LABORATORY WORK 213 



Anatomy of the Eye. Take the fresh eye of an ox. Note 



The transparent cornea in front, continuous with the opaque 



sclerotic. 



The entrance of the optic nerve, opposite the cornea. 

 The coloured iris, with the aperture (pupil) in the middle. 



Divide the eye into front and back halves by a razor. Note 



In posterior half 



Gelatinous vitreous humor in posterior chamber. 



Thin transparent retina, covering the 



Black choroid coat. 



Entrance of the optic nerve and its continuity with the 



retina. 

 In anterior half 



The coating membrane of the vitreous humor attached to 

 the front of the choroid and holding the lens in a 

 capsule behind the pupil. 

 Remove the lens, noting its elastic nature. 



Note the aqueous humor, filling the anterior chamber between 

 the cornea and the lens. 



Take another eye and freeze it in a mixture of ice and salt. 

 When solid, divide it by a section at right angles to the preceding 

 one, namely, perpendicular to the surface of the cornea and passing 

 through the entrance of the optic nerve. 



Study the relative positions of the parts as they become visible 

 on thawing. Note especially the way the capsule of the lens is 

 attached to thickened ridges of the choroid close behind where the 

 cornea and sclerotic meet ; and that the iris is also attached to 

 these "ciliary processes." The fibres of the ciliary muscle in them 

 are firmly attached at the junction of the cornea and sclerotic, and 

 when they contract they pull forward and relax the suspension of 

 the lens, so that the latter takes up more or less its natural more 

 spherical form. Thus the focal length is altered to accommodate 

 for near objects. 



Image on the Retina. Expose the back of the eye in a dead 

 rabbit by cutting away the bone and other tissues. Place an 

 incandescent lamp two or three yards in front of the eye, and 

 examine the back of the eye with a lens. If the pigment of the 

 choroid is not too dense, a minute inverted image of the filament 

 is seen on the retina. It is easily seen if an albino rabbit, which 

 has no pigment in the choroid, be taken. It is possible, with care, 

 in an ordinary rabbit to cut out a little window in the sclerotic, 

 and to brush away the pigment with a camel hair brush wetted 

 with 0.9 per cent, saline. 



When the image is seen, place a microscope slide in front of the 



