EYE IN WHALE LIKE THAT OF FISHES.

103

bearing the same relation to the other as the leaves bear to the back of a book; and the extent of surface thus gained may be easily imagined.

Among the mammiferous animals, the cetaceous tribes, as we should expect from their habits, have eyes very similar to those of fishes; the cornea being comparatively very flat; and the lens almost globular, while they are destitute of proper eyelids-a kind of membrana nictitans alone supplying their place-and of a lachrymal gland. In the other tribes, the comparative convexity of the cornea and lens is intermediate between that of these organs respectively in fishes and birds; while the motions of the iris are the mean, as it were, of those of reptiles and birds: in some quadrupeds, moreover, as the cat, they seem to be in some degree voluntary. The form of the pupil is transversely oval in the pecora and solidungula, and vertically oval in the Feræ. The direction of the eyeballs is in most mammiferous animals outwards; in the ape, however, baboon, monkey, and some few others, it is, as in man, directly forwards: further, in some quadrupeds, as the camel-leopard, the eyeball, though naturally directed outwards, may be turned so far backwards as to enable the animal to see distinctly behind it. Like the nocturnal animals, also, of other tribes, quadrupeds which prowl by night, such as the lion, lynx, cat, bat, &c., have the structure as already more than once described, calculated to enable them to distinguish objects in comparative darkness. On the other hand, where the habits of the animal are such as to exclude it altogether from the light, as no structure of the eye could have compensated for the want of this essential condition of sight, nature has denied them a visual apparatus altogether—as in the case of the mole, which has no optic nerve, and an eye so small, that its existence has been doubted; but whatever be its size, in all animals the eye is a perfect optical instrument, and admirably adapted to the circumstances in which each species is placed. We know it to be composed, as we shall hereafter see, of membranes and humours of different densities, so that they may transmit and refract the rays of light with the greatest regularity and exactness. In the eyes of all animated beings, we see the wisdom and beneficence of the Creator. If the animal dwell in water, the cornea is flat, and the lens spherical; if on the surface of the earth, we find, on the contrary, the cornea more projecting, and the lens more flat; and again, if it wing its airy flight above us, its cornea is the most projecting, and its lens the flattest of all.

Hearing. In the very lowest tribes of animals it appears that this function, like those of smell and sight, is merely a more delicate kind of touch, and performed equally by the whole surface of the body. The greater number of animals of this description have no obvious auditory apparatus, the cuttle being among the few exceptions, and furnishing perhaps the best example of an ear in its rudimental state. In this animal it consists merely of a membranous bag filled with liquid, situated in a tubercle of the cartilaginous ring which surrounds the gullet, and surrounded on all sides by cartilage. Upon the outer surface of this bag is distributed the auditory nerve; while, within the liquid which it contains, are some little pieces of earthy matter, presumed to be necessary to render the vibrations of the liquid, on which sound depends, sufficiently forcible to make the requisite impression on the nerve.

In the greater number of insects, also, the auditory apparatus is very obscure; although it is certain that they do hear, and even very acutely. The immediate seat of the function has been presumed to be the membrane which connects their antenna with the head-but spiders hear which have no antennæ, and grasshoppers after these have been removed. In all likelihood, it is, in the majority of insects, merely a variety of

touch, and common, therefore, to the greater part of the surface. In such animals as present any appearance of a distinct auditory apparatus, as the crayfish, it is very similar in its structure to that of the cuttle; consisting, in like manner, of a bag filled with liquid situated, in this instance, in a bony cylinder at the root of the larger antennæ an auditory nerve expanded upon it, and some pieces of earthy matter in the liquid which it contains. In the cray-fish, however, unlike the cuttle, the bag in question is not surrounded on all sides by the hard mass which contains it, but is near the surface of the body, in contact with a thin membrane-the first approach to the external parts of the auditory apparatus, as met with in the higher tribes of animals.

Nor is the auditory apparatus of most fishes much less simple than that of the invertebrate animals. The membranous bag, however, above spoken of, is connected in general with three semicircular canals, of a similar structure, and furnishing more space for the distribution of the auditory nerve; and the earthy pieces, within the liquid contained in this bag, have begun to assume the appearance of regular bones. Still, in most fishes all these parts are buried within the skull, and send no process to the surface; in some of the cartilaginous tribes alone this bag being prolonged to the upper and back part of the head, where the blind termination of it is covered by the common integuments of the body. One fish alone—the lepidoleprus trachyrynchus—presents any appearance of a canal, proceeding from the surface to meet the internal parts, as in all animals above the rank of reptiles. But the extreme simplicity of the auditory apparatus in fishes and other aquatic animals, is precisely what we should have looked for in beings destined to hear through the medium of water; the vibrations of which, being so much more powerful than those of air, would render the complicated apparatus, requisite in terrestrial animals, in them superfluous.

