THE EAR, THE EYE, THE SKIN, AND THE FOOT 175 



the overflow of fluid to the nasal cavity. Tears assist in re- 

 moving the small particles of dirt from the eyeball and keep it 

 moistened. 



The eyeball is composed of the segments of two spheres of 

 different sizes (Fig. 56) . The anterior is the cornea; the posterior, 

 the sclera. The cornea is transparent, oval in outline, and fits 

 into the sclera like a crystal does into a watch. The sclera, or 

 white of the eye, is the thick, tough membrane which supports 

 the choroid, or colored layer, the ciliary body, and the iris. At 

 the free margin of the iris is formed the granula iridis, "soot 

 ball," or corpora nigra, which is relatively large and easily seen 

 in the eye of the horse. The opening in the iris is the pupil. 



The chambers of the eye are two in number. The anterior 

 chamber is enclosed in front by the cornea and behind by the 

 iris. The posterior chamber communicates with the anterior 

 through the pupil or opening in the iris, and is much the smaller. 

 An aqueous humor of clear, water-like consistency fills these 

 chambers. 



The refractive media of the eye consist of the vitreous humor, 

 a semifluid, transparent substance occupying about four-fifths of 

 the whole interior of the globe ; and the crystalline lens, a biconvex 

 structure composed of concentric layers of tissue. The latter 

 is situated just behind the pupil. 



The optic nerve enters the eyeball from the rear and expands 

 to form a very delicate and extremely sensitive membrane known 

 as the retina. 



The function of the eye is to form an image in its fluids so that 

 the animal may better adjust himself to his surroundings. The 

 retina is the sensory surface which responds to stimulation by 

 light. The image is upside down, but this fact is of little con- 

 sequence for it is as easy for the animal to become accustomed to 

 seeing things one way as another. Any impulses which it re- 

 ceives are transmitted by the fibers of the optic nerve to the visual 

 center in the cortex of the brain. The iris regulates the amount 

 of light which passes to the retina by dilating and contracting 

 the pupil. The refractive media are for the purpose of bringing 

 rays of light to a focus on the retina. The power of accommoda- 

 tion, by which it is possible to see objects at close range as well as 

 at a great distance, lies in the lens, which is flattened or made 

 more convex depending upon whether the object is at a distance 



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