The Nervous System. 99 



The vitreous humor, a jelly-like mass fills up the bulk of 

 the eye, and is located back of the lens, once destroyed 

 is never reproduced. The inner coat, retina, is the dis- 

 tribution of the optic nerve (nerve of sight) and is con- 

 sequently the most important structure in the eye. 



The choroid coat lines the sclerotic, at its front por- 

 tion is attached a muscular curtain, the iris, located in 

 front of the crystalline lens. This curtain (iris) is pierced 

 in its center by 'an opening, which varies in shape in the 

 different animals and is known as the pupil. This open- 

 ing is dilated or contracted, by two sets of muscular 

 fibres, according to the amount of light the sensitive 

 structures of the eye are able to take in. The color of 

 the eyes is due to the iris, in most horses it is of a brown- 

 ish yellow tint, in others white or grey, when the latter 

 color, the horse is said to be wall-eyed; at the upper 

 border of the pupil of the horse little black sooty look- 

 ing dots are seen, these are the corpora nigra, they are 

 said to absorb rays of light. 



The outer covering, sclerotic coat, is hard and fibrous, 

 and receives the attachment of the muscles of the eye- 

 ball, it is opaque; at its front portion is inserted the cor- 

 nea (watchglass), a delicate transparent membrane made 

 up of layers of cells; between these layers as a result of 

 inflammation material may be deposited, such is often 

 thought by people ignorant of the construction of the 

 cornea to be a scum; it is important to remember that 

 this is not the case and that therefore the use of strong 

 irritants such as powdered alum or powdered glass is not 

 only cruel but wrong and never beneficial. The eyelids 

 are two movable curtains having in their free edges hairs 



