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PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



of the arrow must be seen, each in its proper place, and 

 therefore erect. Suppose we stand at night in the open 

 air beneath the star-lit sky. Is it any mystery that the 

 stars are seen each in its proper place ? Now, every ob- 

 ject consists of an infinite number of starlike radiants, 

 and each radiant is referred back along its ray line to 

 its proper place. 



2. THE FOVEA AND ITS SPATIAL REPRESENTATION. 



The fovea, or central spot, is directly in the axis of 

 the eye — the south pole of the globe. It differs from 

 other parts of the retina in several respects: i. The 

 fibrous layer and the larger part of the nuclear layer is 

 wanting, so that the bacillary layer is more directly 

 exposed than elsewhere to the direct action of light. 2. 

 This part of the bacillary layer consists of cones only. 

 3. The cones here are much smaller than elsewhere. 

 From the absence of the other layers this point is 

 depressed, hence the name fovea, a pit. It is evidently 

 the most highly organized part of the retina. 



Its spatial representative is the spot we look at — the 

 point of sight, or rather the line of sight, and a small 

 area about it. When we look at anything the axis of the 

 eye is turned directly upon it, and the image of the thing 

 falls on the fovea. The point we look at and a small 

 area about it are seen distinctly. If we look steadily 

 at the point and at the same time observe our per- 

 ception of objects in other parts of the field of view, we 

 find that while their presence is plain enough, their ex- 

 act form and surface-detail are more and more imper- 

 fectly perceived as we go from the point of sight. This 

 is not the result of imperfect image, but of imperfect 

 perception of the image ; not the result of an imperfect 

 instrument, but imperfect retinal response. We can not 

 have a better illustration of this than the act of reading. 



