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But this, though much, is not the whole by different species of animals the faculty we are describing is possessed, in degrees suited to the different range of vision which their mode of life, and of procuring their food, requires. Birds, for instance, in general procure their food by means of their beak; and, the distance between the eye and the point of the beak being small, it becomes necessary that they should have the power of seeing very near objects distinctly. On the other hand, from being often elevated much above the ground, living in the air, and moving through it with great velocity, they require for their safety, as well as for assisting them in descrying their prey, a power of seeing at a great distance; a power of which, in birds of rapine, surprising examples are given. The fact accordingly is, that two peculiarities are found in the eyes of birds, both tending to facilitate the change upon which the adjustment of the eye to different distances depends. The one is a bony, yet, in most species, a flexible rim or hoop, surrounding the broadest part of the eye; which, confining the action of the muscles to that part, increases the effect of their lateral pressure upon the orb, by which pressure its axis is elongated for the purpose of looking at very near objects. The other is an additional muscle, called the marsupium, to draw, on occasion, the crystalline lens back, and to fit the same eye for the viewing of very distant objects. By these means, the eyes of birds can pass from one extreme to another of their scale of adjustment, with more ease and readiness than the eyes of other animals *.
The eyes of fishes also, compared with those of terrestrial animals, exhibit certain distinctions of structure, adapted to their state and element. We have already observed upon the figure of the crystalline compensating by its roundness the density of the medium through which their light passes. To which we have to add, that the eyes of fish, in their natural and indolent state, appear to be adjusted to near objects, in this respect differing from the human eye, as well as those of quadrupeds and birds. The ordinary shape of the fish's eye being in a much higher degree convex than that of land animals, a corresponding difference attends its muscular conformation, viz. that it is throughout calculated for flattening the eye.
The iris also in the eyes of fish does not admit of contraction. This is a great difference, of which the probable reason is, that the diminished light in water is never too strong for the retina.
In the eel, which has to work its head through sand and gravel, the roughest and
eye may be moved in any of the intermediate angles. By the succession of such actions it may be moved rapidly round in the orbit. In all these cases the action of one muscle is moderated by its opposite. The motions of rotation inwards and outwards, motions in which the eye does not move from its place, but only on its axis, are executed, the first by the superior, the last by the inferior oblique. By the united action of the six, we are enabled to preserve the eye in the same relative position with regard to the object, whether it be at motion, or at rest; and whether the head is fixed or moving in any direction, so as to alter its position with respect to any object; in short, we can by their means direct the eye to any point, and keep it fixed there under any change of the situation of either. To use the expressive words of Mr. Hunter, "the object becomes as it were the centre of motion, or a fixed point, commanding the direction of the actions of the eye, as the north demands the direction of the needle, let the box in which it is placed be moved in what direction it may."-(Recs' Cyclopædia. Art. "Eye." An admirable essay.) Now all this arrangement is very mechanical, and it appears upon deeper investigation that the mechanism is arranged and applied, just as any one perfectly skilled in mechanics would direct; there is not even the most common of all the attributes of chance connected with it, it is not even occasionally misplaced. For instance, these muscles that regulate the motions of the eye are placed considerably before the transverse vertical diameter of its globe, thereby obtaining an extent of power which would have been denied them, if attached behind that line. Thus the muscles not only happen
to be where they were required, but also happen to be in the best possible place.
The human eye is calculated for motion in almost every direction; indeed, the idea of the universal joint must have been suggested by it. To those animals who have not this mobility of eye, various compensations are given. The snail, the lobster, and others, have the eye placed upon a column, that can be bent towards the object to be observed. Others have the number of their eyes increased. Spiders have from four to eight; scorpions one hundred, and in the minute ephemeron, Swammerdam observed two thousand.
The mole, so long supposed to be blind, is now known to have eyes of a very admirable structure. Borrichius, Blasius, &c. have shewn that these organs are preserved from injury during its underground operations by being deeply seated in the cranium, and the orifices are well protected by powerful eyelids and its fur. When vision would be of service, the animal has the power to protrude them in the same manner as the snail.
