangement, however, in other respects is the same as in the generality of reptiles.
(2001). The lower tribes of Amphibia, as we might be led to expect from their close approximation to Fishes, have neither tympanum nor columnella, and thus, like Fishes, can only hear in an aquatic medium.
(2002). The membranous labyrinth of Reptiles (fig. 350, a, b, c) corresponds in its general conformation with that of Fishes, presenting the same semicircular canals, ampullae, and vestibular cavity, and moreover the sacculus contains cretaceous concretions or otoliths of a similar character; but in this class the membranous canals become enclosed in a bony sheath, moulded as it were upon their outer surface, which is another very important step towards perfecting the auditory-apparatus.
Fig. 349. Ear of the Tortoise.
Fig. 350. Auditory and olfactory apparatus of the Turtle.
(2003). Neither must we omit to mention that in the highest of the Reptilia, as for example in the Crocodile, the first rudiment of a cochlea makes its appearance, although as yet in a form of extreme simplicity. This portion of the organ of hearing, which, from the elaborate structure that it presents in the higher Vertebrata, must be regarded as being importantly connected with correct audition, is seen in this the earliest stage of its development to be a simple conical appendage to the sac of the vestibule; and on opening it, it is found to be divided by a central cartilaginous septum into two compartments, which are, however, continuous with each other at the apex of the cone. One of these compartments or canals opens at one extremity into the vestibule, while the other communicates with the tympanic cavity by a very small aperture closed with a thin membrane. Thus, therefore, although the entire organ resembles a simple canal bent upon itself, the representatives of the scala vestibuli, of the scala tympani, and of the fenestra rotunda of the human ear can be distinctly identified.