From aeulenbe@indiana.edu Ukn Feb 27 19:40:56 1995 Date: Mon, 27 Feb 1995 17:24:48 -0400 (EST) From: Alex Eulenberg Subject: Corneal accommodation revisited Status: RO X-Status: The modern theory that the lens is responsible for accommodation, and not the cornea, is usually credited to Thomas Young, who, at the age of 28, delivered a lecture in 1801, and published among the Transactions of the Philosophical Society of London. This article, according to every source I have come across, definitively proved that 1) the cornea does not change during accommodation to near and far distances, and furthermore 2) the crystalline lens, located behind the pupil, is responsible. It appears, however, that most people have not read this article. For if they did, they would be led to the article that Young was trying to refute, and they would conclude that Young's disproof was incomplete, especially since an extensive rebuttal was reprinted the following year, to which there was no response from Young in the Transactions. It seems likely that it was Young's reputation as the originator of the modern physical concept of energy, his research on color vision, his study of astigmatism and other important areas in optics, not to mention his help in deciphering the Rosetta stone, that convinced people that Young had the right idea -- not a careful comparison of the relevant studies. As a side-note, I add that corneal accommodation has recently been discovered to occur in chicks.. I reprint from the much-neglected article: * * * Final paragraphs of Croonian Lecture of 1795, by Howe (printed in the Philosophical Transactions of the Royal Society of London) August the 28th, the former experiments were repeated by Sir Henry Englefield, Mr. Ramsden, and myself, on the eye of a young lad, and the result was similar to the others, the motion of the cornea was uncommonly distinct. Sir Henry now became the subject o the experiment, and changed the adjustment of his eye from one distance to another in a very irregular manner, without giving the smallest information, with a view to embarrass Mr. Ramsden who was the observer, but without effect, for Mr. Ramsden was able to tell every change in distance he had made, without a single mistake; this exceeded our expectation, and appeared to us so satisfactory that we required no further proofs of the truth of our former observations. Before we concluded our experiments, every mode that could be devised was put in practice to see how far there might be any deception; the eye was moved on its axis, and in different directions, but these motions did not give at all similar appearances to those seen in the adjusting of the eye to different distances. >From the different experiments which I have had the honour to lay before the R. S., I shall consider the following facts to have been ascertained. 1st, That the eye has a power of adjusting itself to different distances when deprived of the crystalline lens; and therefore the fibrous and laminated structure of that lens is not intended to alter its form, but to prevent reflrections in the passage of the rays through the surfaces of media of different densities, and to correct spherical aberration. 2d, That the cornea is made up of laminae; that it is elastic, and when stretched, is capable of being elongated 1/11 part of its diameter, contracting to its former length immediately on being left to itself. 3d, that the tendons of the 4 straight muscles of the eye are continued on to the edge of the cornea, and terminate, or are inserted, in its externial lamina: their action will therefore extend to the edge of the cornea. 4th, That in changing the focus of the eye from seeing with parllel rays to a near distance, there is a visible alteration produced in the figure of the cornea, rendering it more convex; and when the eye is again adapted to parallel rays, the alteration by which the cornea is brought back to its former state is equally visible. Having supported these facts by the evidence of anatomical structure, and absolute demonstration, I shall consider them to be established; and make some observations on the muscular and elastic power by which so very curious an effect as the adjustment of the eye is produced. The 4 straihgt muscles of the eye are attached to the bottom of the bony orbit near the foramen oppticum; they become broader as they pass forward, and when arrived at the anterior part of the eye-ball, are insensibly changed for tendons; these adhere to the sclerotic coat, and terminate in the external lamina of the cornea, which appears to be a continuation of them. When we consider the situation of these muscles, it is evident that their action will produce 3 very different effects on the eye, according to circumstances. When they act separately, they will move the eye in different directions; when together, with only a small quantity of contraction, they will steady the eyeball; and when this is increased they will compress the lateral and posterior parts of the eye. This compression of the eye will force the