166 AMERICAN FARMER'S HORSE BOOK. 



The inner coat of all is the retina (see o), the beautiful ex- 

 pansion of the optic nerve spread over the internal surface 

 of the eye. This receives the impressions conveyed by tht 

 rays of light, which, when they fall upon it, trace on its deli- 

 cate and susceptible expansion a wonderfully minute yet 

 perfect image of the object in view from whence the rays 

 have proceeded. By the optic nerve the sensation thus ex- 

 perienced is instantly conimunicated to the brain, the seat 

 of intelligence, and in this way the animal is conscious of 

 seeing. 



The three humors of the eye are the aqueous and vitreous, 

 and the crystalline lens, which comes between them. An- 

 atomists do not always call the lens a humor, but such it 

 really is. 



The aqueous (watery) humor (see q) occupies the space be- 

 tween the cornea and crystalline lens. It is a perfectly trans- 

 parent and limpid fluid, secreted by the lining of the cham- 

 ber in which it lies. K the attenuated walls of this chamber 

 are punctured, so that the humor escapes, ISTature rapidly 

 renews it, and the sight is restored. 



The crystalline lens (see g) consists of a number of concen- 

 tric layers, arranged like the coats of an onion. It has a 

 jelly-like consistence, and in shape is double-convex, as rep- 

 resented in the cut. It is the chief agent in so modifying 

 and refracting the rays of light, which are continually pro- 

 ceeding from every object within the range of vision, as to 

 cause those rays to clearly trace the miniature image of that 

 object upon the retina. Without this lens sight would be 

 impossible. 



The vitreous (glass-like) humor (see p p) fills the great bulk 

 of the globe of the eye. It is a limpid fluid, but, being 

 bound up in a net-work of transparent cells, it has the ap- 

 pearance and consistency of a thin jelly. 



In addition to these three membranes and three humors, 

 there are three other important things to be mentioned in 

 our description of the horse's eye. The first of these is the 

 conjunctiva, which is the very delicate membrane covering the 



