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and an activity consequent on volition. Nothing less than perceptions of the mind, and voluntary acts, suited to a thousand circumstances of relation, could preserve the higher classes of animals, and man above all others, from destruction.
All these provisions proceed from an arrangement of nerves and muscles. The mechanical adjustment of the muscles and tendons is perfect according to the principles of mechanics. The muscles themselves possess a different property; they are irritable parts; motion originates in them. This living property of contraction is admirably suited, in each particular muscle, to the office it has to perform. In some it is necessary that the muscles should act as rapidly as the bowstring on the arrow; in others their action is slow and regular; in others it is irregular, and after long intervals, according as the functions to which they are subservient require. The motions of the limbs, the motions of the eye, those of the heart and arteries, stomach and bowels, are all different. This appropriation of action is not in the muscles themselves, but as they stand in relation to the nervous system, and the sensibilities which impel them.
We hope, then, that by the course we have taken, we have carried the reader to a higher sense of the perfection of the animal structure. We first drew him to observe provisions in the strengthening of the bones, the adjustment of their extremities to the joints, the course of the tendons, and other such mechanical appliances; proving to him the existence of design in the formation of the solid fabric of the body. We have then explained how that motion is produced which was at all times familiar to him, but even the immediate causes of which he did not comprehend. We have, in the last place, shown him that under the term Life, he has a still more admirable subject of contemplation, in the adjustment of the living properties; in the sensibilities, which differ not so much in degree as in kind; and in their appropriation, both to the operations of the internal economy, and to the relations external and necessary to safety.
It is not possible to examine these things without having the full proofs before us of the power of the Creator in forming and sustaining the animal body. As a man with gutta serena may
turn his eyes to the sun, and feel no influence of light, so may the understanding be blind to these proofs. With the celebrated Dr Hunter, we may say, that he who can contemplate them without enthusiasm, must labour under a dead palsy in some part of his mind; and we must pity him as unfortunate.]
A COMPARISON OF THE EYE WITH THE HAND.
"And the eye cannot say unto the hand, I have no need of thee."
Ir in quest of an object which shall excite the highest interest, and at the same time afford the most convincing proofs of design, we naturally turn to the Eye, as the most delicate of all the organs of the body. And some consideration of this organ is appropriate to our present purpose, which is to show how much the sense of vision depends on the Hand-how strict is the analogy between these two organs.
From the time of Sir Henry Wotton, to the latest writer on light, the eye has been a subject of admiration and eulogy. But on a former occasion,* I have ventured to say, that this admiration is misplaced, if given to the ball of the eye, or the optic nerve, exclusively. The high endowments of this organ belong to the exercise of the whole eye-to its exterior appendages of muscles, as much as to its humours and the proper nerve of vision. It is to the muscular apparatus which moves the eye, and to the conclusions we are enabled to draw from the consciousness of muscular effort, that, in combination with the impression on the retina, we owe our knowledge of the form, magnitude, and relations of objects. One might as well imagine that he understood the uses of a theodolite by estimating the optical powers of the glasses, without looking to the quadrant, level, or plumb-line, as suppose that he had learnt the whole powers of the eye by confining his study to the naked ball.
Let us begin by some observations on the minute structure, and the sensibility, of the retina. The retina is the internal coat of the eye; it consists of a delicate, pulpy, nervous matter, which is contained between two membranes of extreme fineness; and these membranes both support it and give to its surfaces a smoothness mathematically correct. The matter of the nerve, as well as these supporting membranes, is perfectly transparent during life. In the axis of the human eye, there is a small * See Philosophical Transactions.
portion which, after death, when the rest of the membrane becomes opaque, remains transparent; and has thence been mistaken for an opening in the retina.* Surprising as it may be after all the industry employed to demonstrate the structure of the eye, it is only in the present day that a most essential part of the retina has been discovered-the membrane of Mr Jacob. From observing the phenomena of vision, and especially the extreme minuteness of the image cast upon the retina, I had conceived that the whole nerve was not the seat of vision, but only one or other of its surfaces. That could not be well demonstrated until this exterior membrane of the retina was known; now we see, when it is floated in water under a magnifying glass, that this membrane is of extreme tenuity: and its smooth surface is calculated to correspond to the exterior surface of that layer of nervous matter, which is the seat of the sense.
The term retina would imply that the nerve constituted a network; and the expressions of some of our first modern authorities would induce us to believe that they viewed its structure in that light, as agreeing with their hypothesis. But there is no fibrous texture in the matter of the nerve: although, when floated and torn with the point of a needle, the innermost of the membranes which support the retina, the tunica vasculosa retina, presents something of that appearance.
Vision is not excited by light, unless the rays penetrate through the transparent retina, and reach its exterior surface from within.
We all know that by pressing upon the eye-ball with a key or end of a pencil-case, zones of light are produced: and they are perceived as if the rays came in a direction opposite to the pressure. It may be said, that here the effect of the pressure is assimilated to that of light; and as light can approach and strike the part of the nerve pressed upon from without by the key, only by entering the interior of the eye and coming from within, that the zones of light produced by the mechanical impulse must appear in the usual direction of rays impinging upon that part: and that, consequently, they will give the impression of their source being in the opposite quarter. Contrast, however, this phenomenon with the following experiment. Let the eyelids be closed, and covered with a *It is this part which is called the foramen of Soemmerring.
piece of black cloth or paper, with a small hole in it; place this hole, not opposite to the pupil, but to the white of the eye; then direct a beam of light upon the hole: this light will be seen in its true direction. Why is there this difference in the apparent place from which the light is derived in these two cases? Is it not because the rays directed through the hole upon the white part of the eye-ball, after penetrating the coats and striking upon the retina at this part, pierce through it, and through the humours of the eye, and impinge upon the retina again on the opposite side? That explains why light transmitted in such a manner shall appear to come from a different quarter. But it does not explain why there should not be a double impression-why the beam of light should not influence the retina while penetrating it in the first instance; that is, in passing through it from without inwards, as well as when it has penetrated the humours and impinged upon its opposite part, from within outwards.
Another fact, which has perplexed philosophers, is the insensibility of the optic nerve itself to light. If it be so contrived that a strong beam of light shall fall upon the bottom of the eye, so as to impinge on the end of the nerve where it begins to expand into the delicate retina, no sensation of light will be produced. This ought not to surprise us, if am correct in my statement that the gross matter of the nerve is not the organ of vision, but the exterior surface of it only; for in the extremity of the optic nerve there is, of course, no posterior surface. Indeed, nothing can better prove the distinct office of the nerve itself, as contrasted with the expanded retina, than this circumstance, that when a strong ray of light strikes into the nerve, the impression is not perceived: it seems to imply that the capacity of receiving the impression, and that of conveying it to the sensorium, are two distinct functions.
Is not this opinion more consistent with the phenomena, than what is expressed by one of our first philosophers,—that the nerve at its extremity towards the eye is insensible, and forms what has been called the punctum cœcum (blind spot), because it is not yet divided into those almost infinitely minute fibres, which he considers can alone be fine enough to be thrown into tremors by the rays of light?
Independently of this "punctum cœcum," we have to observe