aqueous humour forwards against the centre of the cornea, while the circumverence is steadied by the muscles, so that the radius of curvature of the cornea will be rendered shorter, and its distance from the retina increased. That the eye-ball cannot be made to recede in the orbit by any of these actions, is sufficienly proved by its not having done so an any of the experiments. These muscles are uncommonly large, and come much more forwaredxd than appears necessary for the purposes generally assigned to them; but when applied to so important an office as that we have just stated, their size, and anterior insertion, are easily explained. It may be imagined that I have allotted to these muscles a greater variety of uses than is compatible with the simplicity of the general laws of the animal economy: but to prove this not tob e the case, I shall only bring the biceps flexor cubiti as an instance of a similar kind. That muscle is attached ot the scapula by both its heads, one of which passes through the joint of the shoulder, they afterwards unite, and their common tendon is inserted into the radius; when the muscle contracts, the first effect will be to steady the joint of the shoulder; if the contraction be increased, it will rotate the radius, and if still more increased, bend the fore-arm. There are many instances in animal bodies of elasticity being substituted for muscular action, but this in the eye is by much the most beautiful of those applications. In the vascular system the arteries are composed of muscular fibres, and an elastic substance; in the natural easy state of the circulation, the reaction in the larger vessels is principally the effect of elasticity; but when increased, it is the effect of muscular contraction. The claws of the lion are drawn up, and supported from the ground, by means of elastic ligaments; but they are brought down for use, which is an action not so often required, by muscles. In the adjustment of the eye it is the same; the state fitted for parallel rays is the effect of ielasticity, but that for nearer distances, which is less frequently wanted, is the effect of muscular action. In these different instances, the intention is uniformly to avoid the expence of muscular action whenever the effect ccan be produced in any other way, as muscular actions consume a considerable quantity of blood, which is the nourishment of the body. That the adjusting the eye to near distances is the effect of an action, or exertion, was very evident to every gentleman concerned in these experiments. In changing the focus of our eyes, we were much astonished, particularly Sir Henry Englefield, at the exertion required to adjust the eye to the near distances, and the facility with which it was adapted to distant ones; the first was a strain on the eye, the 2d appeared a relief to it. When the eye was intent on the near object, it required the attention to be constantly kep tup, or the object became indistinct, and if we looked at it beyond a certin time, the eye was so much fatigued as to lise it at intervals. This corresponds with other muscular actions, for whenever muscles are kept long in one state they begin to vibrate involuntarily. These circumstances explain what may be called a coup d'oeil, or the distinctness with which and object is seen when the eye is first fixed on it. This arises from the nice adjustment produced by the muscles when first thrown into action, which they cannot keep up, being unable to remain long in the same state; nor can they, after having been used for any time, return to this adjustment with the same exactness. The change that takes place in the eye at an advanced period of life, by which it loses its adjustment to very near, and at very distant objects, does not arise from any defect in the muscles, as might at first be imagined, since that would not account for the eye being iunable to seee with parallel rays; nor is there any obvious reason why these muscles should lose their powers, while others, which are not apparently so strong, if we may judge by their effects, retain their full action long after the eye has undergone this change. This defect in the eye, I am led to believe, is brought on by the cornea losing its elasticity as we advance in life, neither contracting nor being elongated to its usual extent, but remaining in a middle state. That elastic substances in the body do undergo such a change, may be well illustrated in the vascular system. The aorta is compsed almost entirely of elastic substance, and there is probably no part of the body, at an advanced age, which is so often found to have lost its natural action; it appears to undergo change from age alone, becoming inelastic, and then taking on diseases of different kinds, as being ossified, or becoming aneurismal; but in neither of these diseases is it found to be contracted, though often the reverse, and when disease has not supervened, the artery more commonly remains in the middle state. The cornea, having similar properties must be liable to a similar