THE EAR, THE EYE, THE SKIN, AND THE FOOT 129 



or white of the eye, is the thiclv, tough membrane which supports 

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



The chambers of the eye are tv/o 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 int^erior of the globe; and the crystalline lens, a biconvex 

 structure composed of concentric layers of tissue. It 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. Any impulses which it receives 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. They have the power of accommodation by which it is 

 possible to see objects at close range as well as at a great distance. 

 Each eyeball is afforded varied combinations of movements by 

 special voluntary muscles that are capable of perfectly balancing 

 the eyes for binocular vision. 



THE SKIN 



The shin i& the covering tissue which acts as a mechanical pro- 

 tection to the surface of the body. It consists of two main layers, 

 the epidermis or scarf skin being the outer and the dermis or true 

 skin the inner. The epidermis is composed of several layers of 

 cells, the outer of which are horny, scale-like, and fall off in the 

 form of dandruff when horses are groomed. If contains the 

 coloring-matter or pigment of the skin. The dermis is a vascular 

 structure containing a network of white fibrous tissue with some 

 elastic fibers. Its inner face adheres closely to the strata of fat 

 tissue which Ues beneath it. Its external face is perforated by 