* Mr. Philip Crampton combats the idea that the marsupium is the agent employed in regulating the focal distance in the eyes of birds; but in doing so he endeavours to establish, and apparently successfully, the employment of another muscle more decidedly connected with the bony hoops of their visual organs, and better calculated to depress their convexity.-Annals of Philosophy, 1813.
Supposing him to be correct, this does not weaken Paley's argument; there is still a contrivance for a par tic: lar purpose.
hardest substances, there is placed before the eye, and at some distance from it, a transparent, horny, convex case or covering, which, without obstructing the sight, defends the organ. To such an animal, could any thing be more wanted or more useful?
Thus, in comparing the eyes of different kinds of animals, we see in their resemblances and distinctions, one general plan laid down, and that plan varied with the varying exigences to which it is to be applied.
There is one property however common, I believe, to all eyes, at least to all which have been examined *, namely, that the optic nerve enters the bottom of the eye, not in the centre, or middle, but a little on one side: not in the point where the axis of the eye meets the retina, but between that point and the nose. The difference which this makes is, that no part of an object is unperceived by both eyes at the same time t.
In considering vision as achieved by the means of an image formed at the bottom of the eye, we can never reflect without wonder upon the smallness, yet correctness, of the picture, the subtilty of the touch, the fineness of the lines. A landscape of five or six square leagues is brought into a space of half an inch diameter; yet the multitude of objects which it contains, are all preserved, are all discriminated in their magnitudes, positions, figures, colours. The prospect from Hampstead-hill is compressed into the compass of a sixpence, yet circumstantially represented. A stage coach, travelling at its ordinary speed for half an hour, passes, in the eye, over one twelfth of an inch, yet is this change of place in the image distinctly perceived throughout its whole progress; for it is only by means of that perception that the motion of the coach itself is made sensible to the eye. If any thing can abate our admiration of the smallness of the visual tablet compared with the extent of vision, it is a reflection which the view of nature leads us every hour to make, viz. that, in the hands of the Creator, great and little are nothing.
Sturmius held, that the examination of the eye was a cure for atheism. Besides that conformity to optical principles which its internal constitution displays, and which alone amounts to a manifestation of intelligence having been exerted in the structure; besides this, which forms, no doubt, the leading character of the organ, there is to be seen in every thing belonging to it and about it, an extraordinary degree of care, an anxiety for its preservation, due, if we may so speak, to its value and its tenderness. It is lodged in a strong, deep, bony, socket, composed by the junction of seven different bones ‡, hollowed out at their edges. In some few species, as that of the coatimondi §, the orbit is not bony throughout; but whenever this is the case, the upper, which is the deficient part, is supplied by a cartilaginous ligament; a substitution which shews the same care. Within this socket it is imbedded in fat, of all animal substances the best adapted both to its repose and motion. It is sheltered by the eye-brows; an arch of hair, which, like a thatched penthouse, prevents the sweat and moisture of the forehead from running down into it ||.
But it is still better protected by its lid. Of the superficial parts of the animal frame, I know none which, in its office and structure, is more deserving of attention than the eyelid. It defends the eye; it wipes it; it closes it in sleep. Are there, in any work of art whatever, purposes more evident than those which this organ fulfils? or an apparatus for exe
The eye of the seal or sea-calf, I understand, is an exception. Mem. Acad. Paris, 1701, p. 123.
The perfection of the excellence exhibited by this mode of insertion is not fully noticed by our author.
"There is a manifest advantage in the optic nerves being inserted on the inside of the optic axes: for if they had pierced the eye in the axis, then the centre of every object would have been invisible; and where all things conduce to make us see best, there we should not see at all. We must likewise have lost some part of an object, if the optic nerves had been placed on the outside of the optic axes: because an object may be so placed, as that all the rays which come from one point, may fall upon the outside of both eyes: but it is impossible they should fall upon the inside of both eyes, therefore that point which is lost in one eye is visible by the other."Taylor's Mechanism of the Eye.