change; but its action being less constant, and the power which to resist being weaker, the change will be probably more graudal and less in degree, but sufficient to account for the alteration we find in the focus of the eyes of old people. There are many other circumstances respecting vision, and many which occur in disease, that may be explained by a knowledge of these facts; but as this lecture is only intended to establish the facts themselves, in doing which I ave already taken up too much of the time of the R. S., I shall at some future period consider their application to the phenomena of vision in health and disease. Fig. 10, p. 5, shows portions of the four straight muscles of the eye, with their tendons insensibly lost in the external lamina of the cornea, stretched out and dried. The tendons become broader as they approach the cornea, and form a circle of which the cornea appears to be a continuation. ========================================================================= From aeulenbe@indiana.edu Thu Sep 18 20:55:44 1995 Date: Mon, 18 Sep 1995 18:24:05 -0700 From: mccollim@ix.netcom.com (Richard Mccollim) Subject: Flashes of clear vision Status: RO X-Status: D Alex (and anyone else interested): As you well know, the occasional posts to sci.med.vision on flashes of clear vision are answered with condescending remarks by the professionals. They are probably unaware that there are a few reports in the literature on this phenomenon. On re-reading a paper on "The resting state of accommodation" (Meredith Morgan, Am. J. Optom. and Arch. Am. Acad. Optom, Monograph 214, July 1957), I came across the following: "Le Grand, using skiametry, found five subjects who showed negative accommodation during "flashes of clear vision" while wearing too much convex lens power....Morgan and Olmstead, using skiametry to measure changes in the refractive state of the eye, reported that sudden sensory stimuli, such as an electric shock or a loud noise, may cause a sudden decrease in the refractive power of the eye, usually not more than 0.25 D." I noticed the same effect from a hard fall when hiking in the mountains--a flash of clear vision. I wonder if anyone else has had a similar experience. The comment about "wearing too much convex lens power" suggests a way to provoke flashes. How about wearing strong convex lenses while exposed to a loud explosion and receivng a strong electric shock! :-) (Would that produce a .75 D. reduction in lens power?) Rich The references are: LeGrand, Y, The presence of negative accommodation in certain subjects. Am J. Optom & Arch. Am Acad. Optom, 29:134, 1952 Marg, E. "Flashes" of clear vision and negative accommodation with reference to the Bates method of visual training. Am J. Optom & Arch. Am Acad. Optom. 29:612, 1939 Morgan, M.W., Jr. Olmstead, J.M.D. Response of the human lens to a sudden startling stimulus. Proc. Soc. Exp. Biol. & Med., 42:612, 1939 ========================================================================= From owner-i_see@indiana.edu Mon Sep 18 22:55:44 EST 1995 Date: Tue, 19 Sep 1995 12:41:56 +1000 From: Rene Malingre Subject: Flashes of clear vision Status: RO X-Status: Richard McCollims' post was quite interesting. I don't believe I have seen those references, but I'll look them up. The findings certainly fit with the "dual innervation" model of the ciliary muscle innervation: positive accommodation (increased lens power) is mediated by the parasympathetic branch of the autonomic nervous system. Negative accommodation (relaxation of accommodation) is mediated by inhibition of the parasympathetic innervation (ie it is not an active process), but there is sympathetic innervation to the ciliary muscle, which probably plays a role in modulating sustained accommodation, and "actively relaxes" the ciliary muscle to reduce the ciliary muscle tone. It is believed to act slowly, far too slowly to help at all in relaxing accommodation under normal circumstances. If you are shocked, hormones are released into your blood stream that can "actively relax" the ciliary muscle. These hormones will also cause a constriction of ocular blood vessels, which may also influence the refraction slightly, and interfere with accommodation. If, under normal circumstances, you are not able to fully inhibit the parasympathetic input to the ciliary muscle, this further inhibitory stimulus may cause "negative accommodation," and cause a hyperopic shift in your refraction. I've played around with the effects of topical beta-blockers on the accommodative response, and suspect that the sympathetic nervous system has probably a bigger effect than people think. However, a "shock" stimulus is unlikely to generate a hyperopic shift of more than 0.25 D, unless the person has a degree of pseudomyopia, or a greater than normal accommodative tone. The "clear flash" phenomenon probably has a different explanation in most people, as most myopes have only slight accommodative tone for distance fixation. Rene =========================================================================