Yielding of the sclerotic to intra-ocular pressure does not always occur at the posterior hole of the eye. In most cases of astigmatism there is evidence of this yielding at some portion of the globe, and the corneal astigmatism is secondary to this. Where the astigmatism is very high, the yielding of the sclerotic is chiefly lateral and localized. And, in eases of conical astigmatism in which the surface of the cornea is distorted so as to approach the surface of a cone with its apex to one side, the staphyloma is to be found in the neighborhood of the cornea. R. D. Batten (Oph. Review, Jan., '97).
Measurements of 4270 eyes with the ophthalmometer show that, on the aver age, the refraction of the vertical merid ian is about 0.78 dioptre greater than the horizontal, and that '/, of all eyes show corneal astigmatism of between 0.25 and 1.25 dioptres. Astigmatism of high degree is apt to he associated with other abnor malities, is unequal in the two eyes, and has its meridians obliquely placed. There is, also, a marked tendency to heredity of the corneal curvature and astigma tism. A Steiger (Arch. of Oph., p. 254, '97).
The direction of the principal merid ians of astigmatism have been frequently studied in the hope of throwing light on the etiology of the defect.
In 1000 cases of binocular astigmatism the meridian of greatest, curvature was vertical in 60 per cent. and symmetrical in the two eyes in 84 per cent.. while. if differences of 5 or 10 degrees in direc tion had been disregarded. the propor tion of cases of approximate symmetry would be considerably higher. H. Knapp (Trans. of Amer. Oph. Soc., '92).
In 2500 cases the direction of the prin cipal meridians were found: symmet rical in 1307 cases; asymmetrical in 458; heteronymous—that is, one meridian at zero and a corresponding meridian of the other eye at 90 degrces--Lin 173 cases; and homonymous—that is, with the prin cipal meridians parallel, but neither ver tical nor horizontal—in 41 cases. In the cases of symmetrical astigmatism the meridian of greatest curvature was found vertical in 57 per cent.; within 15 degrees of vertical in 20 per cent.; horizontal in 12 per cent.; and within 15 degrees of horizontal in 4 per cent. S. D. Risley and J. Thorington (Trans. of Sec. on Oph., Amer. Med. Assoc., '95).
Extensive wounds or incisions of the cornea give rise to permanent change in the corneal curvature and astigmatism.
This is most noticeable after cataract ex traction. The astigmatism is highest a few days after the corneal wound has closed, and from then on slowly dimin ishes until usually within three months, but sometimes later, it becomes station ary. The changes of corneal curvature are flattening of the cornea at right angles to the incision, and increased curvature in the direction of the line joining the ends of the incision.
This is the natural result of dimin ished resistance along the line of the in cision, allowing cornea to give before the intra-ocular pressure and bulge at this point. E. Jackson (Trans. Pan-Amer. Congress, vol. ii, p. 1430).
After operation the astigmatism dimin ishes by the increasing approximation of both meridians to their curvature before operation. With complicated wounds the astigmatism is always greater, and the decrease from the origi nal amount is less than in eases of normal healing. The chief causes of trau matic astigmatism are the intra-ocular pressure and the tonic contracture of the extrinsic ocular muscles. W. Dolganoff (Arch. of Oph., p. 250, '97).
Case in which a marked temporary change in the astigmatism had been pro duced by pressure of a chalazion. Nor ton (Phila. Med. Jour., July 16, '93).
Belief that cocaine has the effect of producing transient astigmatism. Grigg (Phila. Med. Jour., July 16, '98).
Attention called to the temporary as tigmatism, often considerable in amount, that accompanies blepharitis and is due to lid-pressu•e. Wurdemann (Phila. Med. Jour., July 16, '98).
Diagnosis. — Astigmatism is detected and measured by all of the various meth ods of determining the refraction of the eye, and should be sought by more than one method in any given case. The chief reliance is to be placed on the keratom eter (ophthalmometer), skiascopy, and the test-lenses.
The keratometer measures only the corneal astigmatism, which commonly predominates and approximately corre sponds to the total astigmatism. Its value is mainly that it makes an impor tant suggestion as to the presence, de gree, and direction of the astigmatism, which, when followed up by other meth ods of measurement, effects a saving of time. Skiascopy measures the total as tigmatism of the eye, usually with the greatest accuracy of any method. Com mittee on Objective Tests for Ametropia (Trans., Section on Oph., Amer. Med. Assoc., '94).