Heister, sect. 89.
§ Mem. R. Ac. Paris, p. 117.
"The eye," says Robert Dingley, in his "Divine Optics," written with all the quaint eloquence of his age, "the eye hath the greatest variety of objects to feed and delight in; it ranges through the world, and in a moment pierces the skies even to the fixed stars. In number the eyes are two, the better to give direction to the body; in figure they are round, and most capable thereby of objects and motion; in situation, they are placed very high, in the most royal place of the body, far above the rest of the senses to direct our motion, and espy danger. ous evils. The eye-lid is a case for this jewel, and the eye-brow a bulwark to defend it. If the face be the palace of beauty, the eye is her throne; if the face be as a ring of gold, the eyes are the diamonds set therein."
cuting those purposes more intelligible, more appropriate, or more mechanical? If it be overlooked by the observer of nature, it can only be because it is obvious and familiar. This is a tendency to be guarded against. We pass by the plainest instances, whilst we are exploring those which are rare and curious; by which conduct of the understanding, we sometimes neglect the strongest observations, being taken up with others, which, though more recondite and scientific, are, as solid arguments, entitled to much less consideration.
In order to keep the eye moist and clean (which qualities are necessary to its brightness and its use), a wash is constantly supplied by a secretion for the purpose; and the superfluous brine is conveyed to the nose through a perforation in the bone as large as a goosequill. When once the fluid has entered the nose, it spreads itself upon the inside of the nostril, and is evaporated by the current of warm air, which, in the course of respiration, is continually passing over it. Can any pipe or outlet, for carrying off the waste liquor from a die-house or a distillery, be more mechanical than this is? It is easily perceived, that the eye must want moisture: but could the want of the eye generate the gland which produces the tear, or bore the hole by which it is discharged,—a hole through a bone?
It is observable, that this provision is not found in fish,- the element in which they live supplying a constant lotion to the eye.
It were, however, injustice to dismiss the eye as a piece of mechanism, without noticing that most exquisite of all contrivances, the nictitating membrane, which is found in the eyes of birds and of many quadrupeds. Its use is to sweep the eye, which it does in an instant; to spread over it the lachrymal humour; to defend it also from sudden injuries; yet not totally, when drawn upon the pupil, to shut out the light. The commodiousness with which it lies folded up in the upper corner of the eye, ready for use and action, and the quickness with which it executes its purpose, are properties known and obvious to every observer; but what is equally admirable, though not quite so obvious, is the combination of two kinds of substance, muscular and elastic, and of two different kinds of action, by which the motion of this membrane is performed. It is not, as in ordinary cases, by the action of two antagonist muscles, one pulling forward and the other backward, that a reciprocal change is effected; but it is thus: The membrane itself is an elastic substance, capable of being drawn out by force like a piece of elastic gum, and by its own elasticity returning, when the force is removed, to its former position. Such being its nature, in order to fit it up for its office, it is connected by a tendon or thread with a muscle in the back part of the eye: this tendon or thread, though strong, is so fine, as not to obstruct the sight, even when it passes across it; and the muscle itself, being placed in the back part of the eye, derives from its situation the advantage, not only of being secure, but of being out of the way; which it would hardly have been in any position that could be assigned to it in the anterior part of the orb, where its function lies. When the muscle behind the eye contracts, the membrane, by means of the communicating thread, is instantly drawn over the fore-part of it. When the muscular contraction (which is a positive, and, most probably, a voluntary effort) ceases to be exerted, the elasticity alone of the membrane brings it back again to its position *. Does not this, if any thing can do it, bespeak an artist, master of his work, acquainted with his materials? "Of a thousand other things," say the French Academicians, "we perceive not the contrivance, because we understand them only by the effects, of which we know not the causes: but we here treat of a machine, all the parts whereof are visible; and which need only be looked upon, to discover the reason of its motion and action †.”
