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DEFECTS OF DIOPTRIC APPARATUS. In common with all dioptric instruments the eye has certain optical defects which tend to interfere with the distinctness of the image.
Chromatic aberration is due to the breaking up of white light into several colors, owing to the different colored rays of which ordinary light is composed, possessing different degrees of refrangibility. We see this in the spectrum and in the colored rings of the marginal part of a biconvex lens made of a single kind of glass. This form of aberration can be corrected by making lenses of two kinds of glass, one of which counteracts the dispersion caused by the other. Optical instruments may thus be made achromatic. This defect is minimized by the iris, which cuts off the marginal rays in which it is most apt to occur. Ssibly the different density of the various parts of the dioptric media may have a correcting effect on the chromatism of the eye. Further correction takes place in the nerve centres which receive the sensation, for just as we mentally reinvert the image, we are unconscious of the color. At any rate, the chromatic aberration is so slight that it needs certain artifices to make it observable.
Spherical aberration depends upon the fact that luminous rays, on passing through a convex lens; strike the various parts of its surface at different angles, and hence are differently refracted. The rays striking the margin of the lens are more bent than those passing through the centre, and hence the former come sooner to a focus. Thus, a luminous point gives rise to a diffused figure, which is circular in perfectly centred dioptric systems, but is stellate in our eyes where the centering of the lenses is not absolutely accurate. Spherical aberration causes us no inconvenience, as the iris only allows the more central rays to pass, in which its influence is not noticed.
Another optical defect in our eyes is astigmatism, depending upon some irregularity of the curvature of the cornea, which may be bent more horizontally than vertically, or vice versa. In either of these cases the light in the vertical and horizontal planes will be differently refracted, so that lines drawn in the two directions will require different adjustments to see them distinctly. This may be recognized if we gaze with one eye at a centre from which many sharply-defined lines radiate; some of the lines cannot be seen distinctly, unless we move the eye or change its accommodation. When the greater curvature extends evenly over the whole diameter of the cornea it gives rise to what is called regular astigmatism, and when the unevenness is localized to one part of the cornea surface it is called irregular astigmatism.
The astigmatism which may be called physiological is not noticed by the individual, but pathological astigmatism often occurs and requires cylindrical glasses to correct it.
Entoptic images are those which depend on the presence of some opacity or difference in density in the transparent media of the eye itself. They look like variously-shaped specks moving over the field of vision. They are only remarkable when we look at an evenly-colored object or through a pin hole in a black card. In using the microscope they often annoy the unpracticed observer.
THE IRIS. Functions.-It has already been mentioned that the motions of the iris alter the size of the pupillary aperture through which
of light must pass, and while it regulates the amount of light admitted, it also acts like the diaphragm of an optical instrument, and always cuts off a large amount of the marginal rays. The great importance of not allowing the rays which would traverse the margin of the lens to enter the eyeball can be understood after what has been said of spherical and chromatic aberration. The iris also contracts when the eye is adjusteid for near vision, independent of the amount of light by which the object is illuminated. This action is of advantage, because the more convex the lens becomes in viewing near objects, the greater is the aberration of the marginal rays. If the iris did not contract in near vision, the closer an object was brought to the eye the greater would be the tendency to indistinctness caused by spherical aberration.
In structure, the iris consists of a framework of delicate connec
tive tissue, like that of the choroid coat, containing many blood vessels. On its posterior surface is a dense layer of pigment cells called the uvea, which gives the eye its color. The act of contracting the pupil is performed by a very definite set of instriated muscular fibres, forming the sphincter which surrounds the margin of the pupil. The sphincter muscle seems always to be more or less in action, because if it be paralyzed, the pupil dilates. The muscular character of the dilator mechanism has been doubted from the fact that radiating muscular fibres have not been satisfactorily demonstrated under the microscope. Certainly the sphincter
Section through the ciliary region, showing the relation of the iris (f) to the choroid (5)
and the ciliary muscle (al, which arises from the margin of the cornea at (e), and passes toward the choroid to the right, where it separates the latter from the sclerotic. Some
bundles of circular fibres are shown, one marked (d). is the most distinctly contractile, for strong electric stimulation always causes contraction of the pupil, and shortly after death the pupils dilate from the relaxation of the sphincter. If we admit the active dilatation of the pupil, we must assume that the power of the sphincter dies more quickly than that of the dilator, or that it at once relaxes when it has lost the stimulus reflected from the retina. There apears to be no difficulty in explaining the dilatation of the pupil as the effect of the elasticity of the tissue and the changes in vasomotor influences.
