514 * VISION 



A. DIRECT AND INDIRECT VISION 



When we wish to scrutinize an object very closely, we so direct the eye 

 that the middle point of the object is pictured on the fovea centralis of the 

 yellow spot in the retina. This point is therefore designated as the center of 

 exact vision. The diameter of the fovea according to Fritsch is 1-1.5 mm., 

 so that it corresponds to a visual angle (see page 517) of 4 -6. 



The nervous elements of the retina, however, reach all the way to the 

 or a sermta, and, being also sensitive to light, can produce conscious sensa- 

 tions from all parts. But these sensations, as compared with those aroused 

 from the fovea, are more and more indistinct the farther the retinal cells 

 affected lie from the fovea. 



The reader can convince himself of this by a very simple experiment. If 

 one eye be closed and the other be directed intently at some object, he will find 

 that of all the objects in the room only that one directly regarded and those 

 lying nearest it are seen distinctly, others appear less and less distinct the 

 farther they are situated from the line of vision. Vision with those parts 

 of the retina lying outside the fovea centralis is called indirect vision. 



Indirect vision is of very great service, for by it we obtain some idea of the 

 space in which the object directly regarded is situated. Especially is it of 

 service in walking, as anyone can prove to himself by trying to walk over an 

 unfamiliar path with one eye closed and with indirect vision of the other 

 excluded by looking through a tube or through the half-closed hand. He finds 

 it difficult either to perceive or to avoid obstacles. In fact even close work, 

 such as reading a printed page, is much more difficult under such circum- 

 stances, because only a small part of the print can be seen at one time. 



B. THE LIGHT-PERCEIVING LAYER OF THE RETINA 



The retina consists of several different elements, part of which are nervous 

 in nature and part serve as a supporting substance for the nervous structures. 

 Ramon y Cajal has published not long since a detailed investigation of the 

 structure of the retina. His chief results so far as the nervous elements are 

 concerned, may be summarized briefly as follows (cf. Fig. 



The rod fibers (bb) whose bodies together with those of the cones, consti- 

 tute the outer granular layer (#) end inwardly in little knots embraced by the 

 terminal fibers of the outer processes of the definitive bipolar cells (e). These 

 cells together with those belonging to the cones constitute the inner granular 

 layer (E) ; their outer tuft of dendrites is directed vertically. Below the bipolar 

 cell rests upon a ganglion cell (n) and clasps it with fingerlike branches. These 

 ganglion cells form the so-called ganglion-cell layer (G). 



The cone fiber (a) ends in a broad base, from which short basilar dendrites 

 are given off. With these the dendrites of the spinal bipolar cells (e} belong- 

 ing to the cones come into contact. The outer tuft of dendrites of these bipolar 

 cells, in contrast with that of the bipolar cells belonging to the rods, is quite 

 flat, and widely spread out. The inner process ends at various levels of the 

 inner plexiform layer (F} in a terminal arborization which comes into relation 

 with the outwardly directed branchlets of definite ganglion cells. 