In the configuration of the muscle which, though placed behind the eye, draws the nictitating membrane over the eye, there is what the authors, just now quoted, deservedly call a marvellous mechanism. I suppose this structure to be found in other animals; but, in the memoirs from which this account is taken it is anatomically demonstrated only in the cassowary. The muscle is passed through a loop formed by another muscle; and is there inflected, as if it were round a pulley. This is a peculiarity; and observe the advantage
Phil. Trans. 1796.
+ Memoirs for a Natural History of Animals, by the Royal Academy of Sciences at Paris, done into English, by order of the Royal Society, 1701, p. 249.
of it. A single muscle with a straight tendon, which is the common muscular form, would have been sufficient, if it had had power to draw far enough. But the contraction, necessary to draw the membrane over the whole eye, required a longer muscle than could lie straight at the bottom of the eye. Therefore, in order to have a greater length in a less compass, the cord of the main muscle makes an angle. This, so far, answers the end; but still farther, it makes an angle, not round a fixed pivot, but round a loop formed by another muscle, which second muscle, whenever it contracts, of course twitches the first muscle at the point of inflection, and thereby assists the action designed by both *.
One question may possibly have dwelt in the reader's mind during the perusal of these observations, namely, Why should not the Deity have given to the animal the faculty of vision at once? Why this circuitous perception; the ministry of so many means; an element provided for the purpose; reflected from opaque substances, refracted through transparent ones; and both according to precise laws; than, a complex organ, an intricate and artificial apparatus, in order, by the operation of this element, and in conformity with the restrictions of these laws, to produce an image upon a membrane communicating with the brain? Wherefore all this? Why make the difficulty in order to surmount it? If to perceive objects by some other mode than that of touch, or objects which lie out of the reach of that sense, were the thing proposed; could not a simple volition of the Creator have communicated the capacity? Why resort to contrivance, where power is omnipotent? Contrivance, by its very definition and nature, is the refuge of imperfection. To have recourse to expedients, implies difficulty, impediment, restraint, defect of power. This question belongs to the other senses, as well as to sight; to the general functions of animal life, as nutrition, secretion, respiration; to the economy of vegetables; and indeed to almost all the operations of nature. The question, therefore, is of very wide extent; and amongst other answers which may be given to it, besides reasons of which probably we are ignorant, one answer is this: It is only by the display of contrivance, that the existence, the agency, the wisdom, of the Deity, could be testified to his rational creatures. This is the scale by which we ascend to all the knowledge of our Creator which we possess, so far as it depends upon the phenomena, or the works of nature. Take away this, and you take away from us every subject of observation, and ground of reasoning; I mean, as our rational faculties are formed at present. Whatever is done, God could have done without the intervention of instruments or means: but it is in the construction of instruments, in the choice and adaptation of means, that a creative intelligence is seen. It is this which constitutes the order and beauty of the universe. God, therefore, has been pleased to prescribe limits to his own
• A parallel instance occurs in connection with the human eye, and even still more fully demonstrative of contrivance. One of its muscles, the obliquus superior, passes through what may be described as a cartilaginous pulley, fixed to the upper side of the orbit. This pulley is formed by a cartilaginous plate, with its edges turned up and fixed to the orbit, so as to form a complete oblique tube, one fourth of an inch in length; where the pressure to be resisted is greatest, at the two ends of this tube, it is bound to the orbit by ligaments. Now this is all eminently mechanical, as a mechanic would use the same contrivance if his power was to be applied, as in this instance, horizontally, to effect a vertical movement. Morcover to facilitate the motion, that is, reduce the friction, he would remove all asperities from the rope he employed, and grease well the pulley; now in the example before us these requisites are also provided: the muscle is rounded, and the tube is plentifully supplied with synovia, the fluid commonly but ignorantly denominated joint-oil.
Haller emphatically termed the eye-brows, eye-lids, and lachrymal apparatus, " tutamina oculi."
One of the crustacea, the mud crab, having no eye-lid, sweeps off any annoying particles from its protuberant eyes, by rubbing them against a compensatory tuft of hair that grows in their vicinity; an action as intelligible, observes sir C. Bell, as that of a man wiping his spectacles.