NERVOUS MECHANISMS CONTROLLING THE IRIS. When the sympathetic in the neck is cut, the pupil becomes considerably contracted. Hence, it has been argued that the nerves supplying the dilator are derived from the sympathetic.
These fibres are supposed to take origin in the gray matter of the cervical spinal cord. The sympathetic also supplies the walls of the vessels, and thus controls the amount of blood going to the iris, and this contraction of the pupil has been explained as due to vascular engorgement. It is argued that though the vasomotor mechanisms may coöperate in dilatation, they cannot be its only cause, as the widening of the pupil may occur in a blood
Since the other tissues are elastic and antagonize the sphincter, paralysis of that muscle would give rise to dilatation even in anæmic mydriasis.
The constricting nerve mechanism of the sphincter muscle is distinct, and more definitely understood. Its common action is reflex; the stimulus starts in the retina, and travels along the optic nerve as an afferent channel to the corpora quadrigemina, where there is a centre governing the contractions of both irides. The efferent impulses are sent by the third nerve to the lenticular ganglion, and thence by the short ciliary nerves to the eyeball.
In accommodation for near objects three muscles act together, their movements being “associated" by the central nerve mechanisms. The same voluntary effort that causes the ciliary muscle to contract, makes the sphincter of the iris contract, and also causes the internal rectus to move the
eye inward. The voluntary nerve centre must be in intimate relation with the reflex centre, which keeps up the tonic action of the sphincter iridis.
We have then central governors for the ciliary and iris movements. The ciliary muscle and sphincter of the pupil are both caused to act by the will, and the sphincter alone is excited by means of a centre, which reflects the stimulus arriving from the retina by the optic nerve to the branches of the third nerve. The dilator of the pupil, if a muscle, is also kept in gentle tonic action by the impulses sent from the spinal cord with the vasomotor impulses, via the sympathetic; but some think that the blood supply and tissue elasticity explain the dilatation.
Further, from the undoubted facts (1) that some reflex contraction of the pupil may be produced by stimulating the retina even when the eye is cut off from the brain centres, and (2) that the local effect of atropia in dilating, and calabar bean in narrowing the pupil, seem in a measure independent of the central nerve organs, it has been concluded that there must also be some local nerve mechanism in the eye which is capable of reflecting nerve impulses, and is affected by these poisons.
The student must carefully bear in mind all the circumstances under which the pupils contract, namely:
1. The application of strong light to either retina causes reflex stimulation of the ciliary nerves of both eyes.
2. Stimulation of the nasal or ophthalmic branches of the fifth afferent nerve reflexly excites the sphincter.
3. Contraction of the pupil is “ associated ” with accommodation for near objects.
4. Similar “ associated ” contraction always accompanies inward movement of the eyeball.
5. During sleep, or as the result of vasomotor disturbances in the brain (anæmia), the pupil contracts.
6. Under the influence of physostigmin, nicotin and morphia.
7. From any stimulation of the optic or third nerves or the corpora quadrigemina.
The circumstances in which the pupils are found to be dilated are equally important from a practical point of view, namely:-
1. In the dark or with insensitive retinæ. 2. Irritation of the cervical sympathetic. 3
Under the influence of atropin, daturin, etc. 4. In asphyxia or dyspnoea from venosity of the blood. 5. Painful sensations from the skin, etc.
THE OPHTHALMOSCOPE. When we look into the eye the pupil appears quite black, no matter in what position we place the light. The reason of this is that the retina can only be made visible by the light reflected outward from it, and that the portion of the rays which is reflected by the retina is so refracted in passing out of the eye that it occupies exactly the same path as that traveled by the light on its way from the point of illumination to the eye. Consequently, unless the eye of the observer be placed directly in