1894.] The Refractive Character of the Eyes of Horses. 415 



i 



1 and this rotation is produced by the oblique muscles of the eye. As 

 the horse can move its head vertically through an angle of about 90, 

 (the pupil all this time remaining horizontal) it can be seen how 

 ; important is the function of these oblique muscles. 



Though in direct sunlight the pupil closely contracts, yet in or- 



I dinary artificial light it dilates. This phenomenon can be turned to 



1 practical advantage in an ophthalrnoscopic examination. Further, I 



have found that if daylight be thrown into the eye by means of a 



mirror the iris does not, as a rule, contract ; this may be proved by 



; placing a horse in a stable and excluding daylight from all but one 



1 source, and utilising that source to illuminate the eye by means of a 



; mirror; a complete examination of the fundus can now be made, 



owing to the dilatation or, at any rate, non-contraction of the pupil. 



For the last two years I have used no artiGcial light in my 



ophthalmoscopic work, and in the enquiry I am about to detail the 



' whole was carried out in broad daylight. I have met with cases 



I where the pupil has contracted on throwing a beam of daylight into 



the eye ; such do not amount to more than 6 per cent, or 8 per cent. 



of the eyes examined. 



Horses no doubt possess the power of accommodation, but I have 



never been able to satisfy myself that the instillation of atropine 



i paralyses this power as it does in man ; in fact, I think I can go so 



far as to say that in many cases no action is produced by this drug 



; on the ciliary muscle, though the pupil be widely dilated. In a 



small proportion of cases I believe I have been able to observe some 



defect in accommodation under atropine. 



My attention to this point was especially directed by a remark 

 made by Messrs. Lang and Barrett* to the effect that a cat, the pupils 

 of which they had thoroughly dilated with atropine, had no difficulty in 

 pursuing and catching a mouse. Though it is difficult to apply such 

 a satisfactory test to the horse, yet I have adopted measures which 

 should, undoubtedly, have demonstrated any paralysis of accommoda- 

 tion had such existed. 



The physiological pecularities of the horse's eye, which I have 

 ventured briefly to draw attention to, induced me to examine the 

 nature of the refraction, especially as a knowledge of it was calculated 

 to be of practical importance and utility. 



Disorders of vision are very common amongst horses ; the most 



careful examination of such eyes often fails to reveal any pathological 



condition, but I think it will be possible to show that the nature of 



their refraction is calculated to throw light on an obscure condition. 



We must accept it as practically correct that disorders of vision in 



* " The Eefractive Character of the Eyes of Mammalia," by W. Lang, F.R.C.S., 

 and J. W. Barrett, M.B., ' Eoyal London Ophthalmic Hospital Reports,' rol. 11, 

 Part II. 



