324 



THE HUMAN BODY. 



The Blind Spot, Where the optic nerve enters the re- 

 tina it forms a small elevation (Fig. 90), from which nerve- 



FIG. 90. The right retina as it would be seen if the front part of the eyeball 

 with the lens and vitreous humor were removed. The white disc to the right 

 marks the entry of the opiic nerve (blind spot); the lines radiating from this are 

 the retinal arteries and veins. The small central durk patch is the yellow spot, 

 the region of mobt acute vision. 



fibres radiate. This elevation is quite blind, because it pos- 

 sesses neither rods nor cones. Its blindness may be readily 

 demonstrated. Close the left eye and look steadily with the 

 right at the cross (Fig. 91), holding the page vertically in 



FIG. 91. 



What is meant by the "blind spot"? Describe a method of 

 demonstrating its blindness. 



