MANUAL FOR STABLE SERGEANTS. 39 



fifths of the eyeball and affords attachment for the muscles of the eye. 

 In front of and continuous with the sclerotic coat is the cornea, a 

 transparent tissue which admits the light to the interior of the eye. 



93. The choroid, or middle coat, is a vascular membrane of a 

 dark color. At the juncture of the sclerotic coat and the cornea the 

 choroid coat sends down a circular dark colored membrane, the iris, 

 in the center of which is the -pupil, a small opening for the admission 

 of light. By muscular contraction the pupil can be increased or 

 decreased in size to regulate the amount of light passing into the 

 eye. On the iris above and below the pupil are several small black 

 projections called granula iridis, or soot balls. The use of these 

 bodies is not definitely known. 



94. The retina, or inner coat, is the expansion of the optic nerve. 

 It receives the impressions of sight, and the optic nerve transmits 

 them to the brain. 



95. The crystalline lens is situated behind the iris and pupil. 

 It is a circular, transparent body, thick in the middle and tapering 

 toward the edges. Its function is to focus the rays of light on the 

 retina. 



96. The aqueous humor, a watery fluid, occupies the space 

 between the cornea and the crystalline lens. The vitreous humor, 

 a jellylike fluid, occupies the space between the retina and the 

 crystalline lens. The function of these fluids is to maintain the 

 proper shape of the eyeball. 



The Ear. 



97. The ears are the organs of hearing. They are located one on 

 -either side of the poll. 



98. The ear may be di^dded into two portions, external and 

 internal. 



99. The external ear is funnel-shaped, formed of cartilage, and 

 covered with skin both inside and outside. There are numerous 

 fine hairs on the inside of the funnel which prevent the passage of 

 foreign bodies into the internal ear. The lower portion of the funnel 

 communicates with the internal ear and is supplied with numerous 

 sebaceous glands. There are muscles attached to the lower part of 

 the cartilage by which the ears are moved at will. 



100. The internal ear is inclosed in a bony chamber. Here the 

 nerve endings receive the impressions of sound and transmit them 

 to the brain. Separating the internal ear from the external ear is 

 the tympanum, or ear drum. 



