544 A MANUAL OF VETERINARY PHYSIOLOGY 



of accommodation, and the explanation given above is that of 

 Helmholtz ; it is the one generally accepted. 



When a candle is held opposite to the eye three images of the 

 flame are seen : one, a very sharp bright one, obviously reflected 

 from the cornea ; a second, much duller, but also large, reflected 

 from the anterior surface of the lens ; and a third, very small, 

 brighter than the middle one, and inverted, reflected from the 

 posterior surface of the lens (Fig. 163). In a normal eye these are 

 seen perfectly, and move in a definite direction when the candle 

 is moved, the inverted image passing in an opposite direction to 

 the two erect images, and all are equally visible at any point on 

 the reflecting surfaces. This phenomenon has been taken 

 advantage of in determining the clearness of the media of the 

 eye, and though superseded by the greater accuracy of the 



ophthalmoscope, it is still a valuable 



aid ; in cataract one or more of 



II the reflections becomes blurred, and 



sometimes the image is duplicated. 



j! The first and second images are 



" erect, inasmuch as they are reflected 



from a convex surface, but the 



A • D _ C third image is inverted, being re- 



Fig. 163. — Diagram of the fleeted from the posterior surface 



Katoptric Test. f the lens, which, viewed from 



A, From the anterior surface the front, is concave. During the 



of the cornea; B, from the act f accommodation the relative 



anterior face of the lens; and -,. r ,t_ ,, ,-, 



c. from the posterior face of Position of these images alters ; the 

 the lens. second becomes smaller or larger, 



and advances nearer to or recedes 

 from the first, as the anterior face of the lens becomes more 

 convex or flatter, as the case may be. This observation affords 

 the proof that accommodation is due to the varying convexity 

 of the anterior surface of the lens. Fishes are normally short- 

 sighted, and accommodation for a distant object is effected with 

 them by moving the lens towards the retina. 



The ciliary muscle is governed by the short ciliary nerves 

 derived from the ciliary ganglion, and, therefore, indirectly from 

 the third cranial nerve. In the human subject the constrictor 

 fibres of the iris and the ciliary muscle are paralysed by atropine, 

 but in the cat (as first pointed out by Lang and Barrett*), the 

 dog, and certainly in the horse, there is no evidence that any 

 paralysis of the ciliary muscle takes place under atropine, though 

 the pupil dilates. Under the full effect of atropine all these 

 animals can see objects quite close to the eye, and this they 

 could not do if the ciliary muscle were paralysed. 



* ' The Refractive Character of the Eyes of Mammalia, ' Royal London 

 Ophthalmic Hospital Reports, vol. xi., part ii. 



