Page:The American Cyclopædia (1879) Volume V.djvu/183

 COMPARATIVE ANATOMY 179 into its cavity ; the auditory nerve arises from the side of the spinal bulb, without any distinct ganglion ; in such an ear as this the vibrations of the water are communicated to the skin, then to the fluid, and finally to the nerves. So- norous impressions are conveyed with greater intensity in liquids than in air, so that the sense of hearing in fishes may be tolerably acute. In the sharks the semicircular canals are quite large, and the vestibule of the carcharias ol- scurus would contain the whole internal hu- man ear ; of course in such an organ there is no tympanic cavity, and no external ear. In the aquatic amphibia the organ of hearing is like that of fishes ; but in the frog and sala- mander there is a membrane of the tympanum. In the reptiles there is added a cochlea with its own nervous filaments, the tympanic cavity is larger, and a bone, the columella, makes a com- munication between the vibrating membrane find the fluid of the auditory sac ; a Eustachi- an tube communicates also with the throat, and the tympanum is either bare on the level of the skin or just underneath it. In birds there is no external cartilaginous auricle, as in mammals, and only a portion of the external auditory ca- nal ; but in many, especially in the rapacious families, the feathers are erectile around the meatus, and arranged to catch distant sounds ; the bone-surrounded tympanum has also its columella, and its cavity communicates with the fauces by a Eustachian tube, and with the air cells of the skull ; the cochlea is more de- veloped than in reptiles, but, like the rest of the internal ear, does not reach the perfection of the mammalian type. In the owl there is a crescentic valvular fold of integument around the external ear. In mammals there is a per- fect cochlea, a chain of three tympanic bones, an external canal, and an external movable ear ; in cetaceans and other aquatic families, the ex- ternal ear is either wanting, very small, or pro- vided with a valve ; in ruminants and the tim- id rodents it is large and directed backward ; in the carnivora it is small and inclined for- ward. The use of the external ear in man is not exactly determined ; its small size and di- rection would make it of but little use in col- lecting sounds and transmitting them to the tympanum. The experiments of Savart go to show that it acts not only in reflecting sounds, but as a conductor, by virtue of the elasticity of its cartilage. In general the ear of mam- mals resembles that of man. In all animals living in air the vibrations of sonorous bodies are transmitted to a tense membrane, the tym- panum or drum ; the tympanic vibrations are transmitted to the fluid of the internal ear, in which the nerve floats, by the chain of bones ; and in order that the membrane may vibrate freely, the cavity of the tympanum communi- cates with the throat, the tension being equal on both sides. The most complicated of the organs of special sense is the eye. In all ver- tebrates the eyes are two in number, and with few exceptions symmetrically arranged on the sides of the head ; they are essentially on the same general plan, the differences being chiefly in relation to the density of the medium in which the animals reside. Aquatic animals, whether fishes, reptiles, birds, or mammals, have the lens nearly spherical to compjensate for the similarity of the densities of the hu- mors of the eye and the surrounding water. In fishes the eyes are generally large and on the sides of the head, though they are small in the eel, directed upward in urano&copus, and both on one side in the flounder ; the cornea flattened, the sclerotic thick and sometimes partly ossified, the pupil large and round, the lids rudimentary, and the lachrymal gland wanting. The blind fish (amblyopsu) has been found to possess a sclerotic and choroid coat, a layer of cells beneath the latter resembling a retina, a rudimentary leng, and a nerve ; still there is no trace of eye dots on the skin, though such were found in the mass of areolar tissue occupying the usual situation of the orbit ; such an eye cannot be regarded as an organ of vision, as the skin and tissues beneath prevent the pas- sage of light except in a very faint degree ; but such as it is, it corresponds to the vertebrate, and not to the invertebrate eye, with the last of which it has been sometimes compared ; the deficiency of vision in this species is made up by the largely developed organ of hearing, and by the remarkably sensitive papill on the head supplied by the fifth pair of nerves. In the aqua- tic amphibia the eyes are like those of fishes ; in the higher genera, and in reptiles, except ophidians, there are lids moving vertically, a lachrymal apparatus, and a movable iris ; in the snakes there are no lids, but the skin passes di- rectly over the cornea, as in the eel. In birds the eyes are always well developed ; from the anterior convexity and lateral location, their sphere of vision is very extensive ; the retina is quite thick, and apparently gives origin to a fan-shaped dark membrane, the pecten plica- tum, which extends from the entrance of the optic nerve toward the lens; from its being composed chiefly of vessels, some think it a process of the choroid, and accordingly its use may be either to absorb superabundant rays of light to extend the visual surface, or perhaps, as Owen has suggested, to push forward the lens by its erectile property. The pupil is round, the iris very contractile, the cornea large and transparent, and the sclerotic strengthened in front by a series of bony plates ; there are two horizontal lids, the lower largest and most movable, and a third vertical nictitating mem- brane, semi-transparent, which may be drawn over the cornea from the internal angle; lach- rymal glands are also present. There is some apparatus in the eyes of birds, either the pecten or the muscles, by which these organs can adapt themselves to the very different distances at which it is necessary for them to have dis- tinct sight, by which the curvature of the lens and the focus of vision may be changed. In mammals which seek their food by night, the