79 TROPHIC NERVES. [CH. LVI. 



clearly are all its details seen. Moreover, if we want carefully to 

 examine any object, we always direct the eyes straight to it, so 

 that its image shall fall on the yellow spot where an image of a 

 given area will cover a larger number of cones than anywhere else 

 in the retina. It has been found that the images of two points 

 must be at least 3 fj. apart on the yellow spot in order to 

 be distinguished separately ; if the images are nearer together, 

 the points appear as one. The diameter of each cone in this part 

 of the retina is about 3 /x. 



Visual judgments are not always correct ; there are a large 

 number of puzzles and toys which depend on visual illusions. 

 Two of the best known are represented in the preceding diagrams. 



In fig. 600, A, B, and c are of the same size ; but A looks taller 

 than B, while c appears to cover a less area than either. 



In fig. 60 1, the horizontal lines are parallel, though they do 

 not appear so, owing to the mind being distracted by the inter- 

 crossing lines. 



CHAPTER LVI. 



TROPHIC NERVE?. 



NERVES exercise a trophic or nutritive influence over the tissues 

 and organs they supply. The chemical changes that occur during 

 the nutrition of a living cell may bo summed up in the word 

 metabolism ; and this includes two kinds of changes ; anabolic 

 phenomena, that is the processes of building up protoplasm from 

 food material ; and katabolic phenomena, those in which there is a 

 breaking down of protoplasm, and a consequent formation of 

 simpler waste products. 



Some nerves increase the building-up stage of metabolism ; 

 these are termed anabolic. Such a nerve is the vagus in reference 

 to the heart ; when it is stimulated the heart beats more slowly 

 or may stop, and is thus enabled to rest and repair its waste. 

 The opposite kind of nerves (katabolic) are those which lead to 

 increase of work and so increased wear and tear and formation of 

 waste products. Such a nerve in reference to the heart is the 

 sympathetic. 



There has been considerable diversity of opinion as to whether 

 trophic nerve-fibres are a distinct anatomical set of nerve-fibres, 



