176 PRINCIPLES OF VETERINARY SCIENCE 



or near at hand. Each eyeball is afforded varied combinations 

 of .movements by special voluntary muscles that are capable 

 of perfectly balancing the eyes for binocular vision. 



By cutting the optic nerve total blindness is produced, although 

 light passes through the refractive media of the eye to the retina. 

 This is because the path of conducting nerve-fibers is no longer 

 capable of conducting impulses to the brain and there is no con- 

 sciousness of stimulation at the nerve ending. 



As all are more or less familiar with the parts of a camera, we 

 may compare the eye with a camera and so learn the parts and 

 functions of each part of the eye. The outermost structures of 

 the eye, as has been stated, are the lids which correspond to the 

 shutter of a camera and completely shut out light from the eye 

 when closed. Just behind the lids is found the cornea, which is 

 represented in the compound camera by the front lens. Behind 

 the cornea is the aqueous humor, a structure which has no 

 counterpart in the camera. The iris with its central opening, 

 the pupil, is represented in the camera by the diaphragm; in 

 both the eye and camera it regulates the amount of light that 

 passes by "cutting down" or "dilating" the aperture. On a 

 bright day in winter when the sun reflects intense light from the 

 white snow into the eyes, the pupil of the eye becomes very small 

 so as not to allow the rays to flood the back chamber of the eye 

 and injure the sensitive structures located there. The same is 

 true of the camera — on an extremely bright day the size of the 

 opening in the diaphragm is greatly reduced, to cut down the 

 amount of light admitted and prevent over-exposure. Imme- 

 diately after passing through the pupil, the light rays strike the 

 crystalline lens which refracts or bends them and brings them 

 more closely together as does the back lens in a camera. They 

 then pass through the vitreous humor, which may be compared 

 with the air in the bellows of the camera, but is unlike the latter 

 in that it is a refractive medium. The walls of the fundus of 

 the eye are darkened by the choroid or middle coat to absorb 

 light, as are the walls of the bellows. The choroid is supported 

 by the sclera, which in the camera is represented by the case. 

 The retina, or expanded end of the optic nerve, corresponds to 

 the sensitive plate or film of the camera, on which an inverted 

 image of the object is made. 



Both the eye and the ear have been called distance receptors 



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