836 THE SENSE OF SIGHT 



white paper held at a moderate distance vertically in front of it. By 

 now slowly moving the head of a pin inward along the horizontal 

 plane of this field, a point will presently be reached when it suddenly 

 disappears from the view and later On, a second point, when it reappears. 

 By repeating this procedure along the vertical and oblique planes, the 

 margins of this indifferent area in space may be mapped out in its 

 entirety (Fig. 440). By projection it then becomes possible to calcu- 

 late the position and size of the optic disc. It will be found that ob- 

 jects of the size of a 5 cent piece may be made to disappear when held 

 at a distance of about 25 cm., and objects of the size of the head of a 

 man, when placed at a distance of about 2 m. The shght irregularities 

 in the contours of this projected figure of the blind spot are due to 

 the interception of the light rays by the blood-vessels as they cross the 

 margin of the optic papilla. 



Au 



Fig. 440. — Form of the Blind Spot. (Helmholtz.) 



The Yellow Spot. — About 3.5 mm. or 15 degrees outside the bfind 

 spot and 1 mm. above the level of a horizontal line drawn through 

 the posterior pole of the eyeball, lies the most sensitive area of the 

 retina. It is known as the macula lutea, while its somewhat depressed 

 center is designated as the fovea centralis. The latter measures about 

 0.2-0.4 mm. in diameter, and the entire area about 2 mm. Histo- 

 logically it is noted that the retina of this region is greatly thinned, 

 retaining in the fovea centralis merely the layer of the rods and cones. 

 Moreover, it is observed that this particular area is made up exclu- 

 sively of cones which are much larger than those situated in the out- 

 lying districts of the retina. They are so closely packed that they give 

 the appearance of a mosaic of hexagonal prisms (Heine). The fibers 

 leaving these elements, pursue an oblique course and push the inner 

 layers of the retina farther toward the circumference of the macula 

 lutea, so that the cones are more fully exposed to the entering rays 

 of light. ^ Outside the macula the cones decrease and the rods in- 

 crease in number. When these histological characteristics are com- 

 pared with those of the blind spot and those of the outlying districts 

 of the retina, it must be concluded that the cones are the most im- 

 portant factor in vision. This inference finds further support in 



^ Rochon-Duvigneaud, Archives d'anat. micr,, ix, 1907. 



