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A MANUAL OF VETERINARY PHYSIOLOGY 



of the cornea or lens, generally the former. The result of this 

 condition is that the rays of light passing through one meridian 

 of the eye are brought to a focus earlier or later than those 

 passing through the meridian at right angles to it. The horse 

 is very commonly astigmatic ; the horizontal is generally the 

 meridian of least curvature, and corresponds to the long diameter 

 of the pupil . 



Errors of Refraction. — The amount of error of refraction is small 

 in animals, but it is somewhat remarkable that the nature of the 

 error should have proved so variable in the hands of different 

 observers', and it has been suggested that myopia in horses is more 

 frequent in the northern countries of Europe. 



In fifty-four horses the writer found, of ioo eyes examined : 



i per cent, emmetropic. 

 3 per cent, hypermetropic. 

 90 per cent, myopic. 

 6 per cent, mixed astigmatism. 



All other observers have found a larger amount of hypermetropics, 

 a smaller proportion of myopics, and a much larger proportion of 

 emmetropics.* For example : 



As stated above, the amount of error existing is small, the 

 chief visual defect, exclusive of myopia, being astigmatism ; the 

 number of astigmatic horses is remarkable, about 50 per cent. 

 Low degrees of astigmatism are of no importance, but a markedly 

 astigmatic eye may account for the frequency of ■ shying.' Accord- 

 ing to the observation of Long and Barrett, the cow appears to be 

 hypermetropic, and the eye suffers from astigmatism. In dogs and 

 cats they found the refraction to approach emmetropia closely, but 

 in nearly all the wild animals examined by these observers the 

 refraction was hypermetropic. Recent work on the eye of the dog 

 shows it to be always myopic. f 



Berlin, whose work with Eversbusch on the eyes of animals is a 

 classic, says the horse is slightly hypermetropic, but not sufficiently 

 to prevent the eye being called emmetropic! 



The Movements of the Eyeball are brought about by means 

 of the ocular muscles ; in this way the globe of the eye can be 



* II nuovo Ercolani, 1909. 



f R. Boden, Archiv fur Vergl. Ophthalmologie, January, 1910. 

 \ At the time Berlin wrote, the method of determining the refraction 

 errors of the eye by ' skiascopy ' was hardly known. 