Accordingly, it is in reptiles that we meet with, for the first time, more or less constantly, not indeed a canal leading from the side of the head towards the ear-which none of them have—but one leading from the back of the pharynx, to form a cavity, interior to which all the parts already described are situated. This cavity is called the tympanum, and contains more or fewer distinct bones, moved by proper muscles, and serving to increase the impulse derived from the vibrations of the air, and to convey it to the internal parts, which now take the name of labyrinth. Some additions, also, are now made to this; for, besides the three semicircular canals, already described as branching from the common bag in one direction, there is now a second series of canals, of a very complicated structure, called cochlea, branching in one another, and affording, of course, still further space for the expansion of the auditory nerve. It is true these parts are not common to all reptiles; serpents, for instance, having no tympanum-although they have a small bone, analogous to those which, in other reptiles, are situated in this cavity, but which, in serpents, is lost in the muscles of the jaws-and none but some of the highest orders of lizards, as the crocodile, having a cochlea. The last-named animal, moreover, makes the first approach to the well-known appendage to the ear, technically called the pinna; being furnished with a kind of external flap, with which it closes the auditory apparatus at pleasure. It is in this way, probably, that the animal excludes too intense sounds when under water; but it appears that the greater number of amphibious animals are capable of adapting their auditory apparatus, at least partially, to the medium in which they are, by putting all the parts upon the stretch, by means of the muscles already spoken of, when in the air, so as to qualify them to receive slighter impressions, and by throwing them all into a state of relaxation when under water, so as to prevent them from being stunned by more powerful ones.

In birds at length we meet with constantly a short canal, leading from the side of the head, and meeting that coming from the pharynx, in the tympanum. They have but one bone in this cavity; and the general structure of the parts of their labyrinth is very similar to that of the higher orders of reptiles. Birds in general want a proper pinna, or ear-flap, its place being commonly supplied by a small tuft of feathers: the owl, however, has something very similar to this part as found in mammiferous animals.

a, Peregrine Falcon; b, Day owl; c, Tawny owl; d, Long-eared owl; e, Barn owl.

The auditory apparatus of the mammalia is in general little more than a greater development of the same parts as are found in birds. The bones within their tympanum are from two to six in number; and all have a pinna except the cetaceous tribesin which it would have been superfluous, from the vibrations of water being too strong to require to be collected by this means-and some others, which either dwell much in the water, as the shrew, or burrow under ground, as the mole, in which, for an obvious reason, it is still less called for. The shrew, however, is provided with a kind of flap, like that of the crocodile, the principal use of which seems to be, so far from increasing the intensity of the impression, to diminish it when the animal is under water. The great size of the pinna in some quadrupeds, and the frequency and rapidity with which they move it in any direction, are familiar to everybody; and may well account, in conjunction with the complicated and delicate structure of the internal parts of the ear, for the extremely acute hearing which they enjoy, and which is so necessary, in many instances, to their security. Hence, a frequent and rapid motion of the ears is, in all animals, with justice regarded as indicative of a timid disposition. We do not here allude to the organs of sight and of hearing in man, since their description will more appropriately fall under our treatises on Optics and on Acoustics.

Taste is certainly, not only in the lower, but in all tribes of animals, merely a more delicate kind of touch; and is situated, for the most part, not exclusively in the tongue, palate, or any other individual organ, but in the whole interior of the mouth.

Although, therefore, in many animals, as the snail, cuttle, and fishes in general, as well as in some individuals of the superior classes, the tongue is hard and cartilaginous, and apparently very little adapted to this function-nay, although it is, as in the flyingfish and gar-pike, altogether wanting-we have no reason to believe that they are destitute of taste; and the same thing may be said of the numerous animals in which the tongue is covered, more or less perfectly, with prickles, or even with feathers, like the toucan, or scales, like one kind of bat, which must, in a great measure, obviate the contact

106

TONGUE IN FORM OF A SUCKING TUBE.

with it of sapid substances. The immediate instrument of taste seems to be certain pointed projections, called papillæ, with which the whole membrane lining the mouth is more or less abundantly furnished; and that organ will be, of course, in all animals the principal seat of this function, on which these papillæ are most copious. In man this is the tongue, the papillæ of which are larger and softer than those of the skin, perpetually moist, and performing the office of touch more exquisitely than the small and dry cutaneous papillæ. In the greater number of animals, also, it is unquestionably the tongue; and this organ is in some, as the bee and humming-bird, rolled into a sucking-tube, and therefore not only subservient to taste, but also to imbibition; and, accordingly, when the lips take the same form, as in the wared whelk, and

SUCKING TUBE OF NEMESTRINA LONGIROSTRIS.

various kinds of fly, we may presume they are an organ, not only of imbibition, but of taste. Acuteness of taste seems to be much promoted by a copious flow of saliva, by which the sapid particles are dissolved; and it may be presumed, there

fore, that it is much greater in the herbivorous than in the carnivorous birds and quadrupeds, as indeed the necessity which the former are under, but from which the latter are exempt, of distinguishing wholesome from deleterious herbs, would seem to require. Carnivorous animals, on the other hand, are directed to their food principally by the smell.

