42 PHYSIOLOGY. 



retina : this is a delicate expansion of the optic nerve over 

 the choroid coat, which it accompanies to the margin of the 

 crystalline lens, and there terminates. 



The use of the retina is to receive certain impressions 

 made by the light reflected from objects, so as to produce in 

 the mind an idea of their figure and color ; the optic nerve 

 being the medium of communication between the retina and 

 brain. From the above explanation of the mechanism of the 

 eye, it will readily appear that many circumstances may occur 

 to render vision imperfect, or to destroy it altogether. If the 

 transparent cornea, for example, became white, light could 

 not pass through it, and the animal would be blind^however 

 perfect the other parts of the eye might be. The cornea 

 may be either too convex or too flat ; in the former case, 

 causing the animal to be near-sighted ; in the latter, produ- 

 cing an indistinctness of vision with respect to objects that 

 are near. The iris may, in consequence of disease, become 

 fixed, or lose its power of motion ; in which case, the pupil 

 would be always of the same size, and the animal would not 

 have the power of adapting it to the various distances or 

 objects ; or, as sometimes happens, the pupil may become 

 quite closed, by which light would be perfectly excluded 

 from the retina. Supposing the cornea and iris to be healthy, 

 the crystalline lens, or its capsule, may become opaque, and 

 thereby cause total blindness. But in this part, as in the 

 cornea, we meet with different degrees of opacity : sometimes 

 it is very slight, the pupil appearing of a lighter color, and 

 unusually large : in this state, the pupil is said to look dull or 

 muddy, which causes the horse to start \ but when the opacity 

 is complete, it constitutes the disease termed cataract. There 

 is another disease, to which the reader's attention is called ; it 

 is named gutta serena, or amaurosis. This disease is known 

 by the pupil being unusually large or open, and by its con- 

 tinuing so when the eye is exposed to a strong light. 



Remarks on the Causes of Disease in the Eye. — Young 

 horses, generally, are peculiarly liable to a disease called 

 ophthalmia. Such as are got by a blind stallion, or bred from 



