HISTOLOGY OF THE RETINA 



549 



at the fovea and the ability to react to light of low intensity without colour 

 vision which is possessed by the periphery of the retina. The presence of 

 rods scattered between the cones naturally impairs to some extent however 

 the appreciation of fine detail. At the white papilla where the optic nerve 

 enters the eye, there are neither rods nor cones, and therefore as we should 

 expect this region is quite blind. This fact can be readily proved by looking 





FIG. 278. Look at cross with right eye, hold book at about 10 inches'. 



with the right eye at the cross in Fig. 278. If now the book be held about 

 10 inches from the eye the white disc will be found to disappear. By a simple 

 calculation it is found that the disc 

 corresponds with the papilla of the 

 optic nerve. 



THE VISUAL FIELDS. Since 

 the appearance of an external object 

 will vary to a considerable extent 

 according to the region of the retina 

 on which its image falls, it is a mat- 

 ter of considerable interest to deter- 

 mine the positions at which the 

 appearances undergo change. This 

 is also of practical value because the 

 positions are found to be affected by 

 disease. The determinations are 

 usually made by means of an instru- 

 . ment called a perimeter. 



This will beseen from Fig. 279 to consist 

 of a metal arm bent to the segment of a 

 circle. This is so mounted in relation- 

 ship to a horizontal bearing that the 

 segment always has its centre in corre- 



FIG. 279. Priestley Smith's perimeter. 





spondence with a fixed pointer which is seen on the left of the diagram. If the eye 

 of the patient is placed close to this pointer and looks towards the centre of the 

 bearing, the degrees marked on the metal segment show the actual angle at which an 

 index is situated in relationship to the eye axis, no matter what meridian the metal 

 segment may lie in. The index mark usually consists of a small disc 2 mm. diameter 

 either of white or of coloured paper, according to whether the rod or cone area is to 

 be determined. 



