THE EYE & SOME COMMON AFFECTIONS 217 



the second pair of cranial nerves. The optic nerve forms 

 the innermost hning of the eye — the retina — and through 

 it the impressions are carried to and from the brain.* 



The eyeball itself consists of a white portion known as 

 the sclerotic coat, and into this the transparent portion 

 of the eye— the cornea — is fitted, exactly as a watch- 

 glass is fitted into the rim of a watch. Immediately 

 behind the cornea lies the iris, which is a structure com- 

 posed of circular muscular fibres and having a central 

 opening — the pupil. The iris is the pigmented portion of 

 the eye and confers the colour upon it. The pupil dilates 

 and contracts in accordance with the degree of light 

 admitted into the interior of the eye, so that it contracts 

 in strong light, and dilates in the dark. Likewise there 

 is the same response to various chemical agents, such as 

 belladonna and atropine. 



Immediately behind the iris is the crystalline lens, 

 through which hght is transmitted on to the retina. 

 Between the iris and the cornea there is a small chamber 

 containing a highly retractile fluid — the aqueous humour. 

 This is called the anterior chamber in contradistinction 

 to a much larger, but corresponding, chamber occupying 

 the bulk of the interior of the eye behind the crystalHne 

 lens and technically known as the posterior chamber, 

 which contains the vitreous fluid. 



When the eye is severely injured it sometimes happens 

 that one or both these chambers is punctured. If so, 

 the eye collapses. If the amateur wishes to obtain an 

 elementary knowledge of the anatomical construction 

 of the horse's eye, he can do so by obtaining a specimen 

 from a dead horse, subsequently placing the organ in ice 

 in order to freeze it, and so facilitate its proper examina- 

 tion, after making a vertical section through it. 



The foregoing outlines may serve as a preliminary 

 introduction to such study. 



* The horse has a third eyelid known as the membrana nictitans, and this 

 is a triangular piece of cartilage springing from the inner angle of the eye. 



