Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/849

OPHTHALMOLOGY 785 ing, of one-fifth to one-tenth, and green and red blindness in the centre of the field. The disk may be normal or have a muddy colour. It usually recovers under abstinence from smoking and alcohol and the use of small doses of strychnia. Hemianopsia means loss of one-half of the visual field. It is generally bilateral and affects corresponding halves of the two retinae, e.<j., the two right halves or the two left ; rarely the two inner halves or the two outer are involved. The dividing line is usually vertical, or nearly so, bending so as to avoid the fixation point. It is due to some disease of the brain, or of one optic tract. (It will be remembered that each optic tract divides at the optic commissure, and supplies one-half of each retina, the right tract going to the right halves, the left to the left.) Night-blindness without organic change may be due to exposure to bright sunlight, sleeping in moonlight, or to scurvy. Snow-blindness is a similar condition, with con gestion of the eyelids and intolerance of light. irors cf In Hyper met ropia parallel rays falling on the eye meet in a point efrac- behind the retina. This is due in most cases to shortening of the axis of the eye (axial hypermetropia), in. rarer cases to absence of the lens, and is the physiological condition, after the age of fifty- five, of all previously normal eyes, owing to diminution of the refractive power of the lens. Clear vision is obtained only when the entrant rays are focused on the retina. In the hypermetropic eye this is effected by increase in the refractive power of the lens by the action of the ciliary muscle. In slight hypermetropia this is easily accomplished, and no complaint may be made. In higher degrees the continuous severe strain on the accommodation in read ing and other fine work gives rise to aching, -watering, and misti ness of the eyes (accommodative asthenopia). These symptoms may first appear after an exhausting illness or prolonged strain of the eyes, in both cases from reduction of the tone in the ciliary muscle. Convergent squint is a common symptom. In the highest degrees, where no amount of accommodation gives distinct vision, the attempt is given up, and no complaint made. In most cases the continuous use of the accommodation produces a spasm of the ciliary muscle, which renders some of the hypermetropia "latent," what remains being the " manifest " hypermetropia. Both together constitute the "total" hypermetropia. To ascertain the hyper metropia place the patient 20 feet from Snellen s test-types. If a weak convex lens makes sight no worse try stronger and stronger glasses till the best vision is obtained. This gives the " manifest " hypermetropia. If there be no improvement with convex glasses the hypermetropia may be all latent. Paralyse the accommodation by putting a drop of atropine solution and test the " total " hyper metropia. For the oplithalmoscopic tests, see below. Children witk asthenopia should wear constantly glasses which nearly correct the hypermetropia. For adults it is not usually necessary to wear glasses for distant vision. For reading the manifest hypermetropia should be corrected, the strength of the glasses being increased as often as astheuopic symptoms reappear. In Myopia or Short-sight the retina lies behind the focal point of parallel rays entering the eye ; it will therefore be at the con jugate focus of some point at a definite distance from the eye, its "far point." Objects beyond this point are seen indistinctly. Within this point and up to the "near" point objects are distinctly seen. Myopia is generally due to elongation of the posterior part of the eye, the sclerotic and choroid at the macula being thinned and bulging backwards. In severe cases the bulging is general, thinning all the coats and enlarging the eye. Myopia is highly hereditary. It rarely begins before seven, and rarely advances after twenty-five. It is always aggravated, and may be produced, by using the eyes on fine work, especially in a bad light and in a stooping position. Myopia results also from increased curvature of the cornea, as in interstitial keratitis and conical cornea. The symptoms of myopia are well known. Distant vision is indistinct, and fine work or printed type is held near the eyes. In high degrees after reading for a time the letters seem to be blurred and to run into each other, while the eyes ache and water owing to inability of the internal recti to keep up the necessary convergence (muscular asthenopia). Divergent squint may be present. The eyes are often kept half shut (hence the name "myopia") to exclude the excess of light. Ophthalmoscopic examination often shows the "myopic crescent" at the outer side of the disk, or all round it, or at the macula. The other tests are described below. As regards treatment, all myopics should work in a good light, and not in a stooping position. If desired, concave lenses may be given for dis tant vision. For near vision in high myopia concave lenses not fully corrective may be given, to enable the patient to hold his work at a greater distance and to avoid stooping. If there is muscular asthenopia prisms with their bases inwards may be used to relieve the strain on the internal recti. Myopic eyes are liable to various affections, muscre volitantes, opacity and fluidity of vitreous, choroidal haemorrhage, ami