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A TEXTBOOK OF PHYSIOLOGY 



be seen, generally running horizontally; if a side to side, vertically 

 running vessels will be apparent; if a circular movement, the general 

 distribution of the vessels will be visible. In the direct line of vision 

 there is a small area in which no vessels are seen the macula lutea, 

 or yellow spot. 



FIG. 351. SECTION~OF FROG'S RETINA. (After Engleraann.) 

 A, After exposure to light; B, when kept in the dark. 



In these experiments the movement of the light or the illuminated 

 field i& important. The retina appreciates shifting shadows better 

 than fixed one?. 



2. At the point where the optic nerve loaves the eye there are 

 no rods or cones,Tand this spot is insensitive to light. This blind 



Fio. 352. A, THE NORMAL APPEARANCE OF THE RETINA IN THE RABBIT'S EYE 

 BEFORE EXPOSURE TO LIGHT; a, THE ENTRANCE OF THE OPTIC NERVE; b, b, A 

 COLOURLESS LAYER OF MEDULLATED NERVE FIBRES; c, A LAYER OF DEEPER 



COLOUR SEPARATING THE LIGHTER UPPER FROM THE MORE HEAVILY PlGMENTED 



LOWER PORTION. B, OPTOGRAM OF A WINDOW AFTER EXPOSURE OF EYE. 

 (Kiihne.) 



spot is readily demonstrated by Fig. 353. If the left eye be 

 closed, and the right eye gazes at the dot from the distance of a 

 foot, the line will appear continuous. Normally, we do not notice 

 the blind .spot, but fill in the gap with sensations similar to those 

 falling on the neighbouring areas of the retina. The blind spot can 

 be mapped out as follows: Let the head rest in a fixed position , 



