OPHTHALMIA. 633 



eye ; it consists of a kind of colourless jelly, more fluid than that which forms the crystalline lens, 

 the use of it being to aid in the refraction of the rays of light. We need not describe what is 

 called the hyaloid membrane. The needle next comes upon the most important structure of the 

 eye namely, the retina itself. The retina is simply the optic nerve spread out into a curtain at 

 the inner and back part of the eyeball. This is the curtain on which the picture seen is painted by 

 the light, developed as a photographer would say, and the impression being carried to the brain 

 through the medium of the optic nerve and taken cognizance thereof, sight is the result. 



Onward through the retina, and the needle pierces the choroid coat, and a very important 

 coat this is. It is a thin and delicate membrane of a deep black hue, which lines the inner surface 

 of the sclerotic coat, and its black colour is occasioned by the numbers of pigment cells vvhich it 

 contains. 



Now, the use of this choroid coat is evidently to absorb the rays of light, which would 

 otherwise be reflected, and disorder vision. The choroid, in fact, renders the principal cavity of 

 the eye a dark chamber. Without it the picture on the retina would be indistinct. To use familiar 

 language, then, the sclerotic coat is to the eye what the tin box is to the magic lantern no portion 

 of the apparatus itself, but only its box. Supposing a magic lantern built of china, it would not 

 be of much use, because the rays would pass through ; but if you lined that china box with 

 black it would do well enough. The sclerotic may be called the china framework of this truly 

 magic lantern, the eye, and the choroid coat is its black lining. Without the choroid coat the 

 picture on the retina would be as hazy and indistinct as a positive photograph taken on glass 

 before it is darkened with black varnish. 



The last portion for the needle to transfix is the sclerotic coat the strong, white, fibrous sheath 

 which, as I have already told you, encloses the principal portion of the eye. It is largely supplied 

 with nerves and blood-vessels, and to it are attached all the little elongated muscles that move the 

 eye about in any direction the animal wishes. 



Let us now pass on to the diseases of this delicate organ. 



I . Ophthalmia. 



This disease, which signifies an inflammation of the eye, is sometimes called conjunctivitis. 

 For our own part, we prefer the simple term ophthalmia, for the disease is often more than merely 

 an inflammation of the mucous membrane. The sclerotic may partake, and the areolar tissue of 

 the eyelids always does. 



Ophthalmia may be either traumatic that is, it may be the result of injury either from blows 

 or from foreign bodies, such as sand, particles of straw, or other foreign matters getting into the 

 eye or it may arise idiopathically, or be sympathetic with other diseases, such as distemper. 



Perhaps this is the right place to mention that the eye of the dog, more perhaps than that 

 of any other animal, sympathises with the disorders of the mucous membranes. Given a case of 

 diarrhoea, for instance, or gastorrhoea. Examine the dog's eye, and you will find the capillaries 

 congested, perhaps even the darker veins of the sclerotic shining through the conjunctiva. Even 

 a disordered stomach will have the same effect, though in a less degree. The eyeball of a dog 

 ought to be most beautifully white and pure, although it is not always so even in health, for 

 healthful exercise will temporarily redden the eye. 



Causes of ophthalmia. The first to be mentioned are injuries. Generally speaking, in this 

 case only one eye is inflamed. The worst case of this sort we remember was caused by a bit of 

 chaff. A large dog he was, and, poor fellow, he had been blistered and bled and physicked 

 to a great extent, but nobody had thought of examining the eye. 

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