detachment of retina. Astigmatism is either regular or irregular. Regular astigmatism is due to the fact that the surfaces of the cornea and lens are not .segments of spheres. The principal abnormality is in the cornea, and it is found that the meridians of greatest and least curvature (the principal meridians) are always at right angles to each other, and that the intermediate meridians pass by regular gradation from the one to the other. It is evident that rays of light from a point passing through the plane of greatest curvature will have met before those passing through the plane at right angles to it, which will form a line, that similarly the first set of rays will have crossed and will in their turn form a line by the time the second have reached their focus, and that between these two pcii ts the image will be circular or oval, but blurred. In no case will the image be a point, and hence vision will never be distinct. If one of the principal meridians be emmetropic the astigmatism is "simple" ; if both be either hypermetropic or myopic it is "compound"; if one be hypermetropic and the other myopic it is "mixed." If spherical lenses do not raise the sight of otherwise healthy eyes to the normal standard astigmatism is probable. On oplithalmo scopic examination the disk will be found oval, and altering its shape when the lens is removed from the eye ; with the direct method the vessels are not seen with equal distinctness, and may pass across the field in the two principal meridians in the same direction but at different rates if the astigmatism be compound, in opposite directions in "mixed" astigmatism. If the patient look at an arrangement of radiating lines of equal thickness he will not see them all with equal distinctness. Astigmatism is corrected by neutralizing^ the inequality of the refractive surfaces by means of cylindrical lenses. In many cases the vision cannot be brought up to the normal standard. There are many ways of estimating astigmatism. One method is to find by the test-types the spherical lens which gives the best distant vision ; then, by means of a narrow slit in a metal disk, to find the plane in which vision is further most improved. Spherical lenses are placed in front of this slit till the one which gives the best attainable vision is found ; this gives the cylindrical lens necessary for this plane. A similar inquiry is conducted with the plane at right angles to this. Spectacles are ordered compounded of the spherical lens and the two cylindrical lenses with their axes at right angles to each other. Irregular astigmatism depends on irregularities in the surface of the cornea or in the refractive power of the lens. It can seldom be remedied. Presbyopia. From ten years onwards the "near" point of the Dis- eye gradually recedes, owing to increasing firmness of the lens and orders of probably diminishing power in the ciliary muscle. Presbyopia has accom- been arbitrarily fixed as commencing when the near point recedes moda- to 9 inches, because then discomfort in reading is generally com- tion. plained of. In normal eyes it begins about forty, in hypermetropia earlier, and in myopia later. There is no increased difficulty in seeing distant objects, but ordinary type has to be held incon veniently far away. The treatment consists in giving glasses to enable the patient to read at 9 inches. In practice it is usually sufficient to enable him to read at 12 inches distance. As age advances the presbyopia increases, and it is necessary from time to time to increase the strength of the spectacles. Paralysis of the Accommodation is not uncommon after diphtheria ; it forms one of the symptoms of paralysis of the third nerve. Spasm of the Accommodation is common in hypermetropia, and is sometimes present in the opposite condition of myopia. In strabismus the two eyes are not directed to the same point in Strabis- space. The deviation may be inwards or outwards, downwards or mus or upwards, the first two forms being by far the most common. In- squint ward or Convergent Strabismus is due cither (1) to paralysis of the external rectus muscle or (2) to over-development of the intend. (1) Paralytic convergent squint is the result of some affection of the sixth nerve or its nerve-centre, owing generally either to syphilis or to nervous exhaustion. It varies in amount from slight weak ness to complete paralysis of the muscle. When the paralysis i; complete the eye is turned inwards and cannot be moved outwards beyond the middle of the fissure of the eyelids. In minor degrees where the deformity is not so evident, it may be difficult to tell which is the squinting eye. If the patient be told to look at an object held a short distance in front of him, and a piece of ground glass be placed before the squinting eye, neither eye will move ; if the sound eye now be covered the squinting eye will turn outwards to fix the object, while the sound eye will move a greater distance inwards; the "secondary" is greater than the "primary" squint. The reason of this is that the nerve to the paralysed externus and that to the sound internus of the other eye, which work together, both receive the same stimulus from the will, and that the sound muscle acts more strongly than the other. Owing to the displace ment of the yellow spot double vision is produced, the false image bein< projected towards the side of the paralysed muscle (homony- XVII. 99