140 



ANATOMY AND PHYSIOLOGY OP 



moving the iris, the second humor, or crys- 

 talline lens, appears : this is retained in its 

 situation by a transparent membrane, named 

 its capsule, betvveen which and the lens is a 

 minute quantity of fluid. The third humor 

 of the eye is the vitreous. This humor is not 

 contained in one general sac, but in numer- 

 ous minute and perfectly transparent cells, 

 and resembles pm-e water : this humor 

 serves to produce a small degree of refrac- 

 tion in the rays of light, and occupies and 

 distends all the posterior part of the globe 

 of the eye. The next coat to the conjunctive 

 is the sclerotica, or white of the eye, a 

 strong, thick membrane, which extends 

 from the transparent cornea to the optic 

 nerve. The next coat to the sclerotic is the 

 choroid. This is a delicate and very vascu- 

 lar membrane. In the human eye it appears 

 of a black color, and it is this which causes 

 the pupil of the human eye to appear black ; 

 but the choroid coat of the horse's eye is 

 variegated in color ; in some parts black, in 

 others blue, and in others green. The next 

 coat is the retina : this is a delicate expan- 

 sion 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 cir- 



cumstances may occur to render vision im- 

 perfect, 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, producing an in- 

 distinctness 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 per- 

 fectly excluded from the retina. Supposing 

 the cornea and iris to be healthy, the crys- 

 talline 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 : some- 

 times it is very slight, the pupil appearing 

 of a lighter color, and unusually large : in 

 this state, the pupil is said to look duU or 

 muddy, which causes the horse to start ; 

 but when the opacity is complete, it consti- 

 tutes the disease termed cataract. There 

 is another disease, to which the reader's at- 

 tention is caUed ; it is named gutta serena, 

 or amaurosis. This disease is known by 

 the pupil being unusually large or open, 

 and by its continuing so when the eye is 

 exposed to a strong light. 



EXA3IINATI0NS OF THE ORGANS OF SIGHT AND THEHl APPENDAGES. 



Q. Where are the eyes located? — A. Within the 

 orbits. 



Q. By what foraminae is each orbit perforated? — A. 

 By the optic forapien. 



Q. From whence is the lining membrane of the orbit 

 derived ? — A. From the dura mater and periosteum, 



Q. Enumerate the appendages of the eye. — A. The 

 eycUds, eyelashes, muscles of the eyeUds, tarsal carti- 

 lages, meibomian glands, tunica conjunctiva, membrana 

 nictitans, lachrjTnal gland, puncta lachrjinaUa, lachi-y- 

 mal sac, ductus ad nasum, and the muscles of the eye- 

 ball. 



EYELIDS. 



Q. What parts do the eyelids occujjy ? — ^I, The cir- 

 cumference of the orbits and front of the eyeball. 



Q. What are the eyehds composed of? — ^. In com- 

 position they are cuticular, muscular, cartilaginous, and 

 membranous ; also glandular, vascular, and nervous. 



Q. What muscle enters into the composition of the 

 eye ? — A. The orbicularis palpebrarum. 



Q. How are the Uds separated ? — A. By a transverse 

 fissure, bounded by the angles or canthi of the eye. 



Q. What is. attached to the superior or temporal 

 angle? — A. The tarsal ligament. 