Those who would examine more fully the admirable structure of the human eye, will do well to consult Professor Soëmmering's elegant works, "De oculis humanis," illustrated by his "Icones oculi humani ;" and Dr. Young's "Natural Philosophy." If the research is wished to be pursued concerning the visual organs of animals, Cuvier's twelfth lecture, and Blumenbach's Comparative Anatomy," must be referred to. Yet these are but a small portion of the works that have been composed concerning these interesting subjects. Dr. Young in his "Lectures on Natural Philosophy," enumerates more than seventy publications on the anatomy, comparative anatomy, and functions of the eye.
power, and to work his ends within those limits. The general laws of matter have perhaps the nature of these limits; its inertia, its reaction; the laws which govern the communication of motion, the refraction and reflection of light, the constitution of fluids nonelastic and elastic, the transmission of sound through the latter; the laws of magnetism, of electricity; and probably others, yet undiscovered. These are general laws; and when a particular purpose is to be effected, it is not by making a new law, nor by the suspension of the old ones, nor by making them wind, and bend, and yield to the occasion (for nature with great steadiness adheres to and supports them); but it is, as we have seen in the eye, by the interposition of an apparatus, corresponding with these laws, and suited to the exigency which results from them, that the purpose is at length attained. As we have said, therefore, God prescribes limits to his power, that he may let in the exercise, and thereby exhibit demonstrations of his wisdom. For then, i. e. such laws and imitations being laid down, it is as though one Being should have fixed certain rules; and, if we may so speak, provided certain materials; and, afterward, have committed to another Being, out of these materials, and in subordination to these rules, the task of drawing forth a creation; a supposition which evidently leaves room, and induces indeed a necessity for contrivance. Nay, there may be many such agents, and many ranks of these. We do not advance this as a doctrine either of philosophy or of religion; but we say that the subject may safely be represented under this view; because the Deity, acting himself by general laws, will have the same consequences upon our reasoning, as if he had prescribed these laws to another. It has been said, that the problem of creation was, "attraction and matter being given, to make a world out of them;" and, as above explained, this statement perhaps does not convey a false idea.
We have made choice of the eye as an instance upon which to rest the argument of this chapter. Some single example was to be proposed: and the eye offered itself under the advantage of admitting of a strict comparison with optical instruments. The ear, it is probable, is no less artificially and mechanically adapted to its office, than the eye. we know less about it: we do not so well understand the action, the use, or the mutual dependency, of its internal parts. Its general form, however, both external and internal, is sufficient to shew that it is an instrument adapted to the reception of sound; that is to say, already knowing that sound consists in pulses of the air, we perceive, in the structure of the ear, a suitableness to receive impressions from this species of actions, and to propagate these impressions to the brain. For of what does this structure consist? An external ear (the concha), calculated, like an ear trumpet, to catch and collect the pulses of which we have spoken; in large quadrupeds, turning to the sound, and possessing a configuration, as well as motion, evidently fitted for the office: of a tube which leads into the head, lying at the root of this outward ear, the folds and sinuses thereof tending and conducting the air towards it: of a thin membrane, like the pelt of a drum, stretched across this passage upon a bony rim: of a chain of moveable, and infinitely curious, bones, forming a communication, and the only communication that can be observed, between the membrane last mentioned and the interior channels and recesses of the skull of cavities, similar in shape and form to wind instruments of music, being spiral or portions of circles: of the eustachian tube, like the hole in a drum, to let the air pass freely into and out of the barrel of the ear, as the covering membrane vibrates, or as the temperature may be altered: the whole labyrinth hewn out of a rock; that is, wrought into the substance of the hardest bone of the body. This assemblage of connected parts constitutes together an apparatus, plainly enough relative to the transmission of sound, or of the impulses received from sound, and only to be lamented in not being better understood.
The communication within, formed by the small bones of the ear, is, to look upon, more like what we are accustomed to call machinery, than any thing I am acquainted with in animal bodies. It seems evidently designed to continue towards the sensorium the tremulous motions which are excited in the membrane of the tympanum, or what is better