Touch-the most general of the sensations, and of which all the rest are perhaps only varieties-is, collectively speaking, the whole surface of the bodies of animals; although it is in each much more delicate in certain parts of this surface than in others, owing to the greater number of papilla with which they are furnished, and which are generally the immediate instrument as well of touch as of taste. The common integuments of the bodies of animals in general consist principally of the scarf-skin or cuticle; a substance immediately below this, called corpus mucosum, of which the nails and hairs are merely modifications; and the true skin or cutis, the seat of the papilla in question; and there are few animals, even of the lowest tribes, which have not all these envelopes in one form or another. In the armed polype indeed, the sea-blubber, the slug, the earth-worm, and many similar animals, the cuticle takes the form of mere mucilage; while in the corallines, on the other hand, it assumes that of a calcareous mass, by which their bodies are invested. In others, again, it is the corpus mucosum which gives them their earthy covering, a proper cuticle being found exterior to it, as in the sea-urchin, the star-fish, and all the testaceous tribes: the sharp prickles, also, on the shell of the sea-urchin, as well as the hairs of the earth-worm, and numerous other animals of this tribe, are merely modifications of the same substance. A proper cutis seems, indeed, to be wanting in the corallines, as well as in some other animals of quite the lowest orders; but in the testaceous tribes, as the oyster, the cloak is probably a modification of this part, and it is accordingly upon this, or some corresponding organ, that the tentacula, or immediate instruments of touch, are commonly met with. The perspiration from the surface seems to bear the same relation to touch as the saliva

bears to taste; and there are, therefore, few animals which do not perspire in one form or another. In some of these tribes, as the sea-blubber, the perspired matter is said to be luminous; and it is to this cause that the sparkling appearance of the sea by night in some places has been attributed.

In insects, the cuticle is always membranous; while it is the corpus mucosum which constitutes their horny or calcareous sheaths, and forms, also, in some, as spiders, flies, gnats, bees, and butterflies, the fine hairs, feathers, or scales, with which they are in certain parts invested. The proper cutis, again, is below this, constituting, in the lobster, for example, its membranous pellicle. This part is, however, so completely defended, for the most part, from the contact of external substances, that to most insects are given in addition antennæ, palpi, cirrhi, &c., called in general feelers, situated commonly about the mouth, and the chief seat, in them, of the function of touch.

SJAVY!

VARIOUSLY FORMED ANTENNE OF INSECTS.

The cuticle is membranous also in fishes, and immediately invests their scales, as well as the bristles of the stickleback, the tubercles of the sturgeon, &c., all which are formed by the corpus mucosum. Under this is the cutis.

Not only are the smell and taste of fishes very acute, but their touch not less so than that of animals in general. It is astonishing, however, what an extreme degree of heat some fishes can bear. In the thermal springs of Bahia, in Brazil, many small fishes were seen swimming in a rivulet which raised the thermometer eleven degrees and a half above the temperature of the air. Sonnerat found fishes existing in a hot spring at the Manillas at a hundred and fifty-eight degrees Fahrenheit; and Humboldt and Bonpland, in travelling in South America, perceived fishes thrown up alive, and apparently in health, from the bottom of a volcano, in the course of its explosions, along with water and heated vapour that raised the thermometer to two hundred and ten degrees, being but two degrees below the boiling point.

The bodies of most fishes are covered with small brilliant plates of a horny nature, called scales; but in certain kinds these are wanting, as in the turbot, in place of which are found osseous or cartilaginous protuberances in some species, and in others a very smooth skin, without scales or rugosities, but covered with a thick gelatinous secretion. It was observed by Steno, in the skate, that this slimy matter was poured out from numerous orifices regularly placed near the surface; and Dr. Monro has recorded his discovery of a very elegant structure for the preparation of this mucus between the skin and muscles. The secretion is so viscid that it is with great difficulty pressed out. There is a species of carp-the rex cyprinorum of Linnæus-that seems to hold a middle place between the rough and smooth skinned fish; the upper part and back is covered with scales, while these are altogether wanting on the lower part and belly.