THE SENSE OF SIGHT. 287 



oblique arises, like the recti, at the deeper part of the eye socket and 

 passes forward through a fibre-cartilaginous loop or pulley near the 

 inner, upper angle of the orbit, and then runs outward and is attached 

 to the upper surface of the eyeball. 



Movements of the Eye. These six pairs of muscles move the 

 eyes to right and left, up and down, and give rotary movements. 

 Normally the two eyes move in the same direction at the same time, 

 though in looking at near objects the two eyes both point inward, so 

 that one appears cross-eyed, and in looking at an object that is moving 

 away from one, the eyes are gradually diverging, though this is slight. 



Dissection of an Eye. The muscles and external parts of the eye 

 may readily be seen by examining the eye of a rabbit in its natural 

 position and then dissecting it out. A beef eye should be obtained 

 from the butcher and the structure of the eye learned by following the 

 description below. 



The Coats of the Eye. There are three coats, the outer 

 or sclerotic, the middle or choroid, and the inner called the 

 retina. 



The Sclerotic Coat. This is of a dull white color, con- 

 stituting the " white of the eye." It is thick and tough, 

 holding all the contained parts firmly and furnishing suffi- 

 cient strength for the attachment of the muscles that move 

 the eyeball. 



The Choroid Coat. The middle layer of the eye coat 

 is the choroid. It is thinner than the sclerotic and of much 

 more delicate structure. It is permeated by blood tubes, 

 and has an inner lining of dark color to prevent the reflec- 

 tion of light in the eye, just as most optical instruments 

 are painted black on the inside. 



The Retina. The retina is a continuation and expan- 

 sion of the optic nerve and forms an inner coat that lines 

 all but the anterior part of the eye. It is a thin, translu- 

 cent film, somewhat like the film that forms over the white 



