THE EYE 



391 



image of this spot changes on the retina. When the spot cannot be 

 seen, it is because the image falls on the blind spot. 



Dissection of the Eyeball. Procure from the butcher two or three 

 eyeballs obtained from cattle. After separating the fat, connective 

 tissue, and muscle, place them in a shallow vessel and cover with water. 

 Insert the blade of a pair of sharp scissors at the junction of the scle- 

 rotic coat with the cornea 

 and cut from this point 

 nearly around the entire 

 circumference of the eye- 

 ball, passing near the op- 

 tic nerve. Spread open 

 in the water and identify 

 the different parts from 

 the description in the text. 

 Open the second eyeball 



in water by cutting away 



_ . FIG. 167. Model for demonstrating the 



the cornea. Examine the 



eyeball, 

 parts in front of the lens. 



To illustrate Accommodation. Paste together the ends of a strip of 

 stiff writing paper (two by five inches) making a ring a little less than 

 three inches in diameter. This is to represent the crystalline lens. 

 Now paste a piece of thin paper (two by seven inches) upon a second 

 strip of the same size, leaving an open place in the middle for the in- 

 sertion of the paper lens. A flexible piece of cardboard (three by twelve 

 inches) is now bent into the form of a half circle and to its ends are 

 fastened the strips of paper containing the ring. Make a small hole in 

 each of the four corners of the bent cardboard. Through these holes 

 pass two loops of thread, or fine string, in opposite directions, letting 

 the ends hang loose from the cardboard. 



When everything is in position, the tension from the cardboard 

 flattens the paper lens, while pulling the strings releases this tension 

 and permits the lens to become more rounded. With this simple 

 device the changes in the curvature of the lens for near and distant 

 vision are easily shown. 



