Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/479

Rh EHVES AND SKNSE ORGANS.] REPTILES 461 junction between the spinal nerves and the sympathetic cords there are ganglia, and from these sympathetic nerves proceed to the iscera, the heart, and the various blood-vessels. The Ear. The auditory organ may consist of three parts the external, median, and internal ear. The ex- ternal ear, however, hardly exists in Reptiles, for there is no external fold of skin representing the external ear of Mammals in any save in the Crocodiles, and in them it is little more than a rudiment. The true internal ear always exists, and is composed of two parts (1) a membranous labyrinth, containing fluid, enclosed in (2) a bony labyrinth, which has also fluid con- tents. The former consists of three membranous semi- circular canals which open into a membranous sac, the vestibule, which is connected with a tubular structure, the membranous cochlea. It is to these parts that the ultimate fibres of the auditory nerve are distributed. The bony labyrinth is formed of the otic bones (prootic, epiotic, and opisthotic), which enclose it completely save at the points where openings are left called the fenestra ovalis and the fenestra rotunda. The latter is entirely sur- rounded by the opisthotic. The fenestra ovalis is partly enclosed by the prootic and partly by the opisthotic. The prootic specially protects and shelters the anterior vertical semicircular canal of the membranous labyrinth, and cor- respondingly constitutes that of the bony labyrinth. The posterior vertical semicircular canal is similarly related to the epiotic, and the external horizontal semicircular canal is sheltered by the prootic and opisthotic. The membranous cochlea helps to almost divide the bony chamber in which it lies into two parts called scake, which communicate at the apex of the cavity. Such being the innermost ear of Reptiles and an outer- most ear existing only in rudiment in Crocodiles, there is also a median ear which may or may not exist in this class of animals. This median ear is the tympanic cavity or chamber. It is into this cavity when present that the fenestra ovalis looks, while the fenestra rotunda looks into the cochlea. Both fenestrse are closed with membrane, so that the fluid in which the membranous labyrinth is suspended cannot escape through them. The fenestra ovalis has fitted to the outer side of its closing membrane a small ossicle of similar shape to itself, which generally has projecting from its outer side a more or less elongated and slender ossicle, the stapes or columella auris. This rod may be wanting, however, as in Typhlops, Rhinophis, and Tortrix. When a middle or tympanic cavity exists, the stapes traverses it from the ossicle, closing the fenestra ovalis to become attached to a membrane, the tympanic membrane, which forms part of the external wall of the tympanum. Where there is no tympanic cavity, the stapes simply extends outwards amongst the muscles which lie external to the internal ear. In the Oplddia and Amphis- bsenians and some other Snake-like Lacertilians there is no tympanic cavity, and there is none in Hatteria. Its pres- ence is sometimes inconstant in those Lizards ; thus in the Slow Worm, Anguis fragilis, it may exist in some in- dividuals and not in others. When it exists, it may not show any external indication of its presence as in the Chamseleons and some Chelonians, Generally, however, the tympanic membrane is covered by a scale of correspond- ing size, so that its presence is plainly marked externally. The tympanic cavity communicates with the back of the mouth by a wide opening in Lacertilians ; but in Chelonians this communication is contracted into a narrow passage, the Eustachian tube, which opens (one from each tympanum) by a separate aperture on the roof of the mouth. In the Crocodilia these passages become further complicated and connected so that, in addition to the aperture on either side of the mouth, there is also a median aperture which is connected with both the Eustachian tubes, that of the right ear and that of the left. The Eye. All Reptiles have eyes, although in some Eye. forms they are quite rudimentary and hidden beneath the skin, which is not at all or but very slightly modified in structure where it passes over them, as, e.g., in Typhlops and Typhline. The eye consists of the same parts as in other Verte- brates, namely, of a nervous expansion at the back (the retina) derived from the optic nerve, a coloured lining (the choroid), a transparent medium (the vitreous humour) separated from one more anterior (the aqueous humour) by a sort of diaphragm (the iris), and a solid, transparent body (the crystalline lens), whilst the whole structure is enclosed by a fibrous membrane, the sclerotic, to form the " ball of the eye," a superficial part of the sclerotic being transparent and distinguished as the cornea. The ball may have the skin which invests it externally separated in front of it into two folds, the eyelids. When the skin is not so separated, then a closed sac of delicate transparent membrane, the conjunctiva, lies over the cornea between it and the skin, and when the skin is divided into folds or eyelids then the conjunctiva is divided also, one layer lying as before next to the cornea, and the other layer being reflected from the outer margin of the cornea into the inside of the eyelids which it lines. There is often a third eyelid, which can be drawn obliquely over the eye, and is called the nictitating membrane. A glandular structure, the lacrymal gland, lies on the outer side of the front of the eyeball, its secretion passing into the mouth by the lacrymal canal ; another gland, the Harderian gland, may lie on the inner side of the eyeball. The sclerotic may be strengthened by bony plates being formed within it. A vascular coloured membrane, the pecten, may project into the vitreous humour from near the entrance of the optic nerve. The ball of the eye is moved by four straight and two oblique muscles, and may also be suspended by a funnel-shaped muscular sheath, the choanoid muscle. There are also special muscles of the eyelids. In the Ophidia the eye is generally rather large, but very small in the Typhlopidss and Uropeltidw. In no Ophidians is the skin over the eyes divided into eyelids, so that the sac of the con- junctiva beneath it is never subdivided. In the Eurystomatous Ophidians, however, the skin, when it passes over the eyeball, is transparent. The outer (or epidermal) layer of the skin is cast with the rest of the skin of the body. The lacrymal canal is wide, but opens into the mouth by a narrow aperture on the inner side of the palatine bone. In some Trigonocephali the lacrymal gland extends almost all round the eyeball. The sclerotic never develops bony plates in any Serpent. Amongst the Laccrtilia, many, such as the Amphisbaenians, the Geckos, and many Skinks, have the eyelids undivided, as they are in Ophidians. Most Lizards, however, have an upper and an under eyelid, and also a nictitating membrane, and there may be a cartilaginous or bony disk in the lower eyelid. Many Skinks have the lower eyelid more or less transparent. The eye of the Chameeleon is most peculiar. The larger eyelids have. but a minute aperture, and the two eyeballs can be rotated independently, so that their axes may be differently directed simultaneously. There is a rudiment of a third eyelid, and there is a bony plate in the lower part of the large eyelid. The Harderian gland is large, but the lacrymal is small. The lacrymal canal opens below in the outer wall of the posterior nares. In most Lacertilians there are overlapping bony plates in the sclerotic, but these are wanting in Chamseleons and Geckos. There is mostly a pecten. In some species the lacrymal gland appears to be wanting. The nictitat- ing or third eyelid is drawn over the eye by the simultaneous action of two curiously arranged structures. One of these is a muscle which takes origin from the postero-inner orbital wall and ends by forming a fibrous loop. The other is a tendon which passes through the loop to be attached at one end to the nictitating membrane, and at the other to the inner wall of the orbit. In Hatteria there are eyelids the lower with a cartilaginous disk and a nictitating membrane. The sclerotic contains osseous lamellae, and there is no pecten. The Crocodilia have three eyelids, and no bony plate in any part