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Then again, for farther protection, we have the eyelashes on the margin of the eye-lids. These serve directly the purposes of shading the light, of entangling an insect, and incidentally of ornamenting the window of the soul with a beautiful and appropriate fringe.
These are not all the matters to be contemplated and admired. The transparency of the organ is necessary for its uses, and how shall this be maintained amid all the floating particles of dust around us, some of them too minute for observation or avoidance? For this important end there is an organ at the external angle of the eye called the lachrymal or tear-producing gland; every moment it throws out a small quantity of water, which the action of the eye-lids gently presses over the whole surface of the ball, and thus every instant accidental pollution is carefully removed, and whatever could soil the pure transparency of the ball is washed away. Now comes the question, How shall this fluid be conveyed away when it has answered its purposes? The provision of the God of nature for this end is truly admirable. At the inner angle of the eye there is a small opening, leading to a grooved channel through the lachrymal bone, which channel opens at its farther extremity into the nostril; this opening at the inner angle is lower than the gland at the outer angle, and hence the tears flow toward it, and they are directed into it by a few hairs which shoot out from beside the aperture. The tears are exhaled from the inner surface of the nostril in the form of vapour. Look in passing at this farther provision for the transparency of the eyeball. It must be nourished with blood for the sustenance of its animal substance, but red blood would disfigure and discolour the ball; accordingly the vessels are of so minute a calibre as that in a healthy state the red par
ticles are denied admission, and it is only in disease that they are admitted. Inflammation, intolerance of light, and pain, are the consequence. Only one farther particular remains to be mentioned, and that is, the provision that is made to guard against an insect, or other extraneous body insinuating itself within the eye-lids, and to the back of the orbit, where its intrusion might be fatal to vision. The inner covering of the eye-lid proceeds backward to the margin of the anterior hemisphere of the ball, and thence is reflected, being perfectly transparent in its duplicature, over the ball whose outward covering it is, and thus at the edge of the lid there is within a doubling of the membrane, and thus nothing short of the violence that would rupture this coat of the eye could force any substance beyond that line. We have contemplated the eye as a ball securely lodged, and next proceed to state by what mechanism its manifold and delicate motions are effected.
It has six muscles, four of which are called the straight ones, and two the oblique. They are fixed at one of their extremities into the bony orbit, at the other into the ball that which is fixed in the upper part of it is called the muscle of pride, from an idea that its action expresses that sentiment; the muscles whose attachments are to the lower part of the globe and orbit express humility. There is a muscle at each side for moving the ball laterally. The outer oblique muscle is of considerable length, and doubles by its tendon through a pulley, and is inserted in the middle of the eye-ball: the short oblique muscle is directly opposite in situation and action, and is a very short one. The combined action of the straight muscles is to fix the eye, the successive action to roll it. The action of the oblique muscles is, as their name imports, to give a slanting direction
to the ball, and the mixed actions of the different muscles give all the variety of movement required. The straight muscles compress the ball, when acting collectively, and render it more convex; their relaxation renders it less so. The globe is thus endued with a vast power of adapting itself to any direction, and to any distance, within a certain range in which visibles can be contemplated. The bodies of all the muscles are behind the ball. Thus long we have dwelt on the outworks of this wondrous mechanism, and now advance to the inner chambers of the eye. Its walls are composed of three coats; the first is that doubling of that inner skin of the eye-lid whose place, transparency, and use we have touched upon. The next is a thick, tough membrane, (suppose it to be like the layer of an onion,) which is composed of the tendons of the six muscles just spoken of. This coat is altogether opaque, and admits no ray of light except in the front of the ball, where it is completely transparent, in what may be called the open part of the circular window, and which is the coloured portion of the eye. In the back part nearest to the brain is an opening for the admission of a fine silk-like cord, called the nerve of sight, which is the immediate organ of vision. This organ proceeds from the brain into the eye-ball through the bottom of the bony funnel. Within the coat last mentioned is another, which commences from the entrance of the nerve, and spreads itself on the inner surface of the former, all the way forward till you come to that part which is transparent. Arrived thither, it breaks off and hangs down, a little circular curtain, endued with the singular properties of contraction and dilatation, without alteration of its circular form. It is this curtain which forms the colour of the eye, black, or gray, or blue. The opening
in the centre of the curtain is what is vulgarly called the star of the eye, and is in reality not a substance but a vacuity.
Now this coloured curtain floats in a small collection of water, called the aqueous humour of the eye. This humour it divides into two chambers, the posterior and anterior. The curtain has a very peculiar faculty, as just hinted; it can draw itself up, and thus enlarge the opening into the back of the eye, or it can fall down, and thus narrow the aperture into a point. The great benefit resulting from this faculty is to adapt the admission of light to the quantity of it: in a strong light we almost close this inner eye-lid, and yet receive a sufficient number of rays to complete the image of visibles. When we go into a faint light, as, for instance, when we go out of doors on a moonless winter's night, at first we can hardly see at all, and for this reason, the curtain is not yet drawn up. By and by we see every thing more distinctly, and, to quote a remark from the unpublished lectures of Professor Jeffray, (a man eminently accomplished in physiology,) if we get a fright, the curtain is so enlarged, that every thing seems larger in its dimensions than it really is; and the man who tells us he met a ruffian eight feet high, may be understood fairly to represent the matter as it really appeared to him. By what mechanism all this is effected, science hath never yet discovered; it is not even known whether the strings of the curtain are composed of muscles, of blood-vessels, or of nerves.
Although the eye has all the coats we have mentioned, it is, after all, a globe scooped, so to speak, to contain, among other things, the iris or curtain, and the watery humour in which it floats. Behind these is a transparent substance, the segment of two different
humour; if it fall exactly upon the centre of this part of the eye, it passes straight forward, if it fall on the edge or on any portion of the arc of the circle-it is bent in its passage through the door-way of the curtain; it is further bent in its passage through the crystalline, at least in a small degree, but the peculiar faculty of this humour is to bring the object nearer; and lastly, it is still further bent as it travels through the glassy humour, till it is so converged as to assist in forming a little and distinct image on the nerve-so small and so distinct as that the starry heavens are painted on a space hardly so large as the point of one's little finger. Secondly, How does this image suggest the idea to the mind? This question does not at first appear to be one of greater difficulty than the former, but it is one upon which Solomon could not have satisfied the queen of Sheba; and in point of fact, the research of whole ages has not advanced us one step in the inquiry. Nor should this be a matter of wonder: for who by searching can find out the Almighty?—his ways are high as heaven.
Whatever belongs to the connection between matter and mind is beyond our attainment and conception; and perhaps all that is certainly known on the subject under discussion may be comprehended in the two following remarks:-The image on the back of the eye is in some way indispensable to vision; for, if the organ lose its transparency, or any of the humours become opaque, so as to intercept the light in its passage to that spot, blindness is the consequence. But supposing the image to be completely formed, and the eye in all those portions of it just mentioned to be perfectly sound and unobstructed; if it become diseased itselfif disease form in the shape of the black drop